(***
ELECTRONIC
CIRCUIT DEVICES
AVIONICS TECHNICIAN TRAINING COURSE
Digitized by the Internet Archive
in
2012
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ORDER NO.
ELECTRONIC
CIRCUIT DEVICES
AN AVIATION MAINTENANCE PUBLISHERS,
TRAINING MANUAL
By
Frank Harris
:
I
'V
k
J
A P, Inc.
P.a Box 36* 1000 College View Drive
Riverton,
Wyoming 85201-0036
Tel: (BOO) 443-9250 • (307) 856-1582
INC.
EA-192-1
International
Standard Book Number 0-89100-192-1
Aviation
Maintenance Publishers. Inc.
View Dr.. Riverton. WY 82501
P.O. Box 36. 1000 College
Copyright 1983 by Frank Harris
All Rights Reserved
Printed
in
the United States ot America
Table of Contents
Introduction
ix
ELECTRONIC CIRCUIT DEVICES
I.
II.
1
Basic Electronic Theory
1
A.
Atoms, Crystals, and Energy States
1
B.
Conductivity
2
C.
Insulators
3
D.
Semiconductors
3
E.
Controlled Introduction of Electrons and Holes
5
F.
Conduction by Holes
6
G.
Conduction by Doped Semi-conductors
8
H.
Thermistors
8
I.
Photoconductors and Photo Resistors
8
J.
Electron Conduction in
Vacuum Tubes
Diodes
9
13
A.
Diodes
13
B.
Ideal Diodes
13
C.
Vacuum Tube Diodes
14
D.
Semiconductor
15
E.
Diode Applications
19
F.
Filtering Rectifier
G.
Diode Clippers
23
H.
Diode Peak Detector Circuits
23
Output
21
i
III.
I.
DC
J.
Diode Switch
25
K.
Diode Detector
26
L.
Diode Frequency Converters
27
to
DC
Voltage Inverter
Special Purpose Diodes
Diodes
33
33
A.
Stabistor
B.
Silicon Solar Cells
34
C.
Light Emitting Diodes
35
D.
High Voltage Diodes
36
E.
Zener Diodes
37
F.
Varactor and Step Recovery Diodes
38
G.
Pin Diodes
42
H.
Schottky or
HCD Diodes
43
I.
The Tunnel Diode
44
J.
Tunnel Diode Oscillators
46
K.
Tunnel Diode Amplifiers
49
L.
Tunnel Rectifiers
50
M. Gunn Diodes
IV.
24
Transistors and Other Electronic Control Devices
50
53
A.
The
B.
The Basic Transistor Amplifier
C.
The
D.
Vacuum Tubes
60
E.
The Transistor
62
Ideal Control Device
Ideal Transistor Volt- Ampere Characteristic
53
59
60
V.
F.
Bipolar Transistors
63
G.
How
to
Turn Off a Transistor
63
H.
How
to
Turn On a Transistor
64
I.
Why
Transistors
66
J.
Transistor Input Characteristic
68
K.
Limitations in Transistor Performance
69
L.
Transistor Fabrication
70
M.
Testing Transistors
71
AC
Power Control Devices
75
A.
Controlling Alternating Current
75
B.
The Thyratron
76
C.
Thyristors
77
D.
The P-N-P-N Diode
77
E.
Inert
F.
Silicon Controlled Rectifiers
G.
Controlling the
H.
Controlling Full
I.
VI.
Have Gain
Gas Lights and Voltage Regulators
DIACS
and
79
80
SCR
81
Wave AC Power
85
TRIACS
85
89
Field Effect Transistors
A.
Introduction
B.
Junction Field Effect Transistors
90
C.
Field Effect Current Regulator Diodes
92
D.
The Metal-Oxide-Semi-conductor
E.
MOSFET
•
Symbols
FET (MOSFET)
89
92
95
m
F.
NMOS
G.
Complementary
H.
Dual Gate
I.
VII.
VMOS
and
PMOS
96
MOSFETs (CMOS)
97
MOSFETs
98
Power Transistors
99
J.
How
K.
Storing Loose
L.
Installing
M.
Built-in
to Protect
MOS Transistors
MOSFET
and Integrated Circuits
Devices
MOSFETs in Circuits
MOSFET
99
100
100
101
Protection
Transistor Amplifiers
103
A.
Amplifiers
103
B.
Impedance Matching
103
C.
Basic Transistor Amplifiers
105
D.
The Emitter Follower (Common
E.
The Common Base Amplifier
108
F.
Direct Coupled Transistor Amplifiers
109
G.
Basic Field Effect Transistor Amplifiers
110
H.
Alphabet Classification
110
Collector Amplifier)
of Amplifiers
107
I.
Linearity and Distortion
Ill
J.
Class
A Amplifiers
112
K.
Class
B
Amplifiers
113
L.
Class
C Amplifiers
115
M.
Class
AB
120
N.
Class
D
Amplifiers
120
O.
Class
E
Amplifiers
120
P.
Biasing Transistor Amplifiers
Amplifiers
.
.
iv
121
VIII.
IX.
Biasing Class
R.
Static
and Dynamic Amplifier Characteristics
Sine
Wave
Oscillators
Amplifiers
123
125
129
A.
Introduction
129
B.
The Phase
130
C.
Resonant Circuit Oscillators
132
D.
Colpitts and Hartley Oscilators
133
E.
Crystal Oscillators
135
F.
The Armstrong
137
G.
The Regenerative Detector
Square
Wave
Shift Oscillator
Oscillator
Generators and Bistable Circuits
138
141
A.
Introduction
141
B.
The Multivibrator
142
C.
Uses
145
D.
Bistable Flip-flops as Memories
147
E.
Astable Multivibrators
148
F.
Synchronized Multivibrators
149
G.
Monostable Multivibrators
150
H.
Schmitt Triggers, Zero Crossing Detectors, and Comparators
151
Unijunction Transistor Oscillators
154
Integrated Timing Circuits
155
I.
J.
X.
A
Q.
for the Bistable Flip-flop
Operational Amplifiers
161
A.
Introduction
161
B.
The Op-Amp-The
Ideal Amplifier
161
C.
Differential
Amplifiers
161
D.
Inverting and Non-inverting Inputs
163
E.
Operational Amplifier Design
163
F.
The Comparator
165
G.
The Voltage Follower
166
H.
Precision Diode
168
Operational Amplifiers with Controlled Voltage Gain
169
J.
Balancing Operational Amplifiers
171
K.
A
172
L.
Single Power Supply Amplifiers
173
M.
Op-Amp Output Impedance
173
I.
XI.
Applications for Operational Amplifiers
177
A.
Introduction
177
B.
Operational Amplifiers as Differential Amplifiers
177
C.
Summing
177
D.
Active Frequency Filters
178
E.
The Logarithmic Amplifier
180
F.
Integrators and Differentiators
183
G.
Integrator
H.
The Operational
Amplifier
Sweep
185
Circuits
Differentiator
186
The Voltage-to-Current Converter
189
Power Supplies and Voltage Regulators
193
I.
XII.
Thermocouple Amplifier
A.
Introduction
193
B.
Power Supply Design Goals
193
VI
C.
Parallel Voltage Regulators
194
D.
The
196
E.
Current Limiter Circuits
197
F.
Three Terminal Integrated Voltage Regulators
199
G.
Energy Gap Voltage Standards
201
H.
Varistors
202
Switching Power Supplies
202
I.
Series Voltage Regulator
Power Supplies and the Phate
J.
Isolated
K.
Constant
AC
Voltage Transformer
Final
to
211
233
Study Questions
255
Examination
Answers
208
217
Glossary
Answers
Isolator
to the Final
Exam
261
vn
INTRODUCTION
voltages and currents with tiny voltages and cur-
measures 1/4" by 3/4" and 90% of its bulk is
taken up with the 14 pins that enable it to be
plugged into a socket. What used to be a large
module in a costly machine is now a component
available for less money than a single resistor
rents.
cost 25 years ago.
Electronics really began with the invention of
vacuum tube in 1906 by Lee Deforest.
vacuum tube was important because,
first time it was possible to control big
the triode
The
triode
for the
As
a result, circuit designers
today use operational amplifiers in their designs
as casually as resistors were used twenty years
Unlike an electromechanical relay, it could do
without moving parts and at very high
speeds. By controlling voltages and currents we
mean turning them "on'" or "off," or sometimes
just partly "on." The vacuum tube and the transistor are comparable devices and can be thought
ago.
this
The most
example of this trend is
Microcomputers are available now on a single chip for about S5. Although
everyone is familiar with TV games and home
computers which are based on the microcomstartling
the microcomputer.
of as electrically controlled electricity faucets.
what is not generally known is that they
can be programmed to perform almost any elecputer,
Twenty years ago
a book written on elec-
tronic devices would have been limited to a
discussion of discrete electronic components such
as diodes, transistors, and vacuum tubes. Since
that time,
the integrated circuit has
made
tronic function.
They
are, in essence, the
ultimate
Unfortunately they
complex to be covered in this text.
electronic
device.
are
too
it
possible to compress complex circuits and even
Electronic devices are a wide, complex field,
and you aren't going to learn it all here. In fact, if
you spend the rest of your life studying electronics, you will still be ignorant about some
entire instruments into a single silicon chip. Sud-
denly pieces of equipment, such as operational
amplifiers, frequency counters, memory banks,
and even entire computers have become single
devices that can be plugged into a circuit much
aspects of the
field.
the beginner, but
like a transistor or diode.
old hand.
You
will
This
it is
is
very discouraging for
also the salvation for the
never run out of things to learn
and challenge you.
For example,
in
1954 an operational amplifier
was a module found only in analog computers.
Physically it was a metal chassis about the size of
a cereal box which contained half a dozen glowing
vacuum tubes and lots of resistors and wires. It
was mounted in a huge instrument rack with a
large number of similar modules. These ampli-
One of the reasons you will never stop learning in electronics is because it is practically impossible to explain anything completely and precisely, therefore,
were interconnected with test leads to
simulate engineering problems such as the design
of suspension systems in automobiles. Each
amplifier represented one of the design variables
in the system such as a coil spring, the rebound of
fiers
the
tires,
and so
your understanding
will
always
be incomplete.
we make frequent use of running
rubber balloons, childrens' swings and
other simple analogies. This is enough for the
earliest rough approximation, but you should
never let yourself believe that electricity always
In this book
water,
on.
behaves
like the analogy.
Many
of its
more subtle
characteristics have no counterparts in the every-
Today
day world. These crude analogies are just intended to get you started. We often have to go much
four operational amplifiers are avail-
able in a single integrated circuit which sells for
49c retail. This integrated circuit (IC for short)
deeper to reach a useful level of understanding.
IX
When you
reach something you can't under-
stand, just get the general idea and take
it
on
Later on, read it again or better yet, read
about it in some other book and get a fresh outlook on it. Most abstract ideas can be explained
"correctly" in more than one way. Every time you
go over a subject you will find that your understanding will improve.
faith.
Electronic books are generally written on one
of
two
levels.
When
written for the beginner, they
are usually so trivial that they do not give the
reader enough depth of knowledge to use the
in-
formation in practical circuits such as found in
avionics equipment.
The other
level
level of writing is the engineering
where the high priests have carefully con-
cealed the knowledge behind a smokescreen of
abstract generalizations and mathematical gibberish. It
may
that
possible to graduate with good grades
it is
be a surprise for the reader to learn
and a degree in electrical engineering and not
understand enough about electronics to explain it
to anyone.
The author was living proof of this statement. Engineers are taught how to calculate
answers at breakneck speed in order to pass tests.
However, they are rarely taught how anything
works or why it is important. It is no wonder that
good engineers are such a rarity.
This book is an attempt to describe electronic
devices on a middle level in which we will explain
how each
device works and
why
it is
important.
SECTION I
Basic Electronic Theory
It
is
possible
to
explain
what
A good place
atom and how its
electrons can sometimes be coaxed away to form
uum
electronic
devices do without a detailed explanation of
how
as found in a
TV
picture tube.
to start is a description of the
they do it. In the long run though, it is better to
have a qualitative idea of what is going on inside
a useful electric current.
a device. The devices are usually named with
strange words and letters based on their theory
Atoms, Crystals and Energy States
A.
An atom
and construction. If you know the vocabulary,
you can keep the devices separate in your mind
and know how they are used. For example, an
"N-channel MOSFET" is a transistor which can
be easily damaged if you handle it carelessly.
Once you know what initials like "MOSFET"
mean, you will know what characteristics to expect from the transistor, how to avoid damaging
it, and how it should be wired into a circuit.
consists
of
a
tional forces are insignificant.
the science of controlling the
flow of electrons through solids, such as wires
and transistors, and sometimes through a vacElectronics
held
is
HYDROGEN DONATES
ONE ELECTRON
dense,
in
orbit
by the
The
attraction
electrons are
between the
negatively charged electrons and the positively
charged atomic nucleus.
CARBON DONATES
OR ACCEPTS
FOUR ELECTRONS
Fig. 1-1
positively
nucleus surrounded by a cloud of
negatively charged orbiting electrons. The electrons are sometimes compared to planets orbiting
the sun. The force which keeps the planets from
leaving the solar system is the gravitational attraction between any two masses. In order for
gravity to be significant, at least one of the
masses must be huge. So, in the atom, gravita-
charged
SILICON DONATES
OR ACCEPTS FOUR
ELECTRONS
Diagrams of atoms showing
1
electron shells.
Most
of the electrons in a typical medium-to-
large sized
atom
are orbiting close to the nucleus
and are too tightly bound to leave the atom under
ordinary conditions. With very energetic persua-
In general, a molecule is stable when each
in the molecule has access to 8 electrons in
outer shell. When oxygen combines with only
atom
its
such as atoms in the midst of an atomic explosion, even the inner electrons can be removed
from the atom. The electrons which are most distant from the nucleus are not held tightly and can
be lost to other atoms during a chemical reaction
or under the influence of the low voltages used in
one hydrogen, the two atoms together have only 7
electrons to share. This substance, called the
hydroxyl radical, will react with a great many
substances to try to capture an 8th electron.
Sodium hydroxide (lye), releases this hydroxyl
radical easily. That is why the lye used in toilet
bowl cleaner can dissolve the debris out of plumb-
electronics.
ing.
sion,
From
the analogy of planets orbiting the sun,
and from the diagrams
in this section,
you could
easily get the idea that electrons are sedate, quiet
objects that park in specified positions where
they are associated with other nearby electrons
and atoms. In fact, electrons whirl frantically
around their atom and cover a spherical shellshaped pathway rather than a simple circular
orbit. Moreover, they go so fast that according to
physics theory, they are too indistinct and
"ghostly" to be certain where one is going at a
given instant. In summary, electrons are very
esoteric objects and one must be wary of descriptions that
make them sound like billiard balls. In
number of electrons that can be
spite of this, the
gained or lost by a given voltage level directly explains many of the chemical and physical properties of each kind of atom.
For example, oxygen will share two electrons
with two hydrogen atoms to form a stable cluster
atoms which make up a single molecule
of three
water (H2O).
When we
mean
the
around
that
all
outer
say "shares electrons,"
electrons
three atoms, binding
actually
them
Some atoms, such as neon and argon gases,
already have 8 electrons in their outer orbits.
These atoms not only react poorly with other
kinds of atoms, they don't even associate closely
enough with
orbit
together.
own kind
to
form
solids!
Flourine has 7 electrons in its outer orbit.
flourine reacts with hydrogen, you would
When
it
to form a very stable compound.
However, the flourine atom holds onto the
hydrogen atom's electron so tightly that it does
not share the electron with the hydrogen very
well and the molecule is very unstable. The
hydrogen atom is easily set free from its own electron and will go off to try to capture a new electron from some other molecule. An atom which
has lost or gained electrons is called an ion.
Because the hydrogen ion is so easily released
from the hydrogen flouride molecule, hydrogen
flouride is the most corrosive acid known.
expect
The number of electrons that an atom is able
up or accept is called its valence. For ex-
of
we
their
to give
ample, the valence of hydrogen is plus 1 because
donates an electron easily. The valences of
it
flourine
and oxygen are minus
1
and minus
respectively because they accept those
2
numbers
of electrons easily.
B.
+ 1
-1
+8
HYDROGEN ATOMS
CHARGE IN NUCLEUS
CHARGE IN ELECTRON
OXYGEN ATOM
CHARGE IN NUCLEUS
-6 ELECTRONS IN
OUTER SHELL
Conductivity
The property of materials that most concerns
us in electronics is conductivity. A material that
is a good conductor allows electrons to travel
through it, from atom to atom, with little application of force. The "force" is voltage which pushes
electrons from atom to atom.
Most good conductors
A
are
metals.
Metals
water molecule is made from one oxygen and two hydrogen atoms. The three atoms
generally have just one or two outer electrons
that are easily removed. Once the electrons have
share their outer shell with eight electrons to
make a stable molecule.
been removed, they are easily replaced by other
traveling electrons. Just inside the orbit of these
Fig. 1-2
outer electrons
is
a filled shell of electrons, which
why
metals don't mind losing their outer
electrons. In many heavy metals this filled shell
contains as many as 18 electrons. None the less,
explains
this shell is filled
and these electrons are not
matrix of atoms in the conductor. Conduction
can't occur unless free electrons and resting
places are both present.
C.
Insulators
available for conduction or chemical reactions.
Insulators are very poor conductors.
1.
Crystalline structure
insulators
rocks,
The structure
metals is basically crystalline. Individual atoms in a crystal are stacked
together in a uniform pattern, something like a
neat pile of bricks. The closeness of the individual
metal atoms means that their outer electron orof
bits are practically touching.
As
a result,
it
takes
energy for weakly bound electrons to
wander over and orbit around neighboring atoms.
are
and
also crystals.
salt are
examples.
Many
Crystalline glass,
Some
crystalline
structures keep the adjacent atoms so close that
it requires too much energy for an electron to
enter
or leave the atom. This should sound
backwards to you, but it turns out that when
atoms are closest, the crystal is most likely to be
an insulator.
little
Compared
to other crystals
we
will talk about,
metal atoms form rather loose, flexible crystals.
Many
nonmetallic atoms form tight chembonds with others of their own kind when
they form a crystal. In these crystals each atom
ical
has access to 8 electrons in
locks each
atom
its
outer
shell.
This
tightly to its neighbors. Metal
atoms already have a filled, next-to-outer shell so
the metal atoms do not need to share their filled
shell with other atoms in order to be stable. Only
a few outer electrons are shared in metals and this
tenuous linkage between adjacent atoms explains
why metal is flexible. When under pressure
metals usually bend while glass and other crys-
In a good insulator the kinetic energy of a
is far higher than one which is
resting in an outer orbit. Since so much energy is
needed, we can say that there are effectively no
traveling electron
resting places or free electrons in an insulator. If
you put enough voltage across an insulator, eventually it breaks down and conducts. However,
there is so much energy expended in jamming
electrons into orbits and yanking them out, that
the insulator overheats and is destroyed. For example, wood is a good insulator, but when lightning strikes a wooden house and uses it for a conductor, the heat released usually sets the house
on
D.
fire.
Semiconductors
tals shatter.
2.
Kinetic energy
The energy of motion (kinetic energy) of a
truck doing 90 mph down the interstate is obviously higher than one parked at the truck stop.
Similarly, the energy stored in an electron traveling through a conductor should be higher than
one which is resting in the outer orbit of some
atom. But in a really good conductor, the energy
of some of the traveling electrons is no higher
some
than the energy of
This is because it requires so little energy to
travel, that the energy of some parked electrons
is at
the
same
Semiconductors, logically enough, are halfthe two extremes of good conductors and good insulators. The reason that semiconductors are so important in electronics is their
conductivity can be widely altered from good conductor to good insulator just by applying a small
voltage, heat, or light. In other words, the ability
of semiconductors to change from insulator to
conductor makes it possible to control the flow of
electrical current with subtle application of small
way between
level as
amounts
of energy.
of the resting electrons.
some
of the
wandering
ones.
We can summarize conductors as having two
basic ingredients: free electrons which are easily
lured away from their atoms, and resting places
in the outer electron shells which provide stepping stones for electrons wandering through the
1.
Crystals
Semiconductors are also crystals. They are
made from atoms which have valences of
usually
four. Each atom shares its four outer electrons
with four other atoms in a stable, rigid, crystal.
Molecules and crystals are particularly stable
and chemically inert when there are 8 electrons in
the outer shell of each atom. Because each of the
Fig. 1-3
Diagram of a
silicon crystal
showing how valence electrons are shared with four other atoms.
semiconductor atoms has access to 8 electrons,
there are no electrons to lose and no unfilled
resting places. Pure semiconductor crystals are
actually
2.
3.
Carbon
Carbon
is
an interesting example of how cryscan change the physical and elec-
talline structure
good insulators.
Impurities
Carbon occurs
amorphous carbon, such
trical properties of a material.
in
three different forms:
as
the soot in your chimney; graphite, as in pencil
In order for a semiconductor to behave like a
conductor it must either be impure or extremely
hot. If there are
any different kinds of atoms
trap-
ped in the crystal matrix, these impurities will
have different valences and will donate free electrons and resting places and turn the semiconductor into a conductor.
In semiconductors these resting places are
called holes because each one represents a place
where an electron can go, but which is not filled at
the moment. Heat energy can also form free electrons and resting places by knocking electrons
out of a pure semiconductor crystal structure.
When an electron is knocked out, it leaves a cavity or hole for
in.
some other
traveling electron to rest
lead;
and diamonds.
Amorphous carbon
Amorphous carbon is a poor conductor compared to most metals, but it is a conductor none
the less. The resistors used in electronics are
usually made of carbon. When amorphous carbon
is compressed under enough heat and pressure, it
turns into graphite crystal.
Graphite carbon
Graphite has its atoms closer together than
amorphous carbon and is a semiconductor. It
turns out that
when heated
it
conducts electrons too easily
be used to make tran-
for graphite to
sistors. When graphite is compressed under very
extreme pressure, the graphite crystal collapses
into an extremely dense crystal structure which
is
the diamond. Diamonds are excellent in-
sulators.
Silicon and germanium are the two most important semiconductors used in electronics. There
are others though, such as gallium arsenide and
gallium phosphide, semiconductor molecules
which are used in the light emitting diodes
(LED's) which make up the glowing red or green
numbers
E.
in calculator
Holes
which donate both electrons and
same semiconductor
crystal, the result will be a good conductor. One
reason that semiconductors are so special is that
these two properties, electrons and holes, can be
added separately. When only one of these conduction components is added in small quantities, the
resulting crystal is still an insulator. However,
the crystal is now very sensitive and can now
have the missing component, electrons or holes,
induced into it by voltage, heat, or light. This
makes the semiconductor an insulator on the
verge of becoming a conductor. It is like a valve
If impurities
holes are introduced into the
that can be easily turned on.
Diagram of a
—5 and +3
Holes
Holes can be added by introducing an impurisuch as gallium or indium, which have a
valence of plus 3. Since indium has only 3 electrons to share, it will fit into the crystal, but it
will not have enough electrons to share with each
of its four neighbors. The result is an instability,
a hole, that will readily accept an electron to fill
the place of the missing eighth electron.
ty,
2.
Electrons
and watch displays.
Controlled Introduction of Electrons and
Fig. 1-4
1.
silicon crystal with
introduce extra electrons
and
Electrons can be added by introducing an impurity like arsenic or phosphorus which have a
valence of plus 5. These also fit into the matrix
but have an extra electron which is free for conduction. Phosphorus can also have a valence of
minus 3. This is another way of saying that
phosphorus has 5 electrons in its outer shell.
Phosphorus can accept 3 electrons to make a
stable eight electrons in its outer shell. Alternatively,
it
which case
plus
5.
can share
all 5 with other atoms in
considered to have a valence of
phosphorus is trapped in a silicon
it is
When
semiconductor crystal, four of its 5 electrons are
shared with neighboring silicon atoms, making 8
The ninth electron is
very easily lost. This ninth
electron becomes a conduction electron.
electrons in a stable octet.
out of place and
is
phosphorus and indium impurities showing how valences of
holes into the crystal
and make
it
a conductor.
Doping
3.
The process
F.
adding impurities to semiconductors is called doping. A semiconductor with a
large amount of impurity added is said to be
of
Conduction by Holes
We
have pictured
all
conductivity through
jumping from resting place to
resting place. This is close enough for beginners,
but in the big time this idea is refined into two
separate kinds of conductivity. The idea is that
solids as electrons
heavily doped.
A
semiconductor which has been doped to
is said to be a P-type semiconductor. P
stands for "positive" because negative electrons
are attracted to those holes. This does not mean
that the whole semiconductor is positively charg-
the atom-to-atom hopping that occurs in semiconductors has two mechanisms of conduction.
ed.
volves relatively high energy electrons which are
in the conduction band. Conduction band electrons are the only kind of conduction mechanism
add holes
the atom-to-atom hopping that
we
already described for metals. This conduction
in-
The
A
semiconductor that has been doped to add
is said to be an N-type semiconductor. N
stands for "negative" because the extra electrons
are negatively charged and N-type semiconductor
has them available for conduction. Again, it does
not mean that the whole crystal is negatively
charged. A N-type semiconductor which has been
plus (N+) type
heavily doped is called an
semiconductor. And finally, a heavily doped
P-type semiconductor is called a P+ type semi-
electrons
first is
that metals have.
The second kind
of conduction in semiconduc-
tors is an atom-to-atom
among
hopping that only occurs
the low energy electrons that are orbiting
atoms in semiconductors. This low energy
kind of conduction is said to occur in the valence
crystal
N
band.
conductor.
Valence band conduction
if
You will have the terms pretty well mastered
you can just remember the following:
To make this as confusing as possible, valence band conduction is referred to as conduction
by holes. You aren't going to believe this, but the
accepted way of looking at valence band conduction in semiconductors is that the holes are moving instead of the electrons.
P-type stands for positive and has holes.
N-type stands for negative and has
extra electrons.
The upper
c
floor,
the conduction band,
is like
an expressway.
5
The lower
—
-SPACE
"o
As the cars move
Fig. 1-5
Conduction
in a
valence band,
—
£g
2l
floor, the
to the right, the
is like
bumper-to-bumper
traffic.
xy
^
SPACE
space between them, holes, move to the
left.
semiconductor can be compared to a two level parking garage.
6
Suppose you were in a helicopter hovering
over a parking lot just outside the Superbowl.
The parking lot has more cars than pavement and
the scarce, empty car-sized spaces are quickly filled as cars attempt to maneuver around the lot.
As seen from a great distance it could appear that
the empty places were moving around the lot instead of the cars. Everytime a car moves into a
space it leaves another space behind it which
another car quickly moves into. The important
thing to notice is that as the cars move in one
direction, the spaces seem to move in the opposite
1.
Insulators
Let's go back and apply this analogy to
brought closer and closer, the valence band
energy becomes lower and lower, but at the same
time, the conduction band energy is becoming
higher and higher.
Insulators are
direction.
The parking lot analogy to conduction in
semiconductors has been expanded by Shockley,
Bardeen and Brattain — the inventors of the
bipolar transistor. He pictured the two energy
states as being like two floors in a parking
garage. The upper floor is virtually empty and
represents the conduction energy band. Cars can
drive around unimpeded up there because there
are so few of them. This is the only state there is
in a metal conductor because the conduction
energy band and the va-lence energy band
overlap and there is little difference between
them.
For a pure semiconductor Shockley pictures
the lower floor of the parking garage as being
completely filled with cars so that none of them
can move and none of them can go upstairs where
they could travel about freely.
By
adding a quantity of energy one car can be
from the lower floor up to the upper floor
where it is free to go anywhere its driver wants.
In the process of raising one car from the lower
floor, a parking space is created on the lower
floor. Once a space has been made, the cars can
move around down there from space to space.
in-
As atoms are jammed closer and closer
together, we said that the crystal is more and
more likely to be an insulator. As the atoms are
sulators.
much
like
the situation
we
have been describing for pure semiconductors.
The bottom floor of the garage is packed with
cars so there can be no valence band conduction.
The upper floor representing the conduction band
is totally empty so there can be no conduction up
there either.
The difference between pure semiconductors
and insulators is that insulators have the upper
floor thousands of feet above the lower floor. It
takes huge energies (huge voltages) to lift cars
up there
(electrons)
to get conduction started.
In contrast, the parking garage analogy for
semiconductors would have the upper floor the
minimum distance above the cars on the lower
floor. Since raising a car a few feet requires less
energy (small voltages), conduction is easily
started in semiconductors.
2.
Hole
lifted
In the semiconductor literature the word hole
reserved for valence band resting places in
semiconductors. It seems to the author that this
subject would be less confusing if the world
would acknowledge that the conduction in both
is
bands
The lesson in semiconductors is that when
energy is applied to a semiconductor and it begins
to conduct, electrons have been lifted from the
valence band up to the conduction band. The current carriers, which are conduction band electrons
and valence band holes, are generated in pairs.
Moreover, the conduction can be thought of
as occuring in two ways. In the conduction band
the electrons whizz along as if in a metal. In the
valence band the movement is just as rapid but
the electrons move from hole to hole as if they
were
in
bumper
to
bumper
traffic.
is
basically electrons hopping from resting
place to resting place.
In metals or
N+
type semiconductor, extra
electrons are left out of the outer octet of orbiting
electrons. Therefore they are very easily lured
away from
None
their
home atom.
the less, any
valence electron
is
atcm which has
lost its
positively charged and there-
wandering conducband electrons use positively charged metal
atoms as stepping stones, doesn't this make the
charged atoms a kind of hole too?
fore attracts electrons. Since
tion
Conduction
G.
in
because the conduction band becomes crowded as
the heat energy pushes more and more electrons
Doped Semiconductors
Doping adds holes or extra electrons to
semiconductors and conductivity becomes very
much easier. When holes are added they allow
conduction in the valence band. That is, holes
allow the bumper-to-bumper kind of conduction.
When extra electrons are added, they allow conduction in the conduction band because the ninth
electrons are not welcome in the filled shell of
eight electrons and they need very little extra
energy to leave their atoms and enter the conduction band.
To keep
the crystal from becoming highly
amount of impurity added to a
pure semiconductor is usually miniscule. 1 atom
of impurity in 10 million atoms of semiconductor
crystal is plenty. Otherwise the N-type or P-type
semiconductor would conduct all the time and
there would be no way to turn them off.
conductive, the
into
it.
However, semiconductors usually decrease
by 6 or 8% for every degree of
temperature rise. This happens because at low
temperatures there are very few electrons in the
conduction band, but as the temperature rises,
the heat energy knocks loose hole-electron pairs
and in that way increases conductivity.
their resistance
It turns out that germanium and silicon
semiconductors are too hard to build with identical temperature characteristics because it is
hard to control the exact concentration of impurities. Thermistors are usually made from
sintered nickel, manganese and cobalt oxides
which are easier to make consistent.
A
heavily doped semiconductor can have a
positive temperature coefficient like a metal but
it
Majority and minority carrier
is
much more temperature
sensitive.
These
temperature sensors are called sensistors and
in-
crease their resistance with temperature.
In
holes
P-type
a
the
are
semiconductor,
primary
means
valance
of
band
conduction.
/.
Photoconductors and Photoresistors
Therefore, in P-type, holes are the majority carrier.
Even
in a
P-type semiconductor, some free
electrons enter the conduction band. Therefore, in
a P-type semiconductor, electrons are the minority carrier.
Heat energy introduced into a semiconductor
knocks loose hole-electron pairs and in that way
increases conductivity. Semiconductors respond
to any form of energy that has the end effect of
heating the crystal matrix. This includes visible
light and has led to a variety of photoconductors
and photoresistors.
The opposite observations can be made
N-type
for a
In N-type, conductor
band electrons are the majority carrier while
holes are the minority carrier.
semiconductor.
Semiconductor Thermistors
H.
One
of the difficulties in using semiconduc-
dramatic change in conductivity with
temperature. Like most problems encountered
with materials, this one can be turned into an advantage. Semiconductor thermistors are designed
tors
to
is
their
measure temperature.
All
conductors
change
their
resistance
a
amount with temperature. Most metals increase their resistance a small amount for each
small
degree centigrade of temperature
rise.
This
is
Fig. 1-6
Typical
cadmium
sulfide photoresistor
Cadmium
sulfide cells
Most vacuum tubes resemble an ordinary
They have a glass or metal envelope
that keeps the air out of the vacuum. The heat is
light bulb.
The most common variety is the cadmium
used in camera light meters and door
openers. The cadmium sulfide crystal is deposited
sulfide cell
and has impurities
in a thin layer
of silver, in-
dium, or antimony.
provided by a hot filament just like a light bulb.
In some tubes the filament itself serves as the
cathode, but in most tubes the filament is surrounded by a separate cathode which is made
from a material which
It is the
its color
most popular photoresistor because
spectrum sensitivity
the eye. In the dark
it
is
similar to that of
has a resistance of as
much
as 2 megaohms, but in full sunlight, its resistance
drops to as little as 10 ohms. A lead sulfide cell is
similar but is more sensitive to infrared radiation
and is not as widely used.
J.
Electron Conduction in
Vacuum Tubes
We
said in the beginning that electrons can
also flow across a vacuum as well as through
solids like conductors and semiconductors. For
most applications vacuum tubes have become obsolete as better and cheaper semiconductor replacements for them have been invented. How-
through a vacuum is vital to TV
picture tubes, TV camera tubes, and a variety of
ever, conduction
other useful tubes.
Cathode ray tube
The basic
tubes
is
principle
behind most vacuum
that a very hot object placed in a
will give off electrons directly into the
vacuum
vacuum.
When discussing semiconductors we talked
about how high temperatures can elevate electrons from their orbits around atoms and put
them in the conduction band. If you raise the
temperature of a conductor far enough, and if the
conductor happens to be in a vacuum, then the
valence electrons can be driven clear out of the
metal.
A vacuum means that the hot object, which is
surrounded by nothing. This
nothing to prevent the electrons
called a cathode, is
means
there
is
from leaving the surface of the metal. If the hot
object were surrounded by air, air is a nonconductor and would not let them leave unless
they were exceedingly energetic.
The sun
is
a gigantic example of such a hot,
huge clouds of elec-
electron emitter. It throws off
trons every second which
make
their
way out
through the vacuum of space as the solar wind.
emitting electrons
when
is
particularly good at
heated.
The TV picture tube, a variety of cathode ray
tube (CRT), is a good example of a vacuum tube
that is not yet obsolete.
Anodes
In addition to a glass envelope and a hot
cathode, a cathode ray tube has several metal
plates called anodes which have positive voltages
on them. The positive voltage attracts the negatively charged electrons away from the cathode
and accelerates them toward the viewing screen.
This assembly is called an electron gun and
shoots a stream of electrons at the viewing screen
where they are seen as a lighted spot. The positive voltage on the anodes attracts them away
from the cathode. Since there is no air to stop
them, they readily stream across the void to
reach the positive metal anodes. Most of the electrons streaming toward the first metal anode
strike it and are wasted. However, a small hole in
the center of the anode allows a tiny beam to pass
on toward the screen.
In some designs, like Fig.
third anode focus the electron
pelling
it
1-7,
a second and
beam by
first re-
toward the center of the cylinder with a
slightly negative voltage, then accelerating
again with a high, positive voltage.
it
In some tubes the electrons are focused by
magnets mounted on the neck of the tube. Finally, the electron beam is shot out into the large
bell-shaped end of the picture tube.
In most designs the inside of the bell shaped
is painted with a conductive coating which
serves as the final accelerating anode. In a large
picture tube this last anode can have as much as
30,000 volts DC on it. The electrons slam into the
end
screen of the tube which is coated with a powdery
phosphor which converts the energy of the electrons into visible light.
HORIZONTAL DEFLECTION PLATES
VERTICAL DEFLECTION
PLATES
+
10T FILAMENT
HEATS CATHODE
ACCELERATING
ANODES
(HIGH + VOLTAGE)
-FOCUSING ELECTRODE
(SLIGHTLY NEGATIVE)
Fig. 1-7
Color
Cathode ray tube
TV
while the outer shell has only one electron.
What physical and electrical properties
would you expect lithium to have?
Color TV tubes have three separate kinds of
phosphor arranged in a pattern of colored dots. A
grid behind the screen selects which of the three
dots produces light to produce the correct color
combinations. Some designs have three separate
electron guns, one for each color.
not only the planet where Superit is the name of an element
found on planet Earth. Krypton has four
electron shells. Listing them from the innermost shell to the outermost shell, these
shells contain 2, 8, 18 and 8 electrons respec-
Krypton
In order to "paint"' a picture, the beam of
is swept back and forth in an orderly
pattern to cover the whole screen. In large CRT's
the electron beam is steered by magnetic deflecelectrons
tion using coils
CRT's the
small
anodes which are
shown
tively.
TV
the scanning pattern
What does
conductor?
this tell
you about the
Why
or
why
not?
In a semiconductor or insulator, what
is
the
difference between electrons in the valence
is
band and electrons
rectangular while in radar sets the scanning pattern is usually radial.
in the
conduction band?
In many ways pure semiconductor crystals
and insulator crystals are alike. What is the
essential difference between them that ac-
QUESTIONS
1.
born,
chemical and physical properties of krypton? Do you think krypton might be a semi-
mounted outside the tube. In
electron beam is steered by
at the end of the electron gun as
in Fig. 1-7. In
is
man was
Describe the structure of a typical atom.
counts
for
their
different
electrical
be-
havior?
2.
In
all
known elements,
is filled when
the innermost elec-
Name two ways that holes and conduction
band electrons can be introduced into a
has only two elecan element with only three
electrons. The inner shell has two electrons
tron shell
trons.
Lithium
it
is
semiconductor.
10
7.
The
resistance of a thermistor varies with
temperature.
A piece of metal
resistance with temperature.
10.
What
is
so
dif-
ferent about thermistors?
8.
Why
Because a cold electrode does not release
vacuum while a hot one
does, what relationship would you expect
between the resistance of the vacuum tube
(from cathode to anode) and the temperature
electrons into a
also varies its
are semiconductors so useful in elec-
of the cathode?
amorphous
carbon are poor conductors and poor insulators. Why can't amorphous carbon be
used in place of semiconductor materials?
tronics? Other substances like
9.
11.
When
a positive voltage
is
applied to the
anode of a vacuum tube and a negative voltage is placed on the cathode, a current of
electrons streams across the vacuum. What
do you suppose happens when the positive
voltage is applied to the cathode and the
negative voltage is applied to the anode?
In order for electrons to flow from the
cathode to the viewing screen of a picture
tube, what three conditions must be present?
11
SECTION II
Diodes
A.
Diodes
positive terminal to the negative terminal of the
battery or voltage source. Actually, the electrons
A
ly,
diode
is
a one-way electricity gate. Actual-
are flowing from the negative battery terminal to
means a device that has two elecbut the word has come to mean a one-way
the positive battery terminal.
a diode
trodes,
electricity valve.
some
Later
we
will
confuse you with
CHECK VALVE
exotic diodes which are not one-way valves.
However, garden variety diodes conduct current
one direction. This simple property can be
used to detect radio signals, change AC current
into DC current, shape voltage waveforms, and
even turn signals on and off.
in only
-SPRING
Check valve
DOOR SHUTS WHEN
-TRY TO FLOW IN
OPPOSITE DIRECTION
A
check valve found in plumbing and fuel
lines is a good analogy to a diode. A check valve
contains a spring-loaded flapper or door which
can open in only one direction. When the fluid
pressure is in that direction, the door opens and
the water flows unimpeded. When the water attempts to flow in the opposite direction, the
pressure, which is analogous to voltage, slams the
door and prevents the water from flowing. The
higher the pressure, the more firmly it keeps the
door shut.
DIODE
HX
DIRECTION OF POSITIVE
CURRENT FLOW
A LOAD
RESISTOR
NEEDED.
IS
OTHERWISE
THE CURRENT
MIGHT BE SO
HIGH
IT
WOULD BURN
BATTERY
Fig. 2-1
The circuit symbol for the
shown below the check valve.
FLUID
UP THE DIODE.
Check valve analogy
for a diode
electronic diode is
Positive charge
is
allowed to flow through the diode unimpeded
when its direction is the same as the arrowhead in
the symbol. When positive current attempts to
flow through from the other direction, the diode
blocks the current as suggested by the barrier
facing the arrowhead. Before the flow of electric
current was well understood, positive current
flow was defined as current flowing from the
B.
Ideal or Perfect Diodes
Before
talk about
we study
real d'odes, it is helpful to
what a diode would be
like
if it
were a
ideal or perfect one-way electricity valve. Volt-
ampere characteristics like Fig. 2-2 are a good
way to graph what an electronic device does in
response to voltage and current.
13
+
negative currents are plotted below. You will
notice that there are no negative currents because
the diode conducts only in one direction.
1
RESISTOR
VOLT— AMPERE
CHARACTERISTIC
2.
Resistance
Whenever the voltage across the
is
-V
ideal diode
positive from the arrowhead to the barrier, the
current flows through the diode. Therefore the
current must rise above the horizontal axis in the
first
quadrant where both the voltage and current
are positive.
+
With the ideal diode, the current flow is immediately unlimited with even the slightest forward positive voltage. This is because a perfect
diode would have zero resistance. When you
divide the voltage, V, by the resistance, zero
ohms, you get an unbounded current, an infinite
current, which can't be plotted on the curve. That
is why the arrow just points off toward infinity,
1
PERFECT
R =
DIODE
ZERO OHMS
TURNS FULL ON
Any
would produce an infinite current provided that all the other components in the circuit loop also had zero resistance. Real diodes always have some resistance, and as you will see shortly, their plots are
more interesting.
WITH SLIGHTEST
POSITIVE VOLTAGE
-V
+V
COMPLETELY
TURNED OFF
WHEN VOLTAGE
IS
NEGATIVE
positive voltage
When
the more negative voltage
is
applied to
the arrowhead end of the diode, the diode does not
conduct, therefore the current passing through
-I
is
along the horizontal axis to the left. Perfect
diodes can also resist unlimited voltage across
them without conducting. Real diodes will eventually breakdown or fail when too much voltage is
placed across them.
Fig. 2-2
Volt-ampere characteristics for a "perfect" diode and a resistor.
1.
Linear and non-linear
3.
The volt-ampere characteristic for an ordinary resistor is shown alone. Resistors, even real
life resistors, are very linear devices. The term
linear
means that
it
makes
a straight line
when
On the other hand, diodes are very non-linear
because their resistance varys with the amount of
voltage across them. The current through the
diode is plotted against the voltage across the
C.
The voltage across the diode, V, is plotted
along the horizontal axis. Positive currents, +1,
above
the
horizontal
axis,
Vacuum Tube Diodes
There are a number of ways to construct a
The earliest really first rate diodes were
diode.
plotted
Zero Volts Transition
Another property of the theoretical perfect
diode is that the transition between conducting
and non-conducting is abrupt and occurs at zero
volts. Real diodes, even the most modern ones,
don't achieve this without resorting to a number
of components wired together to attempt to
achieve the perfect zero volts transition.
plotted on a volt-ampere graph.
are
it
zero. In Fig. 2-2 this zero current is plotted
diode.
vacuum
while
II
tubes.
The vacuum diode strongly
re-
sembles an ordinary light bulb. The only additional component is a cylindrical metal anode or
plate which surrounds the filament.
of N-type semiconductor. The resulting P-N junction conducts only when the voltage across the
diode is more positive on the P side of the junction.
When
a positive voltage is placed on the
metal plate, electrons are attracted away from
the hot filament, stream across the vacuum and
strike the plate. So whenever a positive voltage is
Positive voltage applied to the
forward
placed on the plate, the diode conducts. When the
filament or cathode is positive with respect to the
anode, the excess of electrons is now on the plate.
Since electrons can't leave a cold plate, the diode
does not conduct in this direction.
CATHODE
P
side
is
called
the voltage across the diode is
more positive on the N side, only the tiniest
leakage current flows, typically a millionth of an
ampere
bias.
\x
(
a)
If
or less.
When the more negative voltage is applied to
P side, this is called backward bias. The P side
the
and N side are sometimes still called the anode
and cathode, just as if P-N junction diodes were
W
vacuum
diodes.
ANODE
DIODE SYMBOL
HOT CATHODE
FILAMENT
METAL
P-TYPE
HOLES-^,
PLATE
ANODE
EXTRA
y
FPTRnMS
V
P
O
o
WIRE
o
,
o
*o
ANODE
GLASS OR
METAL
ENVELOPE
N-TYPE
CATHODE
o
o
WIRE
o
ELECTRONS STREAM TO
PLATE
IN
VACUUM
Fig. 2-4
A P-N junction
semiconductor diode
Diode and transistor circuits can frequently
be figured out from a diagram by reasoning out
which direction the P-N junctions will allow current to flow. From now on, whenever you can
identify a P-N junction in a circuit diagram, you
DIODE VACUUM
TUBE SYMBOL
mumble
should
Fig 2-3
Vacuum
tube diode
to yourself:
"Positive to
P Conducts"
Loose electrons and holes
D.
Semiconductor Diodes
We
By
far the
most important diodes
are
made
from semiconductors. In Section 1 we said that
one of the reasons that semiconductors are so
special is that conduction band electrons and
valence band holes can be added separately to
form N-type and P-type semiconductors.
tion
for
is
two explanations
for
why P-N
1
we
said that the
two
essential ingredients
conduction are loose electrons which can leave
their
home atom and
resting places in the outer
atoms where those electrons can go.
When either of these two ingredients are missing,
there is no conduction and the material behaves
orbits of
Semiconductor diodes are formed when a
layer of P-type semiconductor
will give
junction diodes only conduct in one direction.
First a simplified explanation: remember in Sec-
joined to a layer
like
15
an insulator.
N-TYPE
P-TYPE
ELECTRONS
DRAWN OUT OF
HOLES IN P-REGION
ELECTRONS
PUSHED INTO
NREGION
o o
O
o*
/
©
ELECTRON
o
FLOW
ELECTRON
FLOW
o-
VOLTAGE PUSHES ELECTRONS
OFF CLIFF INTO HOLES
BATTERY
Forward biased P-N semiconductor diode
Fig. 2-5
A
simple
is
that
to look at type N semiconduchas some loose electrons, but very
few resting places. A similar view of the P
semiconductor is that it has some resting places
{holes), but very few loose electrons.
tor
way
P
the
side, it attracts
In
conduct.
away from empty
holes.
words,
other
the
conductive
On
the
N
side, the
pushing electrons into the
negative voltage
N
those electrons that diffused over to the P side.
So, by applying positive voltage to the P side and
Some
some
up electrons. Since the two components become mixed,
the boundary region becomes a conductor.
N
a
holes diffuse across the border to take
of the junction obtain the electrons
more negative voltage
to the
side,
Now we
bias
it
voltage
doesn
is
will reverse bias the
't
conduct.
applied to the
diode and see
When the more positive
N side of the boundary, it
away from the boundary and out of the crystal. The N material
started off with some loose electrons and no
attracts the loose electrons
will
happens.
forward bias the diode and see what
When
the positive voltage
is
applied to
P-TYPE
N-TYPE
ELECTRONS PUSHED
ELECTRONS DRAWN
OUT OF N SIDE
INTO HOLES
HOLES FILLED SO
P SIDE
ELECTRONS PULLED OUT
SO N SIDE BECOMES
VERY POSTIVE
BECOMES
VERY NEGATIVE
BATTERY
Fig. 2-6
both sides
and holes
they need to become conductors.
why
We
is
region to replace
electrons diffuse over into the holes and
Forward and backward
boundary
region becomes wider until the diode starts to
In the center of the diode where P material is
joined to the N material, there are free electrons
just a few atoms
more electrons across the
N-to-P boundary and into the waiting holes.
it
Reverse biased P-N semiconductor diode
16
resting places.
When
applied to the
N
trons
the more positive voltage
ward biased diode conducts because the electrons
is
side, it attracts the loose elec-
away from the boundary region and out
N
are able to
fall
down
hill.
of
side with no electrons
We can summarize by saying that forward
biasing the diode pushes electrons "off the cliff"
from the conduction band in the N side down to
On the other side of the junction, the more
negative voltage pushes electrons into the P side
thus filling in the holes and destroying the
resting places. So, on the P side too, the diode has
the valence band holes. The forward bias is actually decreasing the energy difference between
the conduction band and the valence band.
the diode, leaving the
and no resting places.
no electrons and no resting places. By back biasing the diode, both semiconductor layers have
been converted into insulators.
Positive and negative charged
P-N diodes
positive voltage
A
more complete way to understand the P-N
is to remember that semiconduchave two modes of conduction. Hole-to-hole
conduction (bumper-to-bumper traffic) occurs
among low energy electrons in the valence energy
band. Atom-to-atom hopping by high energy electrons (expressway traffic) occurs in the conduction band.
The fundamental
these two bands
is
their different energy levels.
and makes
it
when electrons are pushed into the
makes the P side more negatively chargSince both sides become more ionized with
are really talking about voltage dif-
ference between bands. This
means that
it
In order for back baised diodes to conduct,
the electrons would have to enter the P region,
then conduct from hole to hole over to the bound-
takes
energy to raise a valence band electron up to the
conduction band energy level. It is very much like
lifting those cars up to the second floor of the
parking garage.
ary which is at the foot of the energy "cliff." In
order to get up the cliff, the electrons would have
to acquire enough voltage to "climb up the cliff"
to enter the conduction band. The interesting
thing about this phenomenon is that, the more
On the other hand, it takes no external
energy for an electron in the conduction band to
fall down to the valence band level. It is easy to
roll down the ramp to the lower floor of the parking garage. You don't have to burn gasoline or
even start the car!
voltage you back bias across the diode, the higher
the cliff becomes to prevent conduction. This is
reminiscent of the flapper door in the check valve.
The higher the water pressure becomes in the off
direction, the more tightly it holds the door shut.
Now again apply forward bias to the diode
and see what happens to the energy levels. When
the more negative voltage is on the N side of the
diode, it pushes electrons into the N-type material and makes electrons more numerous along
the boundary between the N and P layers. Electrons can fall down, that is, lose energy, as they
fall into the holes along the boundary.
P
Semiconductor forms
in
Semiconductor diodes are usually packaged
two forms. Small diodes usually take the form
of glass, plastic, or ceramic cylinders or beads
with two leads projecting from the ends. Usually
there is a band painted around the diode body
which indicates the cathode end of the diode.
more positive voltage atP material so that the
holes do not become filled by the electrons coming across the border from the N side. The forthe
side
the more
material. This
charge of opposite polarities, the voltage between
them is increased. This increases the voltage between the conduction band and the valence band
respectively. This has to happen because virtually all the electrons doing the conducting on the N
side are in the conduction band. And, virtually all
the electrons doing the conducting on the P side
are in the valence band.
difference between
Another word for voltage is potential energy.
talk about energy difference between
On
N
when
N
side, it
ed.
When we
we
applied to the
Similarly,
P
Potential energy
bands,
is
draws electrons out of the
more positively charged.
junction diode
tors
are back biased
side, the
Large diodes that can handle large currents
without overheating are usually mounted in a
metal bolt-like package with the threaded end
tracts electrons out of the
17
.
#
Fi^. 2-7
Assorted semiconductor diodes
+
serving as the cathode. The diode is bolted to a
heat sink (a large chunk of metal) to keep the
l
(mA)
semiconductor temperature down. The anode for
is usually a terminal which pro-
FORWARD
CONDUCTION
these large diodes
STARTS AT
ZENER
jects out of the top.
ABOUT
BREAKDOWN
Sometimes several diodes are packaged together in arrays or bridges for use as rectifiers or
in
signal processing.
These multiple diode
packages may even resemble integrated circuits
with 14 or more leads coming out of them.
+0.2
VOLTS DC
VOLTAGE
LEAKAGE
CURRENT
a
.AMP
1
Silicon
and Germanium Diodes
GERMANIUM
Most general purpose diodes are now made
from silicon, although germanium diodes are still
useful for some applications. From a physics
DIODE
-I
point of view, the key difference between silicon
and germanium is that the energy level difference
between the conduction band and the valence
band is greater in silicon than it is in germanium.
This means that devices made from germanium
are more likely to increase their conductivity with
heat. If the electrons and holes induced by heat
become more numerous than those embedded in
the crystal by doping, the P-N device will loose its
FOWARD
CONDUCTION
STARTS AT
ZENER
ABOUT
BREAKDOWN
VOLTS DC
+0.6
VOLTAGE
one way characteristics.
«/
"
+V
1.0
(VOLTS)
Volt-ampere characterics
Fig.
istics of
shows the volt-ampere charactergermanium and silicon diodes. If you
2-8
compare these graphs with
Fig. 2-1,
that neither of these diodes
is
For most
you
SILICON
will see
DIODE
very "perfect."
circuits, the principle practical dif
ference between these diodes and the ideal diode
is
that they need a small, positive voltage to turn
them
Fig. 2-8
on.
Germanium and
characteristics
18
silicon diode volt-ampere
In case of silicon, this voltage
is 0.6
volts
DC.
Depending on the diode, the zener voltage
can be anywhere from 5 to 600 volts. This transition often destroys the diode, but as we will see in
the next section, it is possible to design diodes
that survive the zener breakdown.
For germanium diodes, this forward voltage is only 0.2 volts, because the energy band difference is
less. Most of the time these voltage offsets are
not very important if the voltages they are working with are large. For example, when used with a
25 volt signal, it usually doesn't matter whether
the diode begins to conduct at zero, 0.2, or 0.6
E.
veils.
Diode Applications
Diodes as circuit elements are extremely verDiodes can accomplish many tasks that
satile.
When
real diodes conduct,
they always have
with an
ohmmeter, this is typically 5 or 10 ohms for germanium diodes and 10 to 20 for silicon diodes.
Because the forward voltage drop is so constant,
the resistance at high current levels can be extremely low, 0.1 ohm or less. Of course, to have
such low resistances, the diode must be large
enough to tolerate high current levels without
some forward
resistance.
one might assume would require more elaborate
devices, such as transistors or even integrated
When measured
circuits.
Examples include radio
signal detectors,
frequency converters, modulators, peak detectors, regulators, clippers and more.
Rectification
Probably the most
overheating.
common
Rectification
use for diodes
is
the conversion of
alternating current to direct current. Since the
current through a diode flows only in one direcrectification.
Large diodes have more leakage current when
back biased. For example, a small diode might
pass a microampere of current when back biased.
Large, high current diodes might pass a milliampere or more when back biased. A "perfect"
diode would, of course, refrain from passing any
current when back biased.
tion,
by
is
definition, this current is direct current.
Therefore
when
alternating current
is
applied to a
diode, only direct current in the proper direction
will flow
and hence the
Half-wave
rectification.
rectifier
Zener breakdown
When a sine wave voltage, such as household
120 volts AC, is applied to the half-wave rectifier,
the current flows only during the positive half
cycles, hence the name, half-wave rectifier. The
output voltage is taken from across the load resistor. Anytime current flows through a resistor,
voltage appears across it. Although this is probably obvious, resistors are commonly used in
electronic circuits to convert the current coming
out of a device into a voltage.
Another departure from the ideal diode is the
zener breakdown which occurs when a semiconductor diode is back biased past its ability to
resist voltage. At a certain threshold, called the
zener voltage, the P-N barrier is swept away and
the diode suddenly behaves like a good conductor. It is as though the flapper door in the check
valve were caved in and swept aside by too much
pressure.
>
>
t
INPUT VOLTAGE
RESISTOR
V, N
*
>
Fig. 2-9
„
LOAD
A
half-wave rectifier circuit
19
>-
->
RESISTOR
v, N
t
LOAD
>
->
Fig. 2-10
If
the direction of the diode
VquT
Negative half-wave
Oscilloscopes are a kind of high impedance
voltmeter that plot a graph of voltage versus
time on the face of a cathode ray tube.
reversed, then
is
the current will flow through the resistor only in
the opposite direction. The polarity of the output
voltage across the resistor is reversed. Notice
that the "negative voltage" we have generated is
just a matter of which of the two output leads
rectification
Fig.
2-11
shows an oscilloscope measuring
voltage through a typical medium-sized, semicon-
is
ductor diode. Since the oscilloscope draws practically no current, it can see both the positive and
negative parts of the sine wave right through the
defined as "zero."
Voltmeters/oscilloscopes
diode.
It
is
common
diodes only
isn't quite
let
beginners to think that
through positive voltage. This
right.
anode-to-cathode
for
It
is
The lesson here
direction
that
is
allowed
that in order for rectifica-
must be placed on the diode output. Suppose the
diode can pass only 1 microampere in the reverse
direction. If the load were 10 ohms, the voltage
across the resistor on the negative half cycle
to
pass. Voltage, regardless of the polarity, can be
measured through very high resistances, even
back biased diodes.
As you probably know, voltmeters have very
high resistances in them so that they will not
draw any
is
tion to take place, relatively low resistance loads
positive current in the
would
be:
V =
I
R
V=
(1 u
V =
10 millionths of a volt
current. This is called a high internal
impedance. Its purpose is to be sure that the
voltmeter will not draw current and change the
voltage that it is trying to measure.
amp)
(10
ohms)
-
practically zero
m
>— +r
TYPICAL
DIODE
WAVEFORM ON
SCOPE LOOKS
JUST LIKE
INPUT
VOLTAGE. NO
RECTIFICATION!
1
MEGAOHM
LOAD INSIDE
OSCILLOSCOPE
>
Fig. 2-11
An
oscilloscope measuring voltage through a diode
20
So, for a 10
ohm
load, the rectification
Diode misconception
would
be excellent.
Another common diode misconception
has a 1 million
ohm internal impedance, the negative voltage it
would see during the negative half cycle would be:
If the oscilloscope in Fig. 2-11
(1
V=
1
amp)
fi
(1
on
must be "positive" while the voltage
on the cathode must be "negative." Actually,
megaohm)
and negative are just relative terms. All
is which end of the diode is more
that matters
positive.
volt
If the sine
wave had
1
volt peaks, this
that the oscilloscope could
sine
that
the anode
positive
V =
is
in order for conduction to occur, the voltage
wave without the
Both of the diodes shown in Fig. 2-12 are forward conducting because the voltage at the anode
end of each is more positive than the voltage at
means
show the complete
slightest evidence of rec-
the cathode end.
We
should point out that small, high
quality silicon diodes with leakage currents as
low as 0.1 ^ ampere will rectify even in this situatification.
F.
tion.
*r
12
VOLTS
=
—
-WAA/
^^
The current from a half-wave rectifier is DC,
but it is certainly not continuous DC such as you
would obtain from a battery. If this source of DC
were being used to power a Hi-Fi amplifier, these
pulses of DC would be heard as a loud 60 cycle
6
VOLTS
X
Filtering rectifier output
hum
hum
JT
in the loudspeaker, exactly the
same
sort of
that a large transformer makes.
The humps in the rectifier output are called
and to smooth them out, the output is
ripple
WW-T]_
>j
12
.
6
almost always passed through a low pass filter. A
low pass filter passes very low frequencies but
severely attenuates high frequencies. Pure direct
current is the lowest possible frequency. Pure DC
not only does not alternate its polarity, it doesn't
even change its voltage level. Therefore its frequency is zero and when pulsed DC is passed
through a low pass filter, the pulses are at-
VOLTS
-
VOLTS
X
zr
tenuated.
Fig. 2-12
Forward biased diodes
R-C
LOW PASS
FILTER
>—w
Fig. 2-13
Half-wave
DC RIPPLE
rectifier
21
with a low pass
filter
-^
Rload
->
ISOLATION
TRANSFORMER
Full-wave
Fig. 2-14
voltages at the ends of the secondary are opposite
each other, one end or the other of the secondary
Looking at it another way, the capacitor
charges and discharges so slowly that the gaps
between the pulses are not long enough to allow
As
the capacitor to discharge to zero volts.
rectifier circuit
will
always be positive with respect to the center
of the secondary winding.
a
gaps are eliminated. There is always
some up and down variation, ripple, which remains no matter how large the capacitor is. The
larger the capacitor and the larger the resistance,
the purer the DC that will emerge from the filter.
result the
2.
By using the
be half as high as it would be if we used one
of the secondary as "ground." But since we
are using a transformer, the winding can be built
to produce any voltage desired.
will
end
Full-wave rectification
Since
large
capacitors
are
expensive,
The output from the
and
blem. The idea behind full-wave rectification is to
produce useful current out of both the negative
and the positive half cycles. In order to do this,
you need a new "ground" or zero reference for the
output voltage.
Isolation transformer
3.
Bridge
a
rectifier
called a bridge rectifier.
The
AC
to be rectified
fed into opposite ends of the diamond.
is
is
The output
taken off the other two corners.
you look at the direction of each diode
you will see that only positive current is
allowed to go to the positive side of the output
If
carefully,
floating in a balloon.
means that you could touch
is
in
Diodes can be used to establish the new
ground reference for full-wave rectification. The
diamond shaped configuration for the diodes is
of full-wave rec-
but the easiest to understand, is accomplished with an isolation transformer.
Household 60 Hz current is nearly always referenced to earth ground. By passing this current into the primary of an isolation transformer, the
output from the secondary of a well designed
transformer can be as well isolated from ground
as if it were coming from a diesel generator
tification,
Isolation
full-wave rectifier
with no gaps
between.
These pulses are all positive and so close together
that they are easier to filter. There are 120 humps
per second instead of 60 per second for unfiltered
half-wave rectification. If this unfiltered DC were
used to power a stereo, you would again hear a
loud, unpleasant hum, but the frequency would
be noticeably higher.
series of half cycles
diodes are not, full-wave rectification is an inexpensive way to greatly improve the ripple pro-
The most expensive kind
center tap as a ground reference,
the voltage coming out of the full-wave rectifier
while negative current (positive current going the
opposite direction) is allowed to go only to the
negative side of the output. At any given moment
two of the four diodes are conducting, one to the
either
side of the secondary with a bare finger tbut not
both sides) and not be shocked. The new ground
reference can be the center tap of the transformer
secondary winding. Since the polarities of the
positive side and one from the negative side of the
output.
22
LOW PASS
VOUT
FILTER
Fig. 2-15
Diode bridge
rectifier
Choke inductor
with low pass
filter.
voltage of the battery, the diode turns on and conducts current into the battery.
A more complex low pass
filter using a choke
shown. The chokes are not used often
in such filters unless the current is low, because
high current inductors cost quite a lot. The advantage of the inductor is that, unlike the resistor, it doesn't dissipate energy and the output
voltage is higher for the same degree of ripple
smoothing. The input capacitor, Ci, when combined with the forward resistance of the diodes
makes an R-C low pass filter. The C-L-C configuration is sometimes called a pi filter because
the three elements drawn in a circuit resemble the
greek letter n.
inductor
G.
In this way the voltage across the load reprevented from rising higher than the
battery voltage. This clips the sine wave off at V
volts. This clipping action is not restricted to
is
sistor is
positive half cycles.
The
circuit in Fig. 2-17 clips
both the positive and negative peaks of the sine
wave. In effect, it produces a fairly good AC
square wave from a sine wave.
What happens
into the batteries?
to the energy that is shunted
Some
of
it is
lost in heating the
resistance of the battery and diode, but
most
of
it
charges the battery. As we shall see later, there
are easy ways to build clippers that do not require
Diode Clippers
batteries.
Clipping circuits resemble rectifier circuits.
are also called limiters, amplitude selectors,
They
or
slicers.
The
rectifier
circuits
H.
Suppose you had a widely varying DC voltage and you needed to know the highest voltage
that was reached during a certain time interval. A
peak detector consists of a diode charging a
capacitor. The diode allows current (charge) to
flow into the capacitor, but will not let it leave.
As a result, the capacitor charges to the highest
voltage it is exposed to, but doesn't discharge.
"clipped" the AC cycle at zero volts. By rearranging the circuit and adding a reference voltage, it is
possible to clip off either the positive or negative
voltage peaks at any desired voltage level.
Fig. 2-16
shows a diode
clipper circuit
made
from a diode and a battery. Whenever the voltage
across the diode and battery tries to exceed the
>
Diode Peak Detector Circuits
we examined
WW-
">
I
Rload
V VOLTS
"(BATTERY)
>
*
Fig. 2-16
Diode clipper
23
circuit
*-:
>
\AAAr
*
I
>
>
Fig. 2-17
Dual
clipper circuit converts an
AC sine
wave
to
an
AC square
wave.
.PEAK
>— w-
VOUT
->
PEAK
-r
/
i
\
SWITCH
RESETS
CAPACITOR
TO ZERO
!
.
fc
>
Fig. 2-18
/.
Diode peak detector
+5
DC-to-DC Voltage Inverter
The following
+5
volts
volts can be inverted to a
must
minus
be converted into a square
wave. This is done by just switching it on and off
with some sort of electronic switch, such as a
supply,
to the peak detector to provide a small source of
negative voltage. Because power supplies are expensive, engineers try to design electronic equipment with as few of them as possible.
it
first
multivibrator.
Let's assume that the first capacitor, Ci, is
not charged so the voltage across it is zero. As
soon as the square wave jumps up from zero to
+5 volts for the first time, the voltage at the bottom of the capacitor, that is, the voltage across
the diode, CRi, will rise abruptly to +5 volts.
This is because the voltage across a capacitor
For example, most computer circuits use 5
volts DC practically everywhere. However, every
now and then a precision operation amplifier or
an analog to digital converter must be used which
requires a few milliamperes of —5 volts DC. The
circuit in Fig. 2-19 uses diodes and capacitors to
make this transition from +5 volts to —5 volts.
This circuit
—5
volts to
Before
circuit uses a principle similar
can't change instantly.
Since the voltage across the diode
can be thought of as
on the fact that once
a capacitor is charged, a capacitor has no way to
discharge itself. Moreover, a good quality diode
can only conduct in one direction, so between
these two properties, energy is "pushed into" the
capacitors and is given no way to leave except in
the form of a —5 volts supply.
(Fig. 2-19)
two peak detectors.
t
">
is
more
positive on the anode side, the diode will conduct
It relies
and the capacitor
Let's
will
charge toward
+5
volts.
assume that by the time the square wave
drops back down to zero, the capacitor has charged to + 5 volts and the voltage across CRi is zero.
CR2 has not conducted yet, so the voltage across
C2
24
is still zero.
+
5
V
CR 2
-5 VOLTS
cr
c/)
Oh
<O
>
+ 5 VOLTS DC
£
O
w>
=^c
uj
CC
-J c/5
UJ
CC
qj
CD a?
CO
t- 2:
O
+
2
^Rload
a.
iizo
so-
CR,
•
Fi/j.
2-79
As soon as the square wave drops from +5
down to zero volts, it will "push" the charg-
word switch you immediately think
of a device
switch that has three basic parts— the
wire going in, the wire coming out, and the handle
to turn it on. The diode has the first two ingredients, the input and output leads, but where is
the handle ?
like a light
ed capacitor below zero. Remember, the capacitor
can't change its voltage immediately, so its +5
volt side is suddenly connected to zero, while its
old "zero side" is suddenly connected only to the
diode CR2 which connects it to the node where we
want —5 volts. Since CR2 now has a higher
voltage on the right, zero, than it has on the left,
—5 volts, current flows from the zero, right to
left. And, while this current is discharging the
capacitor, Ci, it is also charging C2 in the
negative direction.
The handle
diode is the fact that a small
on top of a large DC signal.
Whenever the diode is turned on by a relatively
large DC signal, a relatively small AC signal can
be added to the large DC signal so that its
waveform is impressed on the top of the DC.
An analogy might be waves on the surface of
a deep river. When the large DC signal is changed
from positive to negative, the diode stops conducting and the AC is turned off along with the
DC. If the river dries up, obviously the waves will
disappear with it.
signal can
Diode Switch
The diode can be used
small
AC
signals on and
as a switch to turn
off.
When you
} VOLTS
*
DC-to-DC inverter
volts
J.
*
•
•
use the
in a
ride
ON
,
V
,\
+
ON
OFF
-
X-
OFF
pK^nON
OFF.
AC RIDING
DC PULSE
\t
RC
>
HIGH
PASS
SV /ITCHIN G
WFORN
F
1
OFF
FILTER
MvW
1*
OUTPUT
WIK>
AC SIGNAL
BEING SWITCHED
WAVEFORM
T
k_
SWITCH
CURRENT FROM LARGE
•ON" FLO//S
*
DOWN TO
SMALL AC VOLTAGE
Fig. 2-20
A
diode switching a small
25
AC signal on
and
off
•
K.
The
Diode Detector
difference
Crystal sets
The
earliest
practical
AM
radio
that the signal going into
we discussed
is
wave, anywhere from, say 100,000
receivers
MHz. The
were called crystal sets and consisted of little
more than a diode, an antenna, and a headphone.
Diode radio detectors are basically rectifier circuits followed by low pass filters. They are not
very different from the half- and full-wave rectifier circuits
is
a high frequency radio signal instead of the low 60 Hz signal found in power supplies. The radio signal is a high frequency sine
the detector
signal varies
Hz
up and down
to 100,000
in
amplitude
at a rate proportional to the modulation.
modulaton can be
whatever.
TV
The
picture, or
the output from the detector were
would remain a series of DC pulses,
the unfiltered output from half- or fullIf
unfiltered
earlier.
voice, music,
it
much
like
wave
rectifier circuits.
Modulation and detection
The modulation and detection process is seen
The upper waveform is the original
audio waveform as it comes out of the studio
microphone at the radio station. The second
waveform is the transmitted radio signal. The
*-t
in Fig. 2-21.
audio signal rides on top of the radio signal, much
the way the small AC signal rode on top of the DC
signal in the diode switching circuit.
Looking
at this another
way,
it is
as though
the amplitude of the radio transmitter signal
is
being increased and decreased in time with the
DJ's voice. The third waveform is the same radio
signal after it has been rectified by a simple halfwave rectifier. After passing through a low pass
TRANSMITTED RADIO SIGNAL
filter,
the resulting signal
is
a good replica of the
original audio signal.
Fig. 2-22
crystal set
shows a
AM
by an L-C tank
circuit
receiver.
The
diagram of a simple
signal is first tuned
circuit so that, hopefully, only
station will be heard at a time.
"-
one
The idea behind
the resonant tank circuit is that all stations except the desired one will be shorted to ground.
SIGNAL AFTER
RECTIFICATION BY DIODE
The
then rectified and passed
pass filter consisting of the
resistance which is the forward resistance of the
diode and the capacitor, Cj. After leaving the low
pass filter, the audio signal is a positive DC signal
which varys up and down at the audio frequency
through
rate.
AM
a
is
low
The R-C
filter
just removes the short radio
frequency pulses. The headphones in this circuit
serve two purposes. The most obvious is, of
course, that they convert the varying DC signal
to sound waves.
REPLICA OF AUDIO WAVEFORM
AFTER LOW PASS FILTERING
Fig. 2-21
signal
As the DC current passes through the headphones, small inductors, coils, produce a varying
modulation und detection process
26
LONG WIRE ANTENNA
RC
LOW PASS
FILTER
~>
FORWARD RESISTANCE
IN
RECTIFIED DC
DIODE
HEADPHONES
CONVERT VARYING
DC CURRENT
TO SOUND
-V*
Rd
HEADPHONES
LC TUNED
ALSO DRAIN
-CURRENT OUT OF
Ci TO KEEP IT
DISCHARGED WHEN
THERE IS NO SIGNAL
CIRCUIT SELECTS
r
T
DESIRED STATION
(SHORTS ALL OTHERS
TO GROUND)
Fig. 2-22
AM detector in a crystal set
magnetic field that pushes and pulls on thin steel
diaphragms. These diaphragms vibrate and produce the sound.
The second purpose of the headphones is
more subtle. They provide a load on the capacitor,
Ci, and discharge it in between audio peaks. If
there were no load on the capacitor, the circuit
would be a peak detector and would charge to the
highest audio peak and stay there. Although detectors are frequently more complex than this,
simple diode detectors are
still
widely used, even
down
to the frequency of the original speech.
The
headphones convert the electrical signal into
sound waves compatible with your ears.
Mixers
It is frequently
necessary in more complex
radio receivers and transmitters to convert one
radio frequency to another radio frequency. Diode
converters or mixers are one
done.
way
this
can be
in sophisticated avionics receivers.
L.
Diode Frequency Converters
The diode
an example of a diode
frequency converter. The radio signal varys in
amplitude at a rate which matches the speech and
music frequencies that are being broadcast. The
human ear can't hear sound waves at a frequency
higher than about 20,000 Hz and it certainly
can't hear electromagnetic (radio) waves at any
crystal set
The basic idea
you want
ly
is
to
mix the
radio signal that
to convert with another different, local-
generated radio frequency signal.
is
together,
other.
the
two signals
One moment they
When mixed
interfere
with each
are both positive
and
they reinforce each other. This makes the
bigger than either of the two signals.
sum
However, the next moment one may be
posi-
tive while the other is negative. In this case they
frequency.
cancel each other and the mixture of the two sig-
Even
directly to
waves were converted
sound waves, say at 1,000,000 Hz, you
if
the
radio
wouldn't be able to hear the modulation
because the sound frequency would be too high
for the ear to respond. Sound waves above the
range of hearing are easy to generate and are called ultrasound. The diode detector serves as a frequency converter which reduces the modulation
still
nals
is
smaller than either of the original two
two signals don't have the same
signals. Since the
it is inevitable that they will be out of
synchronization with each other half of the time.
frequency,
result of this mixing is a new complex signal
which is amplitude modulated with a sine wave
whose frequency is the difference between the frequencies of the original two signals.
The
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LOCAL
OSCILLATOR
Fig. 2-23
A
diode frequency converter
not very sensitive. The antenna
the signal must be very strong
in order to hear a station clearly. Moreover, the
signal going to the earphones is far too weak to
drive a loudspeaker. The second serious problem
with the crystal set is that it is not very selective.
It will easily pick up more than one station at
Beat frequency
First, it is
must be huge and
This difference frequency is called a a beat
The beat frequency could be in the
audio range if the local oscillator and the radio
signal were only a few hundred cycles apart. How-
frequency.
ever, usually the
two signals
are
hundreds of
hertz apart so that the beat frequency itself
radio signal. In summary, mixing the radio
kilois
a
once.
fre-
Both of these problems can be greatly improved by amplifying the radio signal many times
with several sharply tuned amplifiers. We will
discuss amplifiers in detail in Sections 4, 5, and 6.
For now it is enough to understand that amplifiers take small, low amplitude signals and make
high amplitude signals that are like the original,
but very much larger.
quency signal with the local oscillator signal produces an AM modulated signal which is modulated with a
new
radio frequency.
Frequency converter
The next step in the frequency converter is to
detect this modulated radio frequency. This can
be done in the same way as in the crystal set. In
the circuit seen in Fig. 2-23, the diode "shorts
It turns
out that it isn't practical for one
the signal large enough to drive
a loudspeaker or to drive several tuning filters.
out" the negative half cycles by conducting them
to ground. The rectified half-wave signal is
transferred across the transformer where it is
tuned by an L-C filter to exclude all frequency
components except the desired new beat frequen-
amplifier to
make
Practical radio receivers have 4 or
fiers
in
series
more ampli-
to produce signals of sufficient
strength to drive a loudspeaker.
cy.
TFR
You should be asking
yourself,
"What
is
receiver
a
frequency converter good for?" We shall start at
the beginning. Suppose you were to build the
crystal set in Fig. 2-22. You would find that it has
(TRF).
two serious drawbacks.
quency amplifiers
Fig.
28
Another obsolete kind of receiver is seen in
2-24, the tuned radio frequency receiver
The
TRF
receiver uses 4 or
in
series to
more radio
fre-
produce strong,
ANTENNA
ALL FOUR L-C CIRCUITS
TUNED TO RADIO STATION
DETECTOR
LOW PASS
&
FILTER
LOUD-
SPEAKER
Fig. 2-24
selective
radio
signals
prior
to
detection.
A TRF receiver
An
Superhetrodyne receiver
audio amplifier makes the audio signal from the
detector strong enough to drive a loudspeaker.
Fig. 2-25
shows a block diagram
of a super-
hetrodyne receiver. This is a fancy word for a
receiver with a frequency converter. The string of
Note that each radio frequency amplifier is
preceded by an L-C tuned circuit which selects
the proper station. After being tuned four times,
the selectivity is very good. While we are at it,
notice that the low pass filter after the detector
is, in effect, a tuning mechanism for the audio
amplifier. That is, it rejects all the high frequency
half cycles that we do not want to amplify.
tuned RF amplifiers is called intermediate frequency amplifiers or IF amplifiers. After the IF
amplifiers there are the usual detector and audio
amplifier. Sometimes the frequency converter is
called the first detector and the audio detector the
second detector.
As an example, let's say that the intermediate frequency is 455 kHz. As the local
oscillator frequency is changed, the oscillator is
always 455 kHz away from some frequency. So
that is the frequency that is detected and sent to
radio tuned
the IF amplifiers. For a standard
to 1000 kHz with an IF of 455 kHz, the local
oscillator must be tuned to 1455 kHz. When the
The TRF receiver works great, but what happens when you want to change stations? Each of
those L-C tuned circuits must be individually
tuned to the new station. This is very difficult
and explains why TRF receivers are obsolete. Attempts to tune all four tuned circuits simultaneously don't work very well, usually because one
or two of the amplifiers won't tune exactly like
AM
is tuned to 600 kHz, the
tuned to 1055 kHz, and so on.
radio
local oscillator is
the others.
Unfortunately
Modern
the
TRF
there
is
a
catch
to
local
The local oscillator is not only
455 kHz above some frequency, it is also 455 kHz
below some frequency. This means that if there
were not some filtering on the antenna, the radio
could pick up two radio stations at once, even
though the stations were 910 kHz apart! The unwanted station is called an image and is tuned out
by a filter between the antenna and the frequency
receivers achieve the advantages of
oscillator tuning.
receiver without the tuning difficulty.
The radio frequency
signal is converted to a
constant intermediate frequency. After
the signal has been converted, the string of tuned
amplifiers amplify without the need for retuning
them for each station. The idea is that the radio is
tuned by shifting the frequency of the local
oscillator rather than tuning all those amplifiers.
single,
converter.
29
ANTENNA
FREQUENCY
CONVERTER
„
THESE 3 L-C CIRCUITS
ARE PERMANENTLY TUNED
TO THE INTERMEDIATE
FREQUENCY
',
ANTENNA AND LOCAL
OSCILLATOR ARE TUNED
SIMULTANEOUSLY
TO SELECT THE STATION
LOUDSPEAKER
Fig. 2-25
Tuning
two
circuits,
the
filter
and
A
superhetrodyne receiver
voltage regardless of how much current was
passing through the voltage source. Referring to Fig. 2-8, are there any features of
the
than tuning four or more
RF amplifiers, so the superhetrodyne has become
the standard way of building radios. Diode frequency converters are still important at very high
oscillator, is still easier
semiconductor diodes that seem to
fit
this
definition of a "perfect" voltage source?
(like radar) where other more exotic
frequency converters don't work very well.
frequencies
4.
List as
many
reasons as you can why the
is not "perfect."
semiconductor diode
QUESTIONS:
5.
1.
2.
semiconductor diodes have a forward
offset voltage and a forward resistance.
From what you know about the behavior of
thermistors, how would you expect these
two diode characteristics to change as the
diode temperature increases?
All
resistance and power dissipated in the same
diode for 10 milliamperes of forward cur-
Large diodes have more N- and P-type
semiconductor in them. How is this related
to the fact that large diodes pass more
leakage current
small diodl
in the
rent.
6.
reverse direction than
were such a thing, a "perfect
voltage source would provide a constant
It
customary to rate diodes
in terms of the
forward current they can safely
conduct, rather than the maximum power
they can dissipate. This is because the forward voltage drop across the diode is very
constant with different forward currents.
It is
maximum
'
3.
Just because diodes are not linear does not
mean that they do not obey Ohm's law. Referring to the silicon diode characteristics
shown in Fig. 2-8, what is the approximate
forward resistance of the diode when it is
conducting 5 milliamperes? Now find the
there
30
Therefore, the
maximum power is directly
maximum current. For
7.
proportional to the
MR
Electronics
engineer Jones
designed
the
following hybrid power supply for the
new
Mark IV Wonderview TV.
1396 is a silicon
example, the Motorola
diode rated at 30 amperes maximum average forward current. At this current the forward voltage drop is only 1.0 volt. At 0.5
amperes forward current the voltage drop is
0.8 volts. How many watts can it safely
dissipate? What is its forward resistance
It is a
combina-
tion of a diode bridge full-wave rectifier
a center tap full-wave
rectifier.
Wonderview TV's were
and
Over 40,000
built before this in-
novative circuit was noticed. Why was
Jones fired? Was the management jealous of
his inventive genius?
under these two different conditions?
POSITIVE DC
120
VOLTS
AC
VOLTAGE
lAtifc
1
DESIRED HERE
T
/TYYY
1
HINT:
Look
at
what happens to each diode as the transformer secondary processes positive and negative voltage
tapped winding.
in
each
half of the center
When bench
testing some of the 40,000
Wonderview TV's, it was found that some of
them worked fine after initially blowing the
9.
and belching clouds of black
smoke. Looking at the circuit in question 7,
what might have happened that could have
"redesigned" Jones' circuit and left it workcircuit breakers
Electronics engineer Jones gets a new job
with ThunderVista Television Company. He
is certain that his new design will not blow
circuit breakers or smoke. He secretly shows
the new power supply design to you before
building it. Is he right? Will it work?
ing properly?
POSITIVE DC
VOLTAGEDESIRED HERE
T
1
HINT:
How can
the positive current travel from one side of the secondary winding to the other?
31
10.
In each of the following clipper circuits,
13.
figure out the output voltage waveform. If
by a circuit, figure it out
without the battery. That is, replace
the battery with a piece of wire. Then move
you
are confused
first
the clipping action up and
tion (polarity) indicated
>
^t
down
by the
in the direc-
battery.
>
•M-
Engineer Jones has observed that TV commercials are louder than the rest of the
regular program audio. He is designing a circuit that will turn off the TV audio whenever a commercial comes on. He plans to use
a peak detector similar to the one in Fig.
2-18 except that the peak detector must ignore all the audio peaks less than a certain
threshold voltage, V. He wants to add a battery of voltage
V IN
V
to the circuit so that the
output voltage will remain zero until audio
peaks exceed the battery voltage, +V. He
can't figure out where to put the battery, so
he asks you to help him. Draw the new threshold peak detector and sample waveforms.
VOUT
*
14.
11.
Draw
the output voltage waveform.
y
vwv
->
15.
M
,
shown
in Fig. 2-20 a
is
There are about four steps involved in
receiving amplitude modulated radio waves
in the crystal set shown in Fig. 2-22. In your
own
the output voltage waveform.
>
signal
nal rectified?
X
Draw
AC
turned on and off by a
large DC square wave. Why does the AC signal have to be small? Why isn't the AC sig-
VOUT
12.
In the diode switch
"small"
16.
words, describe these steps.
AM
In what ways are a diode
detector and a
diode frequency converter alike? In what
ways are a diode
detector and diode frequency converter different?
AM
>
VOUT
17.
>
What
is the purpose of a local oscillator in a
superhetrodyne receiver? Why is it necessary to tune the antenna signal as well as
the local oscillator?
32
SECTION III
Special Purpose Diodes
In the last section we studied ideal or perfect
diodes and compared them with real semiconductor diodes. We found that real diodes have a
number of flaws that can complicate
one way electricity valves.
many
turns out that
It
i>T
VOLT
BATTERY
0.6
Fig.
of these flaws can be
we
A.
/\AAA/*
DIODE
FORWARD
RESISTANCE
Equivalent circuit of a
3-1
made from
commonly used which capitalize on these unusual
for the
DIODE
their use as
useful. Several types of exotic diodes are
characteristics. First
IDEAL
silicon
diode
"perfect" parts.
will look at applications
forward offset voltage.
the need for real batteries. Several silicon diodes
can be placed in series to produce larger voltage
Stabistor Diodes
offsets.
Stabistor diodes
consist
of
several
silicon
diodes connected in series and packaged as
if they
were one diode. Their combined forward offset
voltages
They
make them
are
For example,
useful for voltage regulators.
sometimes called forward voltage
re-
ference diodes.
All this talk about batteries inside of diodes
given you the idea that you can run
may have
your flashlight on a 3 volt stabistor. This "battery" we are talking about is really a voltage bar-
shows an equivalent circuit for a real
made from "perfect" parts. It conan ideal diode, a resistor, and a 0.6 volt bat-
Fig. 3-1
rier.
silicon diode
It is
not actually a battery that will provide
energy.
tery.
A good analogy would be a dam on a river.
The water must reach a certain depth before it
Resistor
resistor represents
four silicon diodes in
Voltage barrier
Silicon diodes typically have a forward offset
voltage of 0.6 volts. This offset voltage is like
having a 0.6 volt battery built into the diode.
The
we put
bistor diode.
Silicon diodes
tains
if
voltage will be about 2.4 volts.
Fig. 3-2 compares a clipper circuit of the type we
looked at in Section 2 with one made from a staseries, the offset
can flow over the dam. Just because there is a
dam, does not guarantee that there is any water
in the river behind the dam. The energy must
come from outside the diode before current can
flow "over" the voltage barrier.
the forward resist-
ance of the diode when it is conducting. Since
each diode comes equipped with its own 0.6 volt
battery, it is easy to build clipper circuits without
33
R
>
Z>
I
IDEAL
DIODE
VOUT
VlN
_
X
>
>
VOLT
BATTERY
2.4
^
vVv'sA
STABISTOR
'DIODE
VlN
Fig. 3-2
v
J
>
>
A
stabistor clipper circuit
When
Stabistor voltage
it
Stabistors are available in a variety of
voltages up to about 5 volts. From the curves in
Fig. 2-8 you can see that diode forward offset
voltage(s) do vary somewhat with current, so the
nominal regulating voltage of a stabistor is usually specified at some standard current, like 10
milliamperes. Since the stabistor has a voltage
across it and a current passing through it, there
must be power consumed by it. Therefore, the
other important parameter of a stabistor is the
amount of power (heat) that it can dissipate
without damage.
B.
^
EQUIVALENT CIRCUIT
Silicon Solar Cells
the sun shines on the
P semiconductor,
way that heat
frees hole-electron pairs, just the
and any
semiconductor material. The sun's energy knocks
electrons from the valence band in the P material
"up" into the conduction band in the N material.
The forward offset voltage, 0.6 volts, prevents
them from falling back down to the valence band.
frees hole-electron pairs in thermistors
This offset voltage barrier itself acts like a
back biased "ideal" diode as long as the "backward" voltage (the real anode to cathode voltage)
doesn't exceed 0.6 volts. By providing an external circuit for these electrons to flow through,
these
new
useful
work by flowing back
electrons in the
N
material can do
to the
tor layer via the external circuit.
We
said that in order for current to flow over
the forward offset voltage barrier, it had to be
provided by some outside source. The silicon
solar cell is a diode that does this by converting
sunlight to electrical current. As long as the sun
is
shining, this diode really
is
a battery!
voltage in this solar battery
cell
construction
made from delicate wafers of
with a very thin layer of P-type
silicon on the surface. A grid of thin metal win's
on the surface of the solar cell collects the current
from the V semiconductor.
Solar cells are
Vtype
limited to the off-
set voltage, 0.6 volts.
In practice a silicon solar cell provides 0.6
volts only
(when
load
Solar
is
P semiconducOf course, the
when
it
has virtually no load on
it
not powering anything). When a useful
placed on it, the voltage drops down to
it is
is
about 0.45 volts or
less.
Silicon cell battery charger
silicon
To use
silicon cells to
charge a battery,
many
placed in series to give a voltage greater
than the battery you wish to charge. For exam-
cells are
GRID OF
COLLECTING
WIRES
~>
HOLES APPEAR
IN P
MATERIAL
POSITIVE
A
CURRENT
(FLOW OF
.LOAD
HOLES)
SEMICONDUCTOR
ELECTRONS KNOCKED
METAL
LAYER
Fig. 3-3
A
silicon solar cell
charge a 12 volt battery, typically 32 to 36
diode cells in series are used. The amount of current a silicon cell can provide is directly proportional to the area of the cells.
pie, to
C.
Light Emitting Diodes
Light emitting diodes (LEDs) are replacing
incandescent lamps for panel lights in most electronic equipment and are widely used to make up
the number and letter displays in calculators,
digital clocks, cash registers, and avionics instruments. We are describing them here because,
in a way, they are another application of the forward voltage drop in a semiconductor diode.
Fig. 3-4 shows a circuit with 9 silicon cells being used to charge one 3 volt battery. Notice the
extra, conventional diode in series with the battery.
INTO N MATERIAL
This diode allows positive charge to enter
the positive end of the battery but does not let
positive charge go from the battery back to the
Light emitting diodes are the opposite of a
absorb light which
knocks electrons from the valance band up to the
conduction band. Light emitting diodes give off
light when electrons are pushed off the "energy
cliff" and fall from the conduction band down to
the valence band.
solar cells.
silicon solar cell. Solar cells
When the sun sets, the solar cells revert to ordinary silicon diodes and could put a drain on the
battery. This drain is not serious because the
combined forward offset voltages would have to
be overcome before large currents can flow. None
the less, there is a leakage current and it is standard practice to put a protection diode in series
with the solar panel.
In most respects light emitting diodes are ordinary diodes. That is, their volt-ampere characteristics have all the same features as those
shown for silicon and germanium diodes in Fig.
2-8. When ordinary diodes conduct in the forward
direction, they just get hot. The energy dissipated in their forward resistance warms the
diode. When LEDs conduct, most of the energy
becomes heat, but a few percent of the energy is
dissipated in the form of visible light.
THIS DIODE
SUN
PREVENTS
THE BATTERY
(
9
SILICON
\
SOLAR CELL
DIODES
FROM
^DISCHARGING
WHEN THE
SUN SETS
r
VOLT
BATTERY
3
LED
J
We
usually
Fig. 3-4
Silicon solar cells charging a battery.
construction
said in Section
made from
1
that semiconductors are
materials that have 4 elec-
trons in the outer shell. For various physical
35
Fig 3-5 Assorted light emitting diodes. Note the diodes on the
make up numbers and letters.
reasons, silicon and
germanium
are the only pure
right.
They are arranged
in the far infrared
elements that work.
heat.
With LEDs
made from
various mixtures of
aluminum and gallium (which both have 3 electrons in the outer shell) and phosphorus and
are
patterns to
spectrum. Infrared light
is
just
this characteristic frequency is
high because the energy drop
LEDs
in
is
high and the
re-
sult is visible light.
Coherent light
arsenic (which both have 5 electrons in the outer
When mixed
together in the right proporelements produce a crystal that
behaves as though its valence were the average of
3 and 5, which is 4. The energy difference between
valence and conduction bands is large and it
takes about 2 volts to turn on an LED.
shell).
Light emitting diodes can be designed to give
laser light as well as pure light.
Coherent means that the light waves do not interfere with each other, but stay in phase and do
not disperse like ordinary light.
tions, these four
off coherent
These laser diodes are used with flexible glass
sending messages over long distances.
Diode laseroptical fiber systems can carry so
LED light
LEDs
is
fibers for
not only give off visible
limited to one pure color.
red, orange, yellow
LEDs
much
light, the light
are available in
and green. Blue
LEDs
D.
cliff
that
it fell off.
copper
tele-
High Voltage Diodes
have been discussing applications related
forward characteristics of diodes. Now we
are going to look at the backward characteristics
of diodes. When a diode is back biased by a large
voltage, eventually it will reach the zener breakdown voltage, often destroying the diode. To
build a semiconductor diode for rectifying very
high voltages, it is necessary to put many diodes
to the
to the height of
The higher the
cost that eventual-
We
Whenever an electron falls from the conducband down to the valence band, it gives off a
fixed, tiny amount of energy for each electron
that falls. Each little packet of released energy (a
quantum), is an electromagnetic wave with a fretion
the energy
little
phone lines and probably even microwave telephone links. Diode lasers and fiber optics are not
yet used in avionics, but it is inevitable that they
will be used in the future.
exist,
but aren't yet commercially practical. The reason
for the pure colors is the exact difference in
energy (voltage) between the valence band and
the conduction band in these semiconducting
mixtures of elements.
quency (wavelength) proportional
information for so
ly these optical fibers will replace
off-
set voltage, the higher the frequency.
sum of all the zener breakvoltages will be higher than the high volt-
in series so that the
For ordinary diodes these quanta produce
low frequency, long wavelength "light" which is
down
age being
36
rectified.
For example, the Varo VC50X silicon diode
can tolerate over 5000 volts DC back biased but it
requires over 15 volts forward voltage before it
begins to turn on. From this we can conclude that
it consists of about 25 silicon diodes in series.
Each diode must be able to tolerate over 200 volts
the power system becomes overloaded due to gen-
power company
sometimes reduces the voltage rather than cut off
erator failure or hot weather, the
the electricity entirely.
When
the line voltage
drops, the
power supply voltage drops with
unless
regulated.
it is
it,
before zener breakdown occurs in order for the
whole string to tolerate 5000
volts.
Constant load
There are three basic ways of regulating
power supply voltage. Two of them, switching
power supplies and saturable reactors, are complicated and don't directly involve zener diodes.
The most common method of regulating voltage
is to start with more voltage than you need and
then burn up what you don't want in some sort of
variable resistance.
When
voltage,
sates
the power
company decreases
the line
the voltage regulator circuit compen-
by decreasing the power dissipated
in the
resistance. This keeps the voltage across the load
High voltage
Fig. 3-6
By using a zener diode with a breakvoltage equal to the voltage desired across
constant.
silicon diode
down
the load, the zener diode can clamp (or
clip)
the
load voltage to the zener breakdown voltage. In
E.
other words, the zener diode
Zener Diodes
is
an automatic
variable resistance.
By increasing the amount of doping in the
semiconductor that makes up a diode, the zener
breakdown voltage can be decreased. In fact, the
zener voltage can be designed to fall anywhere in
the range of about 200 volts down to zero volts.
Regulator characteristics
An example
shown
back to Fig.
2-8
and you
diode
is
P =
the diode and
is
for regulating
common
at 100
is
needed
mA. The
the load tries to rise higher than 12 volts, the
zener breakdown occurs and clamps the voltage
across the load to 12 volts.
in the
,
Probably the most
DC
up exactly like the clipper circuits we discussed in
Figs. 3-2 and 2-16. Whenever the voltage across
As the unregulated supply voltage rises
higher and higher, more and more current passes
through the zener diode to hold the voltage across
the zener diode constant. The resistor, Rj, is
chosen so that, when the unregulated voltage is
as low as it will ever get, (15 volts here) the zener
diode will be just barely conducting in the zener
Voltage regulator
diodes
is
for a zener
we will start with can vary anywhere
from 20 to 15 volts DC. The zener regulator is set
V Z where I z is the current through
V z is the zener voltage.
IZ
symbol
voltage
in the voltage across the diode. Since the diode
power dissipated
circuit
that will provide 12 volts
the voltage is across a diode in
zener breakdown. Whenever the zener voltage is
reached, the back biased diode can conduct a
great deal of current without a significant change
law, the
The
Suppose a regulated power supply
will see
how constant
must obey Ohm's
of a zener diode regulator
diode is like a regular diode but the cathode bar
has been converted to a sort of "Z."
diode is made large enough to stand the
heat, these zener diodes can be used as clippers
and voltage regulators, just as was described for
If the
stabistors. Refer
in Fig. 3-7.
use for zener
power supply voltage.
Holding power supply voltage constant is necessary because the power company does not
always provide the standard 120 volts AC. When
breakdown mode.
37
27Q
AA/W
>
Ri
UNREGULATED
DC VOLTAGE
ZENER
CURRENT
11
TO
196
LOAD
mA
LOAD
CURRENT
VARIES FROM
15
TO
20
VOLTS
100
ZENER
VOLTS
DC REGULATED
12
RESISTANCE
,(120 Q OHMS)
mA
DIODE
>
A
Fig. 3-7
zener diode in a power supply
under 5 volts, stabistors are usually preferred
because they have less internal resistance under
For example, when Ri is 27 ohms, the voltage
it will be 3 volts and the current through it
will be 111 milliamperes. The load draws the 100
milliamperes it requires and the zener diode
draws 11 milliamperes.
across
(15
-
=
12 volts)
3
V =
volts =
I =
Iz =
I
R
I
(27
the same operating conditions.
F.
Capacitance
ohms)
Any P-N junction
0.111 amperes
I
-
Varactor and Step Recovery Diodes
100
diode has a certain amount
of built-in capacitance. This is usually a nuisance
mA = mA
but varactor diodes are
have high capacitance
which changes with the voltage across them. This
property allows tuned circuits to be tuned without any moving parts. Varactors are also useful
to
circuit
designers,
specifically designed to
When
the unregulated supply rises to 20
drop across the 27 ohm resistor is 8
volts. This means that the current through the
resistor is now 296 milliamperes. It follows then
that 100 milliamperes still go to the load and 196
milhamperes will heat up the zener diode. The
volts, the
power consumed by the zener diode
vz =
P =
iz
P =
2.35 watts
for multiplying frequency.
will be:
(0.196 amp)(12 volts)
A 3 watt,
1
2 volt zener diode should
work
fine
in this circuit.
Zener diode variety
Zener diodes come in a huge variety of
voltages and power handling capabilities; 200
volts down to 2 volts and 1/4 watt up to 20 watts.
There is a zener diode available for practically
any reasonable application. Unfortunately zener
diodes look like any normal silicon diode. When
replacing a zener diode, you have to look up the
part number very carefully to be sure you have
the right diode. For low voltage applications,
A mechanical variable capacitor and the
varactor diode which can replace it.
Fig. 3^8
38
For example, if you had a 100
and wanted to convert it to a 200
MHz
sive
MHz signal
MHz or 300
signal, a varactor multiplier is
way
to
do
this. Finally,
an inexpen-
varactors can even be
used to amplify high frequency signals. They are
widely used in avionics and help make complex
designs reliable and small in size.
SYMBOLS FOR
VARACTORS
When
a P-N diode
is
back biased, the positive
voltage on the N-type semiconductor attracts extra electrons out of the crystal matrix.
On
the
other side of the junction the negative voltage
is
trying to push electrons into the holes in the
200-
P-type semiconductor. The more voltage that is
applied across the diode, the more electron charge
that is pushed into the P side and pulled out of
the N side. This process resembles the charging
of a capacitor in which electrons are pulled off one
metal plate and pushed onto the other metal
VARACTOR
VOLTAGE-CAPACITANCE
CHARACTERISTICS
100-
plate.
IN 5476
.
-
**"
IN 5470
I
'
10
~~~
I
i
20
30
In a capacitor, no current can pass directly
from one plate to the other because of the dielectric insulation. If the applied voltage is suddenly
removed from both the diode and capacitor, the
charge has no easy way to rearrange itself. The
REVERSE VOLTAGE
Fig. 3-9
istic
Varactor voltage-capacitance character-
voltage
and varactor symbols
across
the
diode
(or
capacitor)
change instantly.
POINT IS GROUNDED BY THE
MFD CAPACITOR AS FAR AS
THE RF IS CONCERNED
THIS
»+
VOLTS DC
.01
*
WW
100 K
<
SOURCE OF
.01
VARIABLE
<
RF INPUT TO
L-C FILTER
MFD
DC VOLTAGE
DC VOLTAGE TUNED
L-C
CIRCUIT
/RF
\LC
EQUIVALENT
L-C CIRCUIT
USING
MECHANICAL VARIABLE CAPACITOR
Fig. 3-10
Varactor tuned L-C circuit
39
INPUT TO
CIRCUIT
can't
to the voltage. Saying it another way, the capacitance is not constant and changes as the
voltage changes. The higher the back bias volt-
Varactor diodes are designed to exaggerate
this peculiarity so that changing the voltage
across them can control the capacitance. By varying a DC voltage which is back biased across a
varactor diode, it can vary the capacitance as
seen by a small RF (radio frequency) signal across
the diode. This allows tuned circuits in radio
equipment (L-C circuits) to be tuned by a DC
voltage instead of by a clumsy mechanical capacitor or slug tuned coil.
age, the lower the capacitance because, unlike
metal plates in a capacitor, the number of holes
and electrons in a diode is severely limited and
and at high voltage they become used up.
Half-wave
rectifier
In most high speed diode applications, this
Since the diodes are back biased, they draw
very little power and may be supplied with DC
with rectification.
being used as a half-wave rectifier at high frequency. If the diode can handle a
large current, it usually has a physically large
P-N junction which can store a lot of holes and
built-in capacitance interferes
Suppose a diode
voltage through a very high resistance, typically
100,000 ohms. In fact, a single voltage line can be
used to tune several separate tuned circuits
simultaneously.
is
electrons.
In a normal capacitor, the
that a capacitor can store
is
amount
of charge
Every time the sine wave switches from
minus (nonconducting) to plus (conducting), the
directly related to the
voltage by a constant, the capacitance.
capacitance
must
first
discharge
voltage across the diode can drop.
Q =
CV, where
and
Q
is
the charge,
C
is
the capacitance,
V is
before
the
The input
sine
wave pushes the charged diode below zero in
voltage so that, instead of simple half-wave rectification with only positive half cycles in the output, the real diode produces negative recovery
spikes which are below the zero axis. If the sine
the voltage.
wave frequency
is very high and the recovery
spike lasts very long, the diode will act like a
capacitor and will not rectify at all.
Diodes are not this linear. As seen in Fig. 3-9,
the amount of charge is not directly proportional
IDEAL DIODE
Vr
)
SINE
A
f\
WAVE INPUT
>
->
IDEAL HALF-WAVE OUTPUT
REAL DIODE
—T+r T
i
*
I
I
ii
L-|
I
REAL OUTPUT
•
|- J
I
>
>
-Jtf,
REAL OUTPUT
REVERSE RECOVERY
TIME
Fig. 3-11
How
diode capacitance interferes with rectification at high frequencies. The negative pikes are
caused by the capacitance discharging.
10
Notice in Fig. 3-11 how the negative "'error"
in the half-wave rectified signal has about half the
width of the positive half-wave cycle. If it were
possible to select out these half wave negative
spikes with a resonant tuned circuit, these negative spikes would make a frequency twice that of
the input frequency because they are only half as
wide. Eureka! We have just invented the varactor
frequency multiplier.
Some varactor multipliers can produce an
output of 10 or 20 watts at microwave frequencies. The multiplication efficiency ranges from 70
to 80% at low frequencies to 10% at very high frequencies. All power that does not come out of the
varactor as useful sine wave is wasted as heat.
This heat must be dissipated by the varactor
without the diode junction overheating.
Non-linear devices
Varactor multiplier or step recovery diode
In general, the basic requirement of a
A
tuned
tal
varactor multiplier
filter
on the
frequency,
varactor
f,
makes
left
seen in Fig. 3-12.
is
A
allows just the fundamen-
The
to pass through the diode.
its
quency multiplier
"error" spikes because
A
second tuned
A
rate of
change with time that defines
it,
f.
on the
f, i.e.
right, is
tuned
signal twice the frequency of the fundamental
pure sine wave has a certain
fre-
capac-
maximum
its
frequen-
wave is fed into some device that
the sine wave by rectifying it, chopping
maximum
waveform will be increased
and the new waveform will contain higher frequency components. Since any distortion is defined as non-linearity, any device that is non-
so that only a
2f,
not
or altering the shape in any way, the
rate of
circuit,
to twice the frequency of
itance.
distorts
ing a poor job of half-wave rectifying the fun-
damental sine wave,
non-linearity,
cy. If this sine
do-
it is
is
change
linear can be
fre-
quency can get through to the output. The signal
which appears on the output side is a sine wave at
of the
used as a frequency multiplier.
Low Forward
Resistance
twice the original frequency.
Varactors make good multipliers because
they have low forward resistance so that they
Varactors designed for use as multipliers,
dissipate
little
power and
are quite efficient. If
sometimes called
varactors had high internal resistance, the input
step recover varactors, or step recovery diodes.
signal energy would be converted to heat and we
would have warm varactors and small 2f output
especially large varactors, are
"Step recovery" refers to the negative spike produced by the capacitance discharging.
L-C FILTER
L-C FILTER
TUNED TO
signals. Moreover, because of the capacitance, the
F
TUNED TO 2F
A.
*
INPUT VOLTAGE
v
<
N
THIS PEAK IS TWICE THE
INPUT FREQUENCY BECAUSE
THE WIDTH
IS
ABOUT
1/2
Fig. 3-12
A
varactor frequency multiplier
41
OUTPUT VOLTAGE
varactor stores energy over most of the cycle so
that
it
only passing current
is
through
doing this
become
its re-
on that sine wave
sistance during a small fraction of the total time.
Varactor amplifiers
is
also enlarged.
For example, if the signal being amplified
were a microwave relay of a TV signal, the
microwave signal would be mixed with an unmodulated signal of the same frequency generated by the microwave receiver itself. The
resulting signal would be an amplified version of
Varactors can also be used as amplifiers.
Amplifiers can start with a small high frequency
voltage sine wave and make another that is like
the first, but larger. How a varactor does this is a
abstract, but don't panic.
little
not only does the RF sine wave
but the modulation that is carried
is that,
larger,
the input
In the remote tuning application for varacDC bias on the varactor stays fairly constant so that the varactor capacitance appears
constant, at least as far as the small RF signal is
TV
signal.
PIN Diodes
G.
tors, the
concerned. This small
DC
AC
Voltage variable resistor
We just saw that the varactor diode can serve
signal "rides" on the
can amplify,
To explain how a varactor
we must first think about what
usually designed for use as a voltage variable
would happen
to the voltage across a capacitor
resistor. It is
large
bias voltage.
as a voltage variable capacitor.
if
the capacitance suddenly changed.
commonly used
The PIN diode
is
for controlling the
signal level of very high frequency signals. Other
PIN diode are used as high voltage rectifiers and as charge storage diodes which
are used like varactors for harmonic frequency
versions of the
Suppose you had one
of those
chanical variable capacitors sitting
clumsy meon a wooden
desk, and let's suppose that
it is charged to, say
10 volts. The capacitor is not connected to any
circuit, so the amount of charge, Q, stored in the
capacitor is fixed because it has nowhere to go.
Now suppose you could reach over with insulated
fingers and change the capacitance to half its
original value, say
200 picofarads to 100
multipliers.
METAL LEADS ATTACHED TO SEMICONDUCTOR
P+ AND N +
LAYERS ARE
HEAVILY DOPED
pico-
amount of charge can't change
and the capacitance is now half, the voltage must
become twice as high.
farads. Since the
First:
then
Q = CV =
later:
Q =
3
(200 pfd)(10 volts)
PURE INTRINSIC SEMICONDUCTOR
(1/2C) (2V)
=
(100 pfd)(20 volts)
Fig. 3-13
This
is like
PIN diode
internal structure
squeezing a balloon. The amount
So when the
squeezed, the air is forced into a smaller
volume. In other words, the volume capacity of
the balloon is decreased. The price that must be
paid for compressing the air is higher pressure inside the balloon. This is analogous to increased
of air inside the balloon is fixed.
balloon
The PIN diode
is
"electrical pressure"
which of course
is
(all
capital letters)
is
named
for its internal structure. It consists of a heavily
doped
P+
layer, a central layer of pure, intrinsic
semiconductor, and a layer of heavily doped
type semiconductor.
N+
voltage.
Heavily doped semiconductor conducts quite
and if there were no central layer of pure
semiconductor, the device would act like a zener
diode with a zener breakdown of zero volts. When
Amplification with varactors is accomplished
by mixing the signal to be amplified with a second signal that has the same frequency but differs from the first in phase. This second signal
has the effect of abruptly changing the capacitance of the varactor during each half cycle so
that the voltage abruptly rises. The point of
well
voltage is applied to the PIN diode, holes and
electrons are pushed into the pure semiconductor
layer to establish a temporary, conventional P-N
junction diode in the center.
42
However,
if
the middle intrinsic layer
is
quency
wide,
of the audio modulation. This signal is
then passed on to the audio amplifier and loud-
the transition between reverse bias cutoff and forward bias conduction is gradual instead of abrupt
as
it is
in a conventional diode.
speaker.
When back biased,
a typical PIN diode has a resistance of 8000
ohms. But, as more and more forward bias
voltage is applied, this resistance drops gradually
to about 1 ohm. By adjusting the level of a DC
bias voltage, the resistance of the PIN diode can
be adjusted to anywhere in this range. The small
AC signal which is being controlled rides on top
of this DC level. The resistance encountered by
the small signal depends on the DC bias level.
This same signal is also fed to a second low
pass filter which averages (integrates) the signal
level over a few seconds to make a very slowly
varying DC signal which is proportional to the
overall signal strength. This signal is then
amplified and used to forward bias the PIN diode
variable resistor. So if the average signal
strength drops over a period of one or two seconds, the gain (volume)
compensate
to
Because so much charge has to be pushed inand pulled out of the central layer, this diode
H.
has a very large recovery time, much like a varactor diode. Because of this, it responds too slowly
to rectify signals at high frequencies. So, from the
point of view of a high frequency radio signal, the
PIN diode is a variable resistor, not a rectifier.
A typical application for PIN diodes is in
automatic gain control circuits (AGCs). Modern
radio receivers, even the smallest
transistor
radios, have an automatic gain control circuit
which tries to hold the signal level constant. Simple radios rarely use PIN diodes for this. PIN
diode AGCs are more common in fancy avionics
AM
Schottky or
CATHODE
PIN
diode
AGC,
let in
PIN
DC
signal is
diode to turn
it
DC
signal forward
As
Schottky diode symbol
Silicon Schottky diodes conduct at 0.2 to 0.4
on just enough to
the correct signal strength.
ANODE
Silicon Schottky diodes
proportional to the strength of
the incoming radio signal. This
biases the
a
HCD Diodes
%}
Fig. 3-14
receivers.
is
slowly increased to
Schottky barrier diodes are sometimes called
hot carrier diodes (HCDs). These are semiconductor diodes made from a metal anode, fused directly to a semiconductor cathode. Schottky diodes
have very low capacitance, high switching speed,
less reverse leakage current, produce less radio
noise, and even have a smaller forward conduction voltage than conventional P-N diodes.
AGCs
To build a
generated that
is
for the drop.
volts instead of the usual 0.6 volts.
the radio
preferred for
signal becomes weaker, the DC bias level is increased so that more and more radio signal is
allowed into the intermediate frequency (IF)
amplifiers. This keeps the signal level coming out
of the loudspeaker relatively constant so that you
do not have to be continually adjusting the
volume.
many very
tions such as mixers
They
are
high frequency applica-
and detectors.
HCD construction
Hot
carrier diodes take
advantage of a "pro-
blem" that formerly plagued designers building
integrated circuits. Aluminum and gold are easy
metals to use for printing miniature wires on
The problem arises because
silicon wafers.
aluminum has a valence of plus 3. When it is
bonded to N-type or pure semiconductor, some of
the aluminum atoms diffuse into the silicon and
The DC signal which biases the PIN diode is
usually derived from the detector circuit. In Secyou learned that the output of an AM
is fed to a low pass filter which removes
the radio frequency component. The signal
emerging from the low pass filter is a DC signal
which varies up and down in amplitude at the fretion 2
detector
convert
it
into P-type silicon. This produces an
unintentional
wanted.
43
P-N junction where none was
When gold and other metals are pressed
against semiconductor, a similar P-N junction is
formed but the exact mechanism is not as easy to
Anytime metals, especially aluminum, are
fused or bonded to semiconductor, there is a
possibility that some sort of non-linear diode
junction will be formed. When a diode junction is
not wanted, it can be prevented by adding an intype
termediate layer of heavily doped
semiconductor between the metal and the semiconductor layer that the wire is supposed to connect. The extra N-type doping donates enough
electrons to fill in any holes that may be contributed by the metal atoms diffusing into the
explain. In fact, the diode used in early crystal set
radios
was
lump
a
of
semiconductor with a metal
"cat whisker" pressed against
The P
layer in a Schottky diode
that there are very few holes.
the diode
N+
it.
is
As
is
so small
a result,
when
conducting, the electrons from the
N
semiconductor pass almost directly into the
metal without passing through an extensive P
region. Because the P side of the junction is so
tiny, very little charge is stored in the vicinity of
when
the P-N junction
the diode
is
semiconductor.
For this reason, Schottky diodes are usually
made from three layers: the aluminum (or gold)
layer which is the anode, the pure or N-type
semiconductor layer in which the P-N junction is
formed, and finally an N+ layer which is an intermediary between the metal lead and the semiconductor cathode. Without the N+ layer, the
Schottky diode would have two diode junctions in
series oriented cathode to cathode.
back-biased.
This accounts for the extremely low capacitance
of Schottky diodes.
GOLD WHISKER
GLASS CASE
FUSED TO
SEMICONDUCTOR
CATHODE
ANODE
-VERY THIN LAYER OF PURE
should probably explain the term "hot
P
region
is
is
so tiny in these
almost 100%
elec-
There are many free electrons available in
both the N-type semiconductor and the metal.
Some of these are moving around due to thermal
agitation, but they can't cross the P-N barrier
without forward voltage.
trons.
ALUMINUM ANODE
£
We
diodes, the current carrier
DIFFUSE INTO THIS LAYER
N+
carrier
carrier." Since the
SEMICONDUCTOR
P AND N IMPURITIES
CATHODE
Hot
ANODE
When forward bias is applied, the electrons
that cross the barrier have a velocity higher than
the electrons that were freed by heat but were
-COPPER LEADS-
unable to cross the barrier. Since the current
carrying electrons are moving faster, it is as
though they were hotter. Therefore these forward
conduction electrons are called "hot carriers."
r
— GLASS
\ INSULATING
V LAYER
ALUMINUM
CATHODE
/.
This strange P-N junction diode can oscillate
(make sine waves) at extremely high frequencies
(10,000 MHz), amplify weak high frequency signals, and is used to build extremely fast logic circuits. It can even be used to rectify very low
voltages which are less than the forward break-
ALUMINUM ANODE
zzzzzzzzzzzzz
-N+ REGION
PREVENTS A
SECOND DIODE
JUNCTION
The Tunnel Diode
DIODE
JUNCTION
IS HERE
down
voltage of conventional diodes.
However, when used as a
Fig. 3-15
Three different examples of Schottky
polarity
is
rectifier, the P-N
the reverse of the standard P-N diode.
There seems to be
diode construction.
44
little
agreement about what
e
TUNNEL
RECTIFIER
Fig. 3-16
Tunnel diode
circuit symbols. All are equivalent except the last
signify that the tunnel diode
circuit
is
being used as a
symbol which
symbol should represent the tunnel diode,
first three symbols in Fig. 3-16 are the
With a little forward voltage for encouragement, say 0.05 volts, a significant current can
flow in this manner, anywhere from 1 to 100
milliamperes. This phenomenon is called tunneling under the barrier, hence the name.
Impurity concentration
In a conventional diode the semiconductor P
N regions have a very low impurity concentration, approximately 1 atom of impurity for
every ten million atoms of crystal. By keeping the
concentration of electrons and holes low, it takes
a large voltage to break down the diode in the
reverse direction.
and
As
still
more forward voltage
to zero
is
applied, say
0.3 volts, the short cut tunnel
under the barrier
becomes clogged by too many
go through at once. For silicon,
at a voltage of 0.4
carriers trying to
volts, the tunnel current is cut off
As we know, it takes a small but significant
voltage to make it conduct in the forward direction. As more and more impurity is added, it
becomes easier and easier to break down the
diode in the reverse direction. At an impurity confall
to
able to sneak through the crystal matrix simply
most common.
voltage will
supposed
by random motion, even though they do not have
the necessary energy to go over the barrier.
but the
centration of about one
is
rectifier.
almost entirely.
However, as the voltage is increased still farnormal barrier voltage is exceeded and
ther, the
the diode conducts like a conventional forward
biased diode. This crude explanation of tunneling
is inadequate, but the author's opinion is that a
complete explanation of the physics is even less
atom in 1000, the zener
and the diode becomes
useless for any conventional diode applications.
satisfying!
Volt-ampere characteristics
FORWARD
NEGATIVE RESISTANCE
REGION
+
Tunneling
CONDUCTION
REGION
1
Heavily doped diodes like this have a strange
forward current volt-ampere characteristic that
makes them useful. To begin with, the built-in forward voltage barrier, 0.6 volts for silicon and 0.2
volts for germanium is still about the same.
-V
ZENER
allows small
amounts
new phenomenon
CHARACTERISTICS
(GALLIUM ARSENIDE DEVICE)
of current to pass through
or under the forward offset voltage barrier. There
are so "many" extra electrons and holes looking
-I
at each other across the voltage barrier that there
is
TYPICAL TUNNEL DIODE
ZERO VOLTS-
that
a high probability that a few of
them
will
.35
MILLIVOLTS
BREAKDOWN
OCCURS AT
However, at voltages between zero and 0.6
volts for silicon, there is a
+V
.05
Fig. 3-17
be
nel diode
45
Volt-ampere characteristics of the tun-
to filter it with a low pass filter before it is direct
current with no alternating current component.
What we really had was a mixture of alternating
Negative resistance
The volt-ampere curve
for a typical tunnel
seen in Fig. 3-17. Notice that the diode
practically looks like a short circuit when it is
reverse biased. The most interesting part of the
curve is the S-shaped curve between zero and the
forward conduction region.
diode
current and direct current.
is
High pass
Suppose
extract the
The
posite of ordinary resistors in which increasing
some reason we had wanted
to
Although currents can pass
one direction and then in the other,
in the long run the average direct current passing
through the capacitor will always be zero.
inside the capacitor.
voltage produces increasing current. Because of
this S-shaped characteristic, one current level can
into
be produced by three different voltages. I wonder
what George Simon Ohm would think of that!?
it,
AC
Tunnel Diode Oscillators
first in
Waveform Generation
In electronics whenever an AC waveform
must be generated, it is usually done by
switching DC on and off to make DC pulses. The
pulses are then passed through some sort of high
pass filter to make AC.
Oscillators in general
Before we can discuss how tunnel diode oswork, we have to discuss oscillators in
cillators
make alternating current. Or,
make currents that alternate between positive and negative, they at least make a
general. Oscillators
if
for
AC component instead of the DC com-
ponent. This can be done by using a high pass
filter instead of a low pass filter. This is illustrated in Fig. 3-18. In the R-C high pass filter,
direct current can't pass through the insulation
central region where increasing voltage
produces decreasing current is called the negative
resistance region. This region behaves the op-
J.
filter
they don't
Actually, leaving the
DC
signal riding on a
current or voltage that varies in amplitude
and which can be converted to an alternating cur-
Most
pass
AC in the form of an AC
signal
is
quite convenient.
and electronic devices are
diodes and can only conduct DC currents.
rent.
Low
DC
transistors
Sometimes complex
filter
AC
signals,
such as a
like
TV
picture signal, can go through the entire receiving
process and be projected onto the CRT screen
without ever actually being converted to "of-
For example, you are already familiar with
The output of such a rectifier
half-wave rectifiers.
a series of half cycles of the original
is
waveform. Since
ity,
we
all
AC
ficial"
these pulses have one polar-
call it "direct current."
100%
alternating
positive half of the signal
However, we have
is
current
where
the
equally balanced with
the negative half of the signal.
+ V|N
+v
WL.
>
->
DC CURRENT
THROUGH
CAPACITOR
IS ZERO
>
Fig. 3-18
High pass
I
•>
filter
showing how
46
DC pulses
can be converted to
AC
DCin
>
+v
">
+V
ELECTRONIC
SWITCH
t
>
Fig. 3-19
Another way to look at
">
Basic elements of an
this is to say that the
zero voltage has been redefined so that zero
"zero" for the
AC
DC
or
signal.
current!
The second part of the inconsistency is that a
low energy state must be able to stimulate the
device to produce a high energy state. And conversely, a high energy state must stimulate the
device to turn off or produce a low energy state.
Let's see how these ideas apply to the tunnel
Linear and nonlinear resistors
Now
and
AC
that
we have
established that pulsed
are interchangeable,
we can narrow
the basic issue: what switches the
DC on
DC
in
on
and
off
circuit
Look at the volt-ampere characteristic for the
tunnel diode and you will see that as many as
three different voltages can be related to the same
is ac-
some other convenient
This concept of a signal
having two components, a DC component and an
AC component, is very important and will give
rise to endless confusion if you don't have it clear
in your mind.
tually plus 5 volts
AC generating
diode oscillator.
Tunnel diode
oscillator circuit
into pulses? All electronic devices that can be
used as oscillators have two basic states, turned
on and turned off. In order to be suitable for
oscillators, they must also have an inconsistency,
so that, when they are turned on they will change
their behavior and try to turn themselves back
off. When they are turned off, they will change
again and try to turn themselves back on. It is
like letting the dog out. He immediately wants to
come back in!
we
when
was
The tunnel diode
oscillator circuit
shown
in
Fig. 3-20 is a practical circuit in every respect ex-
cept for the battery. 0.12 volt batteries are hard
to find. Usually the voltage source is made from a
higher voltage source which is divided down with
resistors and made stable with large capacitors
across
it.
L-C
circuit
linear
All electronic oscillators contain an energy
volt-ampere characteristic is
plotted, it is a straight line. When the voltampere characteristic of an ordinary diode is plotted, it is not a straight line and therefore it is
nonlinear. But, even though the diode characteristic is complicated, there is no instance where a
given voltage can be related to more than one cur-
storage element. This can be an inductor, a
capacitor, or both. The energy storage element
determines the rate at which the electronic switch
rent.
bell.
Earlier
because,
In
all
oscillate,
real
said that a resistor
its
devices
that can be
made
switches on and off. The tunnel diode oscillator
uses a resonant L-C circuit.
L-C circuits can be compared to a
is struck with the clapper, the
bell vibrates with a distinct sound frequency. In
order to make an oscillator based on an L-C resonant circuit, pulses must be generated which keep
"striking" the L-C circuit so that the oscillation
will be continous.
Parallel
to
a current through the device can be
caused by more than one voltage. Or, a voltage
level can cause two or more current levels. This is
part of the inconsistency we were talking about.
47
When
the bell
Vin
TINY AC SIGNAL
RIDING ON
*
DC
DC SIGNAL
rwwwm
0.12
0.12
Rload
__ VOLT
t
Vq UT-
4
LC RESONANT FILTER
DETERMINES FREQUENCY
Tunnel diode oscillator
Fig. 3-20
the peak, point A.
In order to generate the current pulses to
make
circuit
the tunnel circuit oscillate, the load resistor
the capacitor
is
Every time the voltage across
low, the current is turned
back on
wave
and voltage source must be carefully chosen so
to charge
that the tunnel diode
cycle, a pulse of current is injected into the
is
forced to operate in its
region of inconsistency. For example,
if
we used
it
up
again. So, during every sine
L-C
circuit to sustain the oscillation.
a
and a 50 ohm resistor, the curwould flow would be far into the forward conduction region of the diode where its
10 volt battery
rents that
behavior
is like
+
1
CURRENT
an ordinary diode.
DC QUIESCENT
-POINT
When
the proper tiny voltage source and load
CURRENT WAVEFORM
resistance are used, the diode can be forced to
operate in the center of the negative resistance
Adjusting the average
region.
DC
voltage on an electronic device to
in
some
current and
make
operate
it
special part of its characteristic
is
called
VOLTAGE
biasing.
Sine wave
FVg,
assume
Let's
-
"ringing'
that the L-C circuit
is
already
we just need to susThe voltage across a ringing
wave. This sine wave is added
or oscillating and
tain the oscillation.
L-C circuit is a sine
to and subt acted from the operating DC voltage
(bias) which is in the center of the negative
resistance region, point B. If there were no
oscillation, the circuit would tend to "rest" at
point H and this is why an operating point like
this is sometimes called the quiescent point.
Linear negative resistance
Let's explain tunnel diode oscillators again
with a different approach. Earlier we said that all
devices which can be used as oscillators must
have two or more operating points that can be
related to the
literally
\> the sine
diode
off
wave voltage
because
very
rises,
little
it
turns the
current
wave voltage falls,
on because the current is high
same voltage
because
all
or current. This
real
devices
fit
is
this
definition.
However,
if there
were a device that had
negative resistance over its entire voltampere characteristic, this device could be made
to oscillate because negative resistance itself is
inconsistent behavior. Low voltage, by definition,
means low potential energy, but with negatixe
linear
negative resistance region.
\^ the sine
dirnle
true
Dows
through the diode in the vicinity of the "vallej
point." point C, which is at the high voltage end
ot the
Tunnel diode oscillator operating char-
.V-1'7
acteristics
it
turns the
at
the top of
l^
resistance, low voltage causes a large current to
flow.
The curve is steep, so that a
small change in voltage will produce a big change
in current. The large change in current will in turn
produce a large change in the voltage across the
load resistance.
the diode to drop.
A large current represents energy which can
increase the voltage across an energy storage
device, such as the L-C circuit.
Once the voltage
rises, the current is turned
Since the L-C circuit has no source of energy
other than the tunnel diode, the voltage across
the L-C circuit eventually must fall again as its
energy is dissipated in the load resistance. This is
off.
throwing a
like
eventually
falling
it
down
up
ball
will
in the air.
You know
part
up
that
have to come back down, so the
automatic. Since the nega-
is
tive resistance property "automatically"
the ball back
The
in the air,
throws
sustained oscillation
circuit for the tunnel diode amplifier is
almost identical to that for the tunnel diode
oscillator (Fig. 3-20) except that the two wires
shown as the output also serve as the input for an
amplifier. Tunnel diode amplifiers are used at
microwave frequencies and a circuit diagram does
not tell you much unless you understand microwaves.
is
inevitable.
Waveguides
Because the voltages across a tunnel diode
wave signal that is produced
Microwaves are such high radio frequencies
are so small, the sine
is
also tiny. 0.07 volts peak-to-peak is typical.
chokes. That
This is such a tiny signal that it hardly seems
worth the trouble. However, tunnel diodes operate well at extremely high microwave frequencies
where transistors work poorly.
The tunnel diode can also be used as an
The diode is provided with a voltage
source and load resistance that will again make it
operate in the negative resistance region. The
amplifier.
signal to be amplified
form of a tiny
in the
is
AC
is,
act like radio frequency
wires conduct microwave signals
wires
very poorly. Since ordinary wires can't be used,
the signals are conducted through microwave circuits using coaxial transmission lines or even
silver-plated pipes called waveguides. Microwave
signals are conducted down waveguides in the
form of actual radio waves. A resonant circuit for
use with a waveguide system looks like a silverplated tin can with bolts which screw in and out
to adjust the resonant frequency.
Tunnel Diode Amplifiers
K.
ordinary
that
applied across the diode
Reflectance amplifier
voltage.
The tunnel diode
As
the input voltage increases, the voltage
amplifier
is
a reflectance
waveguide version, the diode is
the end of a silver plated pipe. The
amplifier. In the
across the diode will cause the current through
mounted
at
is transmitted down
dead-end tunnel and reflects off the diode,
radar signal to be amplified
this
+
much
I
OUTPUT
CURRENT
like a flashlight reflecting off a mirror.
Unlike the flashlight reflection, the signal
is "brighter"
(larger) than the original signal. This is because
the current passing through the diode is bigger
than it would have been for a given amount of
voltage had the diode just been a lump of inert
that returns from this "mirror"
metal.
The input
signal and the amplified output
signal are separated from each other by
means
of
a central chamber or a block of exotic magnetic
material called a circulator. If it were not for the
circulator and the fact that it is operating at
Fig. 3-22
microwave frequencies, none
even be worth the trouble!
Tunnel diode amplification
or
49
of this
would work
USUALLY THINNER
THE CENTER
Tunnel diode construction
IN
Tunnel diodes used for oscillators or amare made from either germanium or
gallium arsenide. Gallium arsenide tunnel diodes
have twice the voltage swing of germanium diodes. They are preferred for oscillators because the
output signal can be larger.
/
plifiers
CONTACTS
PIECE OF
GALLIUM ARSENIDE
Tunnel Rectifiers
L.
TIN
SEMICONDUCTOR
Silicon tunnel diodes
Silicon
oscillators
Fig. 3-22
do not make good
or amplifiers and do not have a protunnel
diodes
nounced tunneling current peak. Silicon tunnel
diodes are used for rectifiers for very low AC
The tunneling
voltages, less than 0.6 volts.
rent peak
is
so low, less than
mA,
1
cur-
that this can
zener breakdown of zero volts.
For
tifiers,
tunnel diode rectifiers are
this reason,
backward diodes. Using tunnel
a rectifying
transition
rec-
exactly zero
at
diode construction
new, high energy state where they lose their
mobility. They are no longer able to conduct
through the material when they are in this high
energy state, so the resistance of the semiconductor increases as the voltage across it increases.
This process sounds something like valence band
electrons being kicked up into the conduction
bana, but physicists never call it that, and it is a
different phenomenon. The diode shows negative
resistance because increasing voltage produces
decreasing current.
approximate being turned off. The "forward"
conducting state of the silicon tunnel rectifier is
really its backward characteristic which has a
also called
Gunn
However, the low "backward" breakdown voltage (0.6 volts) and the
volts can be achieved.
large leakage current,
mA. make
1
this diode far
from perfect.
Gunn Diodes
M.
Gunn
J.B.
diodes are
w
named
Gunn. They are used
after their inventor,
5
<
<J
a
for generating micro-
wave radio signals at extremely high frequencies,
up to 35,000 MHz. In one respect they are comparable
to
a
tunnel
diode:
characteristic includes a
their
segment
NEGATIVE
RESISTANCE
REGION
4.
ui
BE
UI
Q.
2-|
volt-ampere
of its curve that
has negative resistance. By proper biasing the
Gunn diode can be made to oscillate or amplify.
50
100
150
200
250
DC VOLTS
Gunn diodes do not have a deliberate P-N
junction like other diodes and they make lousy
Fig. 3-24
one way valves. They are simply a piece of N-type
gallium arsenide (GaAs) semiconductor with two
Volt-ampere characteristics for a
Gunn
diode
wire leads attached.
Gunn
As you can see. a typical Gunn diode is biased
with hundreds of volts, and large currents
(amperes) flow through it. It should not be sur-
volt-ampere characteristics
mm
When very high voltage (500 volts per
of
semiconductor) is applied across this particular
semiconductor, the conduction electrons gain a
great deal of energy and are "transferred" up to a
Gunn diodes become very hot and
must be cooled with refrigeration
machinery to keep them from being destroyed by
high temperature. Over 100 watts of microwave
prising that
frequently
50
power can be generated by a Gunn diode, but
even with refrigeration it must be pulsed
mittantly to keep from burning it up.
7.
Using a zener diode, design a
DC
voltage
regulator which will supply 5 volts at 100
milliamperes. The average voltage from a
inter-
wave rectifier and low pass filter may
vary anywhere from 6 volts up to 10 volts
full
There are still other diodes that are used to
generate microwaves. These include the IMPact
Avalanche and Transit Time (IMPATT) diodes,
and a number of devices made by combining
Gunn diodes and IMPATT diodes with Schottky
diodes. These devices are difficult to cover in
detail without a good background in microwave
DC. Calculate the zener breakdown
8.
circuits.
An
Compare varactor diodes with capacitors.
How must varactors be biased to behave as
capacitors in tuned circuits?
QUESTIONS:
1.
voltage,
the resistance of the voltage dropping resistor, and the power dissipation of the
zener diode.
How
does the
capacitance change with voltage?
equivalent circuit for a real silicon diode
shown
(not a solar cell) is
in Fig. 3-1.
The
9.
cir-
What
is
the essential characteristic that a
must have for use as a frequency
multiplier? Have we studied any other de-
cuit contains a battery
device
out that this
provide energy. Explain why not. Is there
energy stored in the "battery?" Looking
vices that
and the text points
"battery" cannot be used to
diagram
closely at the
other reason
is
there
in Fig.
why
3-1,
what
10.
the "battery"
How
are
did
may
PIN
be used for this purpose?
diodes get their name?
for PIN diodes?
What
two major uses
can't be used to provide current?
2.
11.
A stabistor is used in a clipper circuit to clip
off the
peaks of a sine wave at about 4.2
How many
volts.
diode junctions are inside
this silicon stabistor?
3.
What do
tifiers,
stabistors, high voltage diode rec-
and a panel
In what ways are light emitting diodes
High voltage
silicon diodes are
ward
6.
What does
this
a Schottky barrier diode construcfrom conventional diodes?
What are its advantages over conventional
diodes?
What do
13.
Why
is
a zener diode and a tunnel diode
are the two most
common? What
in
diode?"
uses for tunnel diodes?
a tunnel rectifier called a "backward
What are tunnel rectifiers good for?
made from
strings of several diodes in series.
this necessary?
different
common
common?
similar to silicon solar cells?
5.
is
tion
have
of silicon solar cells (used
to charge a battery) have in
4.
12.
How
Why
do to the
14.
is
for-
What do tunnel diodes and Gunn
have in common? Suppose you were
diodes
design-
ing a radar system and had to choose between using a Gunn diode and a tunnel diode
for an oscillator to provide microwave power
at 10,000 MHz. What are the relative advantages of one device over the other?
offset voltage?
The zener breakdown voltage can be designed to fall anywhere in the range of zero to
200 volts. What parameter controls this?
51
SECTION IV
Transistors and Other
Electronic Control Devices
The British call vacuum tubes "valves", a
very good description of both tubes and transistors. The purpose of both of these devices is to
enable a very small amount of electricity to control a very large amount of electricity.
1.
A perfect switch
(zero resistance)
Able to turn completely on
and completely off (infinite
-
resistance).
2.
Unlimited gain
-
An
infinitesimal
amount
of
control electricity (voltage or current) should
A
water faucet is a good analogy to a tranbecause the input signal only operates on
the "handle." A person turning the handle has no
direct connection with the water pipes and water
reservoir. The faucet can turn water full on, full
off or anywhere in between. In electronic control
turn the device
sistor
3.
devices, a small voltage or current takes the place
all
the
way on
or
off.
When used with negative
feedback to produce a finite gain, the output
signals should be a proportional, perfect copy
of the input signals.
Perfect linearity
-
of the person turning the handle.
We
4.
started our discussion on diodes with the
ideal diode. We said that ideal diodes don't exist
Complete input-output
must have no
isolation
-
The output
influence on the input signal
and the input signal should control the output signal in only the desired manner.
but available diodes can be used as if they were
ideal with good results. We also showed how the
inherent "defects" in real diodes such as zener
breakdown and forward offset voltages can be
used to advantage. We will use the idea of an
"ideal control device" as a standard of com-
5.
and unlimited slew
able to follow any
should
be
The device
Infinitely fast switching
rate
-
input signal of any frequency.
parison for real control devices.
A.
6.
The Ideal Control Device
The
ideal control device, of course, does not
the voltage which happens to be across the
output side of the device or the current that is
function would be more complicated than a diode and there would be disagreement about the properties for such a "perfect"
device because the "defects" can also be useful.
Most experts would probably include the following features:
exist.
Perfect current source or voltage source output - This means that the gain of the device
will be constant and will not be effected by
Also, its
flowing through the device.
These six characteristics are very abstract
and we will now explain them.
53
1.
sistance. The less heat that is generated, the more
devices that can be packaged in a small space
without the semiconductor material overheating.
Perfect switching
obvious that if you want to turn a curit completely off and not just
nearly off. The reasons for this are accuracy and
power dissipation. Suppose the current through a
device is supposed to represent some value, like
"miles per hour." It is inaccurate if the device
passes a current representing 3 miles per hour
when the car is standing still and the current
should be zero. Second, a leaking device wastes
energy and generates heat.
It is
rent
off,
you want
When
on,
it is
2.
The device must be able to turn a large
voltage or current on and off in response to a very
small current or voltage. An analogy might be the
head gate on a
across a large river. Since
is
turning a head gate wheel. It would be more likely
that he would do this by flipping a switch in the
control room.
the transistor or control device turns
The reasons are again accuracy and power dissipation. When turned on,
the device should have zero volts across it and
this can't happen unless the on resistance is zero.
The largest currents flow through the device
when it is turned "on" and any resistance that
the device may have will cause the device to heat
up as the voltage is dissipated across the reclose to zero as possible.
A dam
dam
a great deal of mechanical advantage in
the machinery that opens the flood gates, a single
man can turn a very large river on and off just by
there
desirable to have the "on" resistance as
Fig. 4-1
Gain
To make
the analogy more like a transistor,
have the head gate opened and closed by a
water wheel driven by a small brook sized ditch.
With such a mechanism a small flow of water, say
a few gallons per second can turn off and on
thousands of gallons of water flow per second.
let's
as an analogy for an electronic control device.
54
a.
Current gain
illustrates
voltage gain and typically have gains of one
you are wondering, operational amplifiers achieve this high gain by put-
A
ting several transistors in series so that one tran-
finite
million or more. In case
This
the concept of gain or
few gallons of flow change in the
ditch makes thousands of gallons flow change in
the river. This would be a "water current gain" of
thousands. In the case of high gain transistors, a
few milliamperes of control current will typically
cause 100 times as much current to flow through
the transistor. The electrical current gain would
be 100 in such a transistor.
amplification.
and so
input control current
b.
gain.
device output
Voltage gain
The gain
3.
of an electrical control device is not
always expressed as current gain. Some control
vacuum tubes or field effect transistors draw so little current on the control input,
it is
more appropriate
to define the gain in
terms of the voltage across the output divided by
the voltage on the input.
voltage gain
output voltage
=
=
control input
X
gain
Amplification
We said earlier that the perfect switch should
be able to turn the current completely on or completely off. In many applications we need to have
the valve perform as a variable resistor, i.e.,
"half-on." If your shower faucet had only the
"on" and "off" positions, you would probably
stop taking showers. The ideal device should
follow the control input perfectly.
devices, such as
that
on another, which turns on a third
on.
In later chapters we will show that an extremely high gain can be shunted with a negative
feedback resistance to produce any lesser finite
gain that might be needed as well as other advantages. For now we will assume that all device
gains are some finite number, say from 1 to 1000
and that the output will equal the input times the
output current
=
current gain
sistor turns
input control voltage
c.
In other words, the gain equation above
would always be true no matter what the input
was. Saying this another way, the gain would
always be constant and the equation above (output = input X gain) would be a linear, first order
Power gain
Another kind of gain is power gain. Since
power = current times voltage, the power gain
would be:
power gain
=
output voltage
—
;
mput
voltage
X
X
equation.
output current
Beginners in electronics frequently have a
input current
hang up about
amplification.
fication as a sort of signal
These are the three commonly used kinds of
which one you
mean when talking about gain. Notice that voltage gain or current gain can be very low (1 or even
less) and still yield a very high power gain.
picture ampli-
comparable to inflating a balloon. The notion that
the original signal "gets bigger" interferes with
understanding how the amplifier works.
gain. It is important to specify
d.
They
enlargement process
They understand that
a small signal enters
Operational amplifiers
the input of an amplifier and that a large signal
comes out. For some reason, they can't get it in
How much
signal.
their
heads that the output
is
not the original
The little signal was not inflated by the
amplifier. The input signal merely turns the flood
gain would be ideal for a control
device? This is a philosophical question, but probably the best answer is that the gain should be
infinite. In our analogy this could be approximated by having a raindrop fall on the water
wheel and turn on the river.
gate on and off. If the fluctuations in the river
flow happen to resemble the fluctuations in the
flow in the little control ditch, then the amplifier
is said to be linear and the output is said to have
low distortion. The original signal is dead and
gone. Only the big representation emerges from
This doesn't sound very practical, but operational amplifiers are designed to approximate in-
the output.
55
4.
Input
The
Frequently, a fast computer can replace a
much more complicated, slow computer simply
because it has time to perform calculations using
output isolation
control
ideal
device
should
be
well
more tedious method. Suppose one computer
uses 16 bit numbers but is very slow. A simple,
fast 4 bit computer may be able to do more work
to the same precision, even though it must use
many more steps to work out the answers to 16
a
between the control signal and the main
isolated
stream
-
it is
means that fluctuamain stream should have no effect
controlling. This
tions in the
whatever on the current
in the control ditch.
Imagine the chaos that might result if water
splashing up from the river were to fall on the
water wheel or control ditch. The river might turn
itself on or off with no regard for the small current in the ditch. If turning the river on and off
were to cause the splashing that was turning the
river on and off, the oscillation would be self-
bits.
perpetuating.
is, its
6.
Power
dissipation
A second reason that high switching speed is
important is power dissipation. When a switch is
full on (closed), it looks like a short circuit. That
resistance
resistance in
This
is
The power dissipated
is zero.
P =
I
2
in a
R.
a serious problem in real amplifiers,
So P
especially high gain amplifiers. Capacitive coupl-
=
I
2
(0)
=
0.
ing can cause positive feedback between the out-
This means there
put and the input and cause the amplifier to
oscillate
by
itself
An example
is
no heat and no power
dissipated, at least not in the switching device
itself.
of positive feedback occurs
when
microphone is placed too near to the loudspeaker of a public address system and the
system whistles and howls. Like most problems,
feedback oscillation can be put to good use. Most
and other
the switch is full off, its resistance is
Since current can't flow through infinite
resistance, the current is zero so,
a
oscillators in radios, transmitters,
When
infinite.
p =
cir-
c.
(0)2
(
oo
)
=
o.
Heat
cuits are essentially amplifiers that amplify their
own
5.
Again, no power is dissipated. In between the
time that a control device is full on and full off, it
must be half on, right? This means that both R
and I are finite and will produce a finite power— in
other words, heat.
The slower the switching time the longer the
time interval when it dissipates power and the
more the device will heat up. If a transistor is
unable to get rid of the heat that is developed inside it, its temperature will rise until eventually it
outputs.
High switching speed and slew
If
the circuit
is
rate
being used as a switch,
it is
desirable to have the device turn on or off instan-
taneously, as soon as the control signal appears
on the control wire. But, of course, none of the
real devices are able to do this. The switching
delay is caused by the circuit as a whole just as
much
as
it is
by the characteristics
fails.
of the device
Present transistor circuits need anywhere
from one nanosecond to several microseconds to
itself.
Usually transistor failures are quite boring.
transistor just stops working. When you test
it out of the circuit with an ohm meter you often
find that all three leads have shorted together internally. On rare occasions a transistor can fail so
suddenly due to overheating that it explodes and
shoots its innards right out through its case.
The
switch.
a.
Speed
Fast switching time
is
three reasons. First, time
important for at least
money. A computer
is
can only work as fast as its individual transistors
switch on and off. The length of time that it takes
to
do a calculation
is literally
the
sum
of
all
Large transistors are physically better at
conducting the heat away, so they are less likely
the
necessary switching times and circuit delays
the computer.
to fail. However, as a rule, the larger the device,
the slower the switching speed, so this is a vicious
circle with the bank account the loser.
in
56
Finally, switching speed is important because high speed information can be lost or garbled if the device is not able to keep up with the
input signal.
be a constant
for a given amplifier or for the condevices that make up amplifiers. This number, the gain-bandwidth product, is a useful way
trol
and compare their performance
high frequencies. Basically, the gainbandwidth product equals that frequency at
which the gain is one.
to rate amplifiers
at
d.
Slew rate
In analog amplifiers, which are supposed to
amplify a continuously varying signal, the equivalent parameter to switching speed is slew rate.
Suppose a HI-FI amplifier were amplifying a
Mozart concerto and during one particular instant the input voltage changed from 0.1 volt to
0.2 volt in just 50 microseconds. If the voltage
gain were 100, the output would be expected to
swing from 10 volts up to 20 volts in 50 microseconds too. This represents a slew rate of 0.2
volts per microsecond.
If the amplifier
For example, suppose that a given amplifier
unable to increase the size of any signal which
has a frequency greater than 10,000,000 Hz. That
is, the gain-bandwidth product is 10,000,000. It
might very well be that at a frequency of
1,000,000 Hz that the gain is 10, since 10 times
is
1,000,000
that
sume
10,000,000,
If
we
control
are talking about a
is
TV
picture ampli-
become blurry
device
looks
like
a
variable
passing through
nearly always dissipating power.
Gain
-
the
if
is
a.
Voltage source
probably the easiest to visualize because it can be
thought of as an infinitely large battery. No matter how much current you draw from a perfect
voltage source, the voltage remains constant.
resistor
it,
so
it is
bandwidth product
Suppose you had a "perfect" 12 volt voltage
Since an ideal control device would be
finitely
Current source or voltage source outputs
The concepts of current sources and voltage
sources are very useful and important, but hard
to become accustomed to. The voltage source is
too slow. In an analog amplifier, the
whenever the signal
6.
7.
will
the picture will literally
amplifier
is 1 Hz, the gain will be
because real devices seldom have
has a slew rate equal to or
be no loss of high frequency
sounds. If the slew rate is slower than needed to
keep up with the input, then the quick transitions
will be blurred or lost by the amplifier. The
amplifier performance will be "low fidelity."
fier,
10,000,000. However, you can't as-
the frequency
gains that high.
better than this, the output will follow the input
and there
is
if
fast
and would have
infinite
gain,
source for a car battery. One cold January morning your neighbor asks you to help him start his
car using jumper cables. With a perfect voltage
init
stands to reason that when gain is multiplied
times the highest frequency it can follow, the product would be unlimited. In real devices there is
usually a trade-off between gain and high frequency.
source you could not only start his car, you could
start every other car in the universe simultaneously. The perfect voltage source would maintain 12 volts across any resistance load, even zero
In other words, the higher the frequency you
to feed into the control device, the less
amplification that comes out of it because it is
unable to keep up with the input. As the frequency is increased with any amplifier, eventually you
will reach a frequency at which the variations in
the output have no greater amplitude than the
Another way to look at this is that the perfect
battery would have zero internal resistance. Real
batteries always have a distributed resistance, R,
resistance.
try
inside them.
variations in the input.
The observed current gain
When
a heavy load
(a
low resistance)
placed across a real battery, there is always a
voltage drop across the internal battery resistance. The battery becomes hot because the resistance dissipates power. Real batteries act as
though they had a perfect voltage source inside
them, but the voltage source can never be sepais
or voltage gain
multiplied times the operating frequency tends to
rated
cw
away from the
internal resistance.
problem passing a certain current through zero
resistance loads. Their departure from ideal
occurs with very high resistance loads.
RL =
A SHORT CIRCUIT
RL =
An arc welding machine is an example of an
attempt to build a current source. In order to
make a neat and uniform weld, the current which
melts the steel needs to be constant. As the
welding rod scratches along the metal, the resistance varies widely so the voltage must fluctuate wildly to try to keep the current constant.
Obviously, if the welding rod is too far from the
metal, there will not be enough voltage to make
an arc jump to the metal. In this case the welding
machine will be unable to produce a constant cur-
WHEN YOU SHORT
rent.
WOULD PRODUCE
INFINITE
CURRENT.
I
=
A PERFECT VOLTAGE SOURCE
A REAL BATTERY,
THE SHORT CIRCUIT
CURRENT
IS
Notice that if you had a perfect current
source it would be very easy to make a good, but
not perfect, voltage source. If the output current
from a current source is pushed through a resistor, a fixed voltage will appear across that resistor in accordance with Ohm's law. If the resistor has a very low resistance, then the voltage
source you have made will have a very low inter-
LIMITED
BY THE BATTERY
RESISTANCE,
R.
THE RESISTANCE,
BECOMES VERY
R,
HOT.
A REAL BATTERY
nal resistance.
Fig. 4-2
A
perfect voltage source
compared
to a
battery.
Ideal
current
source
volt-ampere
charac-
teristics
b.
A
volt-ampere characteristic for an ideal curis shown in Fig. 4-3. Notice that no
matter what the voltage is, the current remains
constant. We are discussing all this because electronic control devices usually have outputs that
Current sources
rent source
Current sources are ideal devices that
will
push a constant current through any load. Real
devices that try to mimic current sources have no
+
1
'cs
FIXED CURRENT LEVEL
TOTALLY INDEPENDENT
OF VOLTAGE
-V
+V
-I
Fig.
4-3
The volt-ampere characteristic
ideal current source.
58
for an
CURRENT
SOURCE
SYMBOL
like current sources. This means that for a
given level of control voltage or control current
input, the current passing through the output
side of the device is proportional to the input current and tends to be unrelated to the voltage
across the output circuit as a whole.
behave
normal conditions the amount of water
coming through the dam is dictated by the
operator controlling the flood gate. The water
flow is not directly controlled by the amount of
water behind the dam. Obviously if there is a
drought and the reservoir is nearly empty, open-
rent. In
ing the flood gates will not produce very
Having the output
Having the output
of a control device
like a voltage source is also useful.
much
water flow.
behave
like a current source is desirable because this
means that the current output of an amplifier will
be independant of variations in the power supply.
For example, suppose there were 120 Hz ripple on
the DC power supply driving an amplifier. If the
output were a perfect current source, the output
current would contain no 120 Hz component at all
even though the voltage that causes the current
to flow is full of 120 Hz ripple!
of a control device
B.
The Basic Transistor Amplifier
Bipolar transistors have a large current gain
which is called beta, the Greek letter (i. The output current equals the control current times the
gain, beta.
I
out
=
w
The voltage which causes the output current
must be supplied with an external power
behave
to flow
A
This property
would insure that the voltage across the device
output would only be related to the control voltage or current input. This would make the output
supply.
load resistor, Rl, is inserted between
the power supply and the transistor. Since the
current passing through the resistor is generated
by a current source, the voltage across the re-
voltage independent of supply voltage or a changing load. No single control device can do this but
the resistor
operational amplifiers, which are a complex
trol current.
made from
cuit
sistor is a
cir-
good voltage source. The voltage across
is
also proportional to the input con-
several transistors, can be wired
The voltage across the load resistor makes a
convenient output for many applications. For example, the voltage across the resistor could be
wired to the deflection plates of a cathode ray
tube in an oscilloscope. The voltage changes in
response to the control current and steers the
electron beam across the face of the oscilloscope
screen to display waveforms.
to achieve this.
Let's apply the idea of current sources to our
dam
analogy. The output sides of real control
devices are wired in series with a power supply
which is analogous to the river or a reservoir. The
control device allows fixed amounts of current to
flow as determined
by the
control voltage or cur-
OUTPUT VOLTAGE SOURCE
Rload
ACROSS RESISTOR
Vcc
POWER
SUPPLY
V
COLLECTOR
'out
>
CONTROL
CURRENT
I.N
>
©
BASE
\
Rbase
l0UT
=
l|
N
ft
EMITTER
'J
Fig. 4-4
A
simple circuit model for a transistor.
59
all, if we had plotted infinite gain, the
would be off the graph, so a finite current
gain of 100 is assumed. To illustrate the control
First of
In a stereo amplifier the load resistor would
be replaced with the loudspeaker itself. The
voltage across the loudspeaker and the current
through it provides the power needed to create
the sound.
curve(s)
that the input current has over the output, sevsamples of volt-ampere curves are drawn to
eral
show how they change with
different levels of in-
put current.
Notice that the power supply voltage must
equal the sum of the voltage across the load plus
A
the voltage across the transistor.
collection of representative curves like this
called a family. Whether to have the family of
curves extend into the negative area at the lower
left (—V and —I) is optional. Most real devices
have families in either the positive or negative
areas, but not both. They are usually turned off or
damaged by operation in the opposite polarity.
is
Vpower supply
=
^ load resistance
+
^transistor
Since the power supply voltage never varies,
is high when the
low and vice versa. It is
quite common to use the output voltage across
the transistor rather than the voltage across the
load resistor and we will cover these possibilities
the voltage across the transistor
voltage across the load
is
D.
Triode tubes
in a later section.
C.
Vacuum Tubes
The Ideal Transistor Volt-Ampere Characteristic
We will mention vacuum tubes just in case
you ever have to fix an ancient radio. The triode
A
vacuum tube
is constructed just like a vacuum
tube diode except that a sieve-like control grid is
placed between the plate and the cathode. The
volt-ampere characteristic for a hypothetdevice is plotted in Fig. 4-5. To
ical ideal control
understand
will require
it
some explanation.
+
'out
OUTPUT
CURRENT
mA
GAIN = 100
"OUT = (100)
l,
600
6
mA ^
500
5
mA
400
4
mA
300
3
mA
200
2
mA
EQUALLY
.SPACED
'LEVELS OF
i
CONTROL
CURRENT
100
1
mA
\
mA
J
NEGATIVE
CONTROL
CURRENT
in
N
-V
llN
/
+V
-1
.100
-2.
^200
-3_
_300
-4
^400
.
-5.
500
r
-6.
h 600
-I
Hypothetical volt-ampere characteristic for an ideal control device. For many applications perin both the positive quadrant 1+ V and +1) and the negative quadrant (— V and —I) would not
be desirable. Therefore, the negative family of characteristics is shown dashed.
Fig. 4-5
formance
60
main current stream of electrons can be controlby placing small voltages on the grid which
Pentode tubes
led
can either turn off the stream or let it through.
Although there is a small control grid current, the
control variable is primarily voltage. A few volts
change on the grid makes a very large change in
the electron stream and turns the tube on or off.
Triode
vacuum vacuum tubes have very
ideal families of volt-ampere curves that
resemble
either
current
sources
or
However, pentode vacuum tubes have two
more control grids and when they are biased with
the proper DC voltages, the pentode vacuum tube
volt-ampere characteristics are very close to the
positive half of the ideal curves in Fig. 4-5. As we
shall see, the pentode vacuum tube behaves very
much like an N-P-N bipolar transistor and the
N-channel field effect transistor is even closer to
the pentode vacuum tube.
non-
do not
voltage
family of curves
resembles a collection of ordinary resistors.
sources.
Instead,
the
triode
r~N
Fig. 4-6
An
assortment of vacuum tubes
61
Modern vacuum
the
tubes
action
of
transferring
across a resistor.
tubes have practically disappeared
from avionics made in the free world, but they are
still used in very high power broadcast transmitters. Vacuum tubes which operate on somewhat
different principles, klystrons, magnetrons, and
traveling wave tubes are still used in radar and
is
The most common type of transistor is based
on a triple layer semiconductor structure, either
N-P-N or P-N-P. Like a P-N junction diode, the
function of these transistors depends on both
kinds of current carriers, holes and electrons.
Because of the interaction of these two pathways
microwave systems.
The Russians have continued to develop
vacuum tubes while the Western world has gone
to semiconductors. Modern Russian avionics are
full of highly miniaturized vacuum tubes and the
or "poles," these transistors are called bipolar
transistors. Field effect transistors only involve
one kind of
sequently
not very different from western
designs made with discrete transistors. However,
integrated circuits can not be built with vacuum
tubes so the future of this technology is severely
is
The
principle
The transistor is a direct descendent of the
semiconductor diode, and was invented by Shockley, Bardeen, and Brattain at Bell Laboratories in
1947. The word transistor was supposed to imply
An
effect
transistors
are
Con-
called
used the point contact
which resembled the cat whisker diodes
first transistors
which served as detectors in crystal set radios.
Point contact transistors worked quite well but
were unreliable and were quickly replaced by P-N
The Transistor
Fig. 4-7
carrier, either holes or electrons.
field
unipolar transistors.
limited.
E.
transistor
a sort of resistor that can vary its resistance to
control the current flowing through it.
Vacuum
performance
current through or
The idea was that the
junction bipolar transistors.
Today P-N junction
by field ef-
transistors are slowly being replaced
fect transistors of various types.
assortment of transistors
62
Josephson junction device
you put a small control current into
has to come out somewhere. In normal
circuit circumstances, the base current joins the
collector-to-emitter current and comes out the
emitter. If
the base,
In the not too distant future, the Josephson
may start replacing both of these
transistors for switching circuits in computers.
This is a very high speed transistor that operates
junction device
it
emitter.
The complexity
operating a circuit at near absolute zero,
(—425° F) is offset by the advantages of very low
power consumption and extremely fast switchat extremely low temperatures.
COLLECTOR
COLLECTOR
of
lc
BASE
BASE
ing.
—
/
t"
3
:>
these devices consume so little
power, very high densities of computer circuitry
Because
It
is
spacecraft will be
inevitable
among
that
aircraft
The
refrigeration
BASE
EMITTER
Fig. 4-8
the
equipment working than the com-
N-P-N and P-N-P
Just as
The bipolar transistor is a three layered
device in which the outer layers are one type of
semiconductor, either N or P, and the center layer
P
or
N
In most applica-
tions the output circuit path that
is
and
their
in diodes, whenever you are trying to
which direction a current will flow
How
to
Turn Off A Transistor
layers of this sandwich are called
the emitter and the collector.
trol
transistors
respectively.
G.
The outer
EMITTER
through a transistor, just remember that the current will flow from the more positive voltage
down to the less positive voltage and that positive charge will always flow in the direction of the
arrow. Remember that positive to P conducts!
Bipolar Transistors
the opposite type,
(P-N-P)
circuit symbols.
figure out
is
Y
and
puter!
F.
COLLECTOR
BASE
(N-P-N)
avionics
may have more problem keeping
technician
COLLECTOR
the first applications for
these super compact computers.
EMITTER
4
can be crammed into far less space than today's
room temperature integrated circuits. Because
they are so small, signals take less time to travel
through them and calculations can be made more
rapidly.
EMITTER
\
from the emitter to the
center layer, the base,
is
we wish
Let's start
collector.
by turning
off
an N-P-N tran-
To turn the transistor off it is only
necessary to make the voltage between the emit-
to con-
sistor.
The
the control element that
ter
and base zero so that there
will
be no voltage
turns the main stream current on and off. This is
done by injecting electrons into the center layer
or pulling them out, depending on whether the
be done by connecting the base to the emitter
which insures that these two leads will have a
transistor is P-N-P or N-P-N, respectively.
voltage difference of zero.
Before we explain this in detail, a lot can be
learned about these two different transistors just
by memorizing the circuit symbols. Note the
family resemblance to diodes in the barrier bar
Since the base and emitter layers are wired
N layer is bypassed and the
circuit becomes equivalent to a back-biased P-N
diode. In other words, the transistor is turned off
because the N layer of a P-N diode faces the
to drive a control current into the base. This can
together, the emitter
and the arrowhead.
positive end of the battery.
In the N-P-N configuration the arrowhead
points away from the barrier line. This arrowhead
indicates the direction of movement of positive
charge through the transistor. Primarily it shows
the direction of positive charge moving between
the collector and the emitter. It also shows the
direction of
movement
of charge
Because the transistor has a high gain, it is
not necessary to keep the base directly connected
to ground to keep it turned off. Connecting the
base to the emitter with even a very high resistance, such as 100,000 ohms is often enough to
keep the transistor turned off.
from the base to
63
Rb
Fig. 4-9
H.
How
to
Turn On
A
Turning off an N-P-N transistor
Transistor
Although the following configuration is
unusual, the transistor can be turned on by simply connecting the collector to the base. If this is
Vcc
done the collector layer is bypassed and the transistor becomes a forward biased P-N diode. Since
only a small current is needed to turn on the transistor, a very high resistance, say 10,000 ohms,
may be enough to bypass the collector.
Fig.
unusual for the control current to come
from the collector. Normally the current would
come from some other circuit, such as the separate battery and switch shown in Fig. 4-11.
It is
4-11
Turning an N-P-N transistor on and
off
to emitter resistor were left out, the transistor
would tend to stay on after the base current, lb,
was switched off.
Switch circuit
The
switch,
The switch circuit in Fig. 4-1 1 needs two base
The one in series with the battery, Vt,,
limits the base current to a small value. The se-
transistor
it is
not only an
ON
and
OFF
range of current control. The size of the collector
to emitter cuirent equals the gain of the transistor times the base current, within practical
limits. Fig. 4-12 shows the family of volt-ampere
resistors.
cond resistor connects the base to the emitter
when the transistor is to be turned off. If the base
characteristics for a real
Rl
Fig. 4-10
is
also an electricity faucet with a wide
Turning on an N-P-N transistor
64
N-P-N
transistor.
TYPICAL COLLECTOR CHARACTERISTICS
between the collector and emitter, V ce can be
significant even though the transistor is turned
full ON. This limitation is seen as the gap between the vertical current axis, (where V ce = 0)
and the family of plotted curves.
,
TYPE 2N1479
50
40
CASE TEMPERATURE =
25°
C
30
1
JO
BASE MILLIAMPERES
J
0.4
1
—
1
=
15
P-N-P Transistor
10
!
!—
-4
">
•
i
;
10
20
30
40
3
50
Now we
4
/
60
Family of curves
for the
same
the polarities are
70
as the
now
reversed.
In other words, the P-N-P transistor operates
in the negative voltage, negative current
2N1479 N-P-N
rant. It is usually destroyed
silicon transistor.
lector to base junction
Notice how constant the collector current is
for each value of base current. The collector cur-
and the base
becomes forward biased
to emitter junction
is
not designed
to withstand high reverse voltage.
almost independent of the voltage across
V ce For each of the base currents
shown, to 50 mA, the transistor varies its resisis
the transistor,
A family of P-N-P characteristic curves is
seen in Fig. 4-14. It closely resembles the negative half of our ideal control device char-
.
tance so that it will act like a current source.
Equal increments in base current produce proportional changes in collector current; that
Ic
quad-
when one attempts
to operate it in the positive voltage, positive current quadrant. Positive to P conducts, so the col-
Collector current
rent
P-N-P transistor. It is
N-P-N except that all
80
COLLECTOR TO EMITTER VOLTS
Fig. 4-12
will look at a
virtually the
acteristic
sfHb.
in Fig. 4-5.
It
may
occur to you that
if
one could combine
the characteristics of an N-P-N transistor with
However, at high collector currents much
more base current is needed to produce the same
change. This means that (1 decreases at high current levels and that real transistors are not
those of a P-N-P transistor, you would closely approach our "ideal" family of curves. This is indeed done as we shall see in Section 7. Using the
two kinds of transistors together is called a complementary transistor amplifier and makes a very
efficient and low cost Hi-Fi amplifier.
perfectly linear.
The
shown
is,
transistor cannot be turned on until the
voltage between the base and emitter at least exceeds the forward offset voltage of the base to
emitter junction, about 0.6 volts for a silicon tran-
Earlier
sistor. When the transistor is passing very high
currents from collector to emitter, the voltage
COLLECTOR
BASE
EMITTER
Fig. 4-13
it
was
said
that,
except
for
the
P-N-P transistors were virtually the
same as N-P-N transistors. For technical reasons
high speed, high power P-N-P transistors are
much harder to manufacture than equivalent
polarities,
The P-N-P transistor
65
V CC
How
TYPICAL COLLECTOR CHARACTERISTICS
TYPE 2N2148
COMMON EMITTER CIRCUIT BASE INPUT
MOUNTING FLANGE TEMPERATURE =
25°
through the diode.
wider this region of displaced minority carriers
becomes. If the base is made very thin, the
minority carriers can diffuse all the way across
the base and "convert" the base layer to the same
semiconductor type as the emitter and collector.
C
BOUNDARY OF
"RECOMMENDED OPERATING"
g -4
s
^^ REGION.
25
O -2
Conversion of the base
-10
-15
-20
-25
For example, to turn on an X-P-X transistor,
is applied to the base so that
the base to emitter junction will be forward bias-
-30
COLLECTOR TOEMITTER VOLTS
a positive voltage
The family of curves for the P-N-P
Fig. 4-14
2X2148 germanium transistor. (It is customary to
plot families of curves for P-N-P transistors with
negative values upuard and to the right.)
P conducts.) Since a positive curflowing from base to emitter, this is the
same as saying that electrons are flowing from
the emitter X material into the holes in the base P
ed. (Positive to
rent
N-P-N transistors. Consequently, the output
stages of powerful amplifiers are nearly always
N-P-N
/.
spreads into X and P
how much current is flowing
The more forward current, the
far this diffusion
regions depends on
is
region.
The
quickly
holes in the
P
region valence band are
with electrons.
filled in
transistors.
Why
As
Have Gain
Transistors
the
emitter
creases, conduction
to
base electron flow
in-
band electrons from the emit-
base P region conducare flowing in the P
region conduction band, the P region is so narrow, it is easy for conduction band electrons to
diffuse all the way across the base without falling
into a hole: that is, without falling down into the
valence energy band. These conduction band electrons can cross right over into the type X collector where there is a large positive voltage to attract them. And, since they are already in the conduction band, there is no energy cliff that they
must climb to enter the collector.
ter begin to diffuse into the
The way we described base currents bypassing the emitter or collector layers it must appear
to you that transistors can be made out of two
P-X diodes wired back to back. Such a transistor
would have no voltage or current gain and would
have little practical use. The transistor is useful
tion
primarily because a small base current can turn
on a large collector to emitter current.
Transistor gain results because the base layer
is very thin,
typically 5 to 25fi meters thick
(0.0002 to 0.001 inches). The idea is that a small
current into or out of the base can temporarily
convert the base layer into a material that is electrically similar to the collector
all
and emitter.
When
more and more base current
X-type or P-type, the resistance from collector to
emitter will be quite low and the transistor will
carriers
all
way
to diffuse minority
across the base.
diodes wired back to back. In other words, the
Carrier Diffusion
said that
the
If the base
region were thick enough, there would be no significant difference between the transistor and two
turn on.
we
Once electrons
To summarize, this conversion in the base
region occurs only because the base is very thin.
As the base is made thicker and thicker, it takes
three layers are electrically the same, either
In Section 2
band.
collector current
when
a diode
current, so there
is for-
would be no bigger than the base
would be no gain.
ward biased there
in
is diffusion of minority carriers
both directions across the junction.
Base layer
Now let's turn the N-P-N transistor off again.
This is done by putting a voltage on the base that
is negative with respect to the emitter. This back-
In other words, holes diffuse from the P side,
where they are a majority, over into the X side
where they are a minority. Also, conduction band
electrons diffuse from the N side, where they are a
majority, over into the P side where they are a
biases the base-to-emitter junction and stops the
flow of electrons into the base valence band. It
minority
also stops the diffusion of electrons into the base
66
As soon
conduction band.
tion
they are interchangeable. They are not. First,
as the region of conduc-
band electrons becomes narrower than the
if
the emitter and collector are reversed, the gain
width of the base, the base will again be acting
P-type semiconductor and the collector-tobase junction will be back biased.
will
be very low.
like a
The thinner the base
layer
Second, the true emitter-to-base junction is
small and could be easily damaged by heat. Tran-
the more sen-
is,
sistors are usually
being overrun by conduction band
electrons from the emitter and the more current
gain the transistor will have. Very thin base
layers also enable the transistor to switch very
fast because relatively few electrons have to be
drawn into the base to turn on the transistor.
sitive it is to
made
so that the collector-to-
base junction is much larger than the base-toemitter junction. A small emitter is usually surrounded by the base layer. The base, in turn, is
usually surrounded by a large collector.
The idea
is
that
when minority
carriers dif-
fuse across the base, a very large base-to-collector
On
the other hand, very thin base layers are
high speed, high voltage, high
junction will give them plenty of opportunity to
enter the collector on their way toward the base.
The emitter is usually more doped than the collector to provide lots of minority carriers to the
a rule, fast power transistors get around
by printing hundreds of small,
However, this makes the base-emitter reverse breakdown voltage very low. Remember
how extra doping lowered the zener breakdown
voltage in zener diodes? Some examples of tran-
more easily broken down by excess collector voltage and damaged by heat. As a result, it is very
difficult to build
power
As
transistors.
base.
this to a degree
high speed transistors in parallel on the same
silicon sub strate. As seen through the magnifying glass, these transistors resemble a comb
where each of the "tines" represents dozens of
low power transistors lined up along side each
shown
sistor construction are
in Fig. 4-18.
Alpha
Transistor gain
other.
is
directly related to the
ciency with which
"emitted" by the emitter.
effi-
the collector "collects" current
Emitter
Since the collector and emitter are the same
kind of semiconductor, you
may have wondered
When
current leaves
the emitter and enters the base region, as much as
possible should go to the collector. The fraction of
the total emitter current going to the collector
if
ONCE THE BASE TO
EMITTER JUNCTION
HAS BEEN FORWARD
N
IE
HIGH +
=
»B
+
lc
MOST
ELECTRONS FLOW
INTO THE COLLECTOR
BECAUSE THEY ARE
ATTRACTED BY THE
HIGH POSITIVE VOLTAGE
BIASED,
Rl-
VOLTAGE
ON COLLECTOR
tyyfc
SMALL
BASE-
VOLTAGE
-^~
VB
Vcc
COLLECTOR
SUPPLY
VOLTAGE
Fig. 4-15
A
bipolar transistor has gain because
the base layer
is
very
thin.
67
is
Greek
called alpha, the
letter a.
This transistor is quite linear and the transfer
curve is quite close to being a straight line. Conse-
Alpha generally
ranges between 0.90 and 0.99 which means that
90 to 99 percent of the emitter current passes into
the collector. Alpha
is
quently, this particular transistor
ed
directly related to the cur-
is
recommend-
low distortion audio power amplifiers.
for
rent gain, beta.
TYPICAL TRANSFER CHARACTERISTIC
TYPE 2N2148
COMMON EMITTER CIRCUIT, BASE INPUT
MOUNTING FLANGE TEMPERATURE
a
=
COLLECTOR-TO-EMITTER VOLTS
£
(T M f)
= 25° C
= -2
(V C e)
-5
w
HI
OL-
and
ft
ID
=
-4
Ql
s
lb
<
<r
O
HO
LU
Substituting the expression for alpha into the
expression for beta gives,
-3
-2
o
u
-1
P=l-a
-10
-20
-30
-40
-50
-60
BASE MILLIAMPERES(Ib)
Fig. 4-16
Beta or hfe
Transfer characteristic for the 2N2148
transistor
Sometimes the term "hfe " is used instead of
beta. For most practical purposes hfe is the same
thing as
ft,
but
it
is
Transistor Input Characteristics
J.
defined for a particular
We
collector-to-emitter voltage.
have talked about families of volt-ampere
characteristics for the transistor from the point
of view of the collector-to-emitter voltage
and
cur-
We have not discussed the volt-ampere char-
hfe is one of four "h-parameters" that are
often given for a transistor to help engineers
rent.
design circuits. With the four standard h parameters, the engineer can draw a relatively simple
equivalent circuit for a particular transistor that
this is just a P-N junction diode, the base-toemitter characteristic looks like a diode.
will allow
behave
him
to calculate
acteristic of the base-to-emitter junction. Since
how the transistor will
The equivalent cir-
TYPICAL BASE CHARACTERISTIC
in a particular circuit.
TYPE 2N2148
COMMON EMMITTER CIRCUIT, BASE INPUT
cuit is similar to Fig. 4-4, but includes the effect
MOUNTING FLANGE TEMPERATURE =
the output has on the input and the resistance of
the collector to emitter pathway.
-50
Transfer characteristics
LU
QC
LU
Q.
The
5
<
relationship between the base current
and the collector current
characteristic.
equation,
I
c
=
called the transfer
,_i
can be approximated by the
lb or given as a graph. Real tran-
2
is
_i
It
ft
LU
U3
<
25°
C
COLLECTOR-TO-EMITTER VOLTS = -2
-40
-30
-20
-10
m
and the transfer
characteristic cannot be expressed by a simple
sistors are not perfectly linear
-0.2
-0.4
-0.6
-0.8
linear equation.
BASE-TO-EMITTER VOLTS
A
transfer characteristic for the P-N-P ger-
manium 2N2148
transistor
is
shown
Fig.
in Fig. 4-16.
4-17
transistor
68
Base
characteristic
for
the
2N2148
Because the 2N2148
tor, its
a
is
germanium
transis-
4.
0.2 volts to be turned on. By dividing the plotted
voltage by the resulting base current, you can
about 11 ohms.
.
Limitations
Transistor Performance
in
One
1.
a zener breakdown of the collector-to-
is
Another is called avalanche
few electrons penetrating the
P-N barrier can ionize the semiconductor. Once an
ionized path is formed, an "avalanche" of elec-
base
Collector cutoff current
junction.
A
multiplication.
Since the transistor is closely related to the
diode, it has many of the same limitations. For example, when the transistor is turn-
P-N junction
ed
off,
trons will follow.
a leakage current, called the collector
The third mechanism is called reach through.
The idea is that the base is so thin that a large col-
cutoff current, I co still flows. It is analogous to
the leakage current that flows through a back
,
biased diode and
is
in the
lector voltage
range of 0.1 to 100
for
Another limit
in transistor
When
performance
is
Small transistors generally have breakdown
base current no longer produce further increases in collector current, the transistor is said
to be "saturated." This
turned "on" as
much
means that the
as
it
will see that
collector cur-
additional increases
in
is
Turn back to Fig. 4-12 and you
and 2 mA of base current, the
1
rent rises sharply as the collector voltage approaches 65 volts. The junction breakdown is occuring at 65 volts, and if very much current is
allowed to flow through the transistor, it will
overheat and be destroyed.
Saturation voltage
the saturation voltage.
can directly act on the emitter and
attract electrons (or holes) across the base.
microamperes. This leakage current results primarily from the hole-electron pairs generated by
heat. So, if having a small leakage current is important, the transistor must be kept cool.
2.
breakdown voltage
There is a limit to how much voltage can be
placed across the collector to emitter without
breaking down the collector-to-base junction.
This is called the collector-to-emitter breakdown
voltage, BV ce
There are three theoretical
mechanisms explaining why the transistor can
break down between collector and emitter.
show that the forward resistance of this diode
junction ranges from about 32 ohms down to
K.
Collector-to-emitter
base-to-emitter junction requires at least
voltages in the range of 20 to 60 volts while large,
modern power
transistor
transistors can have
breakdown
voltages as high as 900 volts. The collector-tobase breakdown voltage (called BV c b) is usually
can be turned on.
measured between the
Real bipolar transistors have a significant
collector to emitter voltage when turned full on
emitter
left
collector and base with the
unconnected. The breakdown voltage
between the collector and emitter, BV ce is usually less than from collector to base because of the
reach through phenomenon.
,
and this is called the saturation voltage, V ce (sat).
For large silicon power transistors passing pulses
of several amperes, the saturation voltage can be
30 volts or more. With a small germanium transistor, the saturation voltage can approach 0.2
volts for low currents.
5.
Power
dissipation
In Fig. 4-12, the family of curves
is
not plot-
ted over the entire area of the graph because the
3.
Maximum
Since there
is
a
minimum
if it is operated off the
In Fig. 4-14 a dashed line is
to fail
it.
shown around the family of characteristics
show the boundary of the safe operating area.
to
must be a
If you multiply the collector voltage, V ce
times the collector current, I c at various points
collector current that can be permitted
,
flow
continuously through the transistor
without overheating it. This is called the maximum continuous collector current and is comparable to the maximum current rating of a diode.
to
is likely
curves shown for
collector to emitter
voltage, the saturation voltage, there
maximum
transistor
continuous collector current
,
all around that boundary you will find that the
power that results is roughly constant (V ce )(I c =
50 watts. This is the maximum power that the
,
)
69
transistor can dissipate without overheating.
This power can be anywhere from 35 milliwatts
for an early 1958 germanium transistor to well
over 100 watts for a modern silicon power transistor. This maximum power is less if the operating temperature is high and greater if the transistor is operated in a low temperature environment. The power dissipation of a transistor can
also be increased by bolting it to a large metal
heat sink.
In the grown type, the semiconductor crystal
formed by slowly pulling a crystal of semiconductor out of a pot of molten silicon or germanium. The N and P regions are created by adding impurities during the crystalline growth prois
cess.
2.
Alloy or fused construction
The
alloy type is also called fused construcThis process starts with a thin wafer of
doped semiconductor which will later become the
base layer. Small "dots" of impurity for making
the collector and emitter are placed on either side
of the wafer. The assembly is then heated until
the impurity melts, but the base layer does not
melt. The impurity diffuses into the wafer until
there is only a tiny barrier of base material left in
tion.
Modern
transistor curves
Fifteen years ago manufacturer's specifications
nearly
always included the
collector-to-
and
sometimes a plot of the transfer and base voltampere characteristics were given. Today the
transistors and the designers have become more
sophisticated and these curves are rarely seen in
emitter, volt-ampere characteristics family
specifications. Instead there are usually a variety
of tables and curves that describe the transistor
performance for the particular job the transistor
was designed to do.
For example, switching transistors have
graphs of time needed to turn on and turn off the
transistor versus the collector current that is being switched. They also have graphs of the capacitance of the P-N junctions versus the reverse
voltage bias across them. Junction capacitance is
important because the transistor can't switch until the charge stored in the capacitance is discharged.
If the transistor is
quency
intended as a radio
The impurity converts regions of the base
semiconductor to semiconductor of the opposite
type. The collector area is made much larger than
the emitter. This is done to encourage as much
emitter current as possible to go to the collector
to insure high gain.
3.
fre-
By
successive
diffusions
and masking, a
three layer transistor can be diffused into the
semiconductor substrate. The surface of this
wafer is sealed with silicon dioxide which is glass.
Using masks, aluminum is deposited on the appropriate places to attach the leads. Since most
operations are on the surface of a plane, these are
called planar transistors.
Transistor Fabrication
4.
1.
Diffusion construction
In the diffusion process one side of a semiconductor wafer is subjected to a gas containing N or
P impurities which convert the surface of the
semiconductor to N or P semiconductor. Using
masks, which are something like photographic negatives, the diffusion is limited to certain areas of
the wafer.
amplifier, then curves are given for signal
gain versus frequency, maximum gain versus frequency, and transistor noise (noise figure) versus
frequency and collector current. If the transistor
is intended as an audio amplifier, curves are given
for output power versus distortion. There are
dozens of specialized kinds of bipolar transistors
and each is described differently.
L.
the center.
Grown construction
named according to the
make them. There are four basic
Transistors are often
methods used
to
techniques for manufacturing transistors, diodes,
and other semiconductor devices. These methods
are grown, alloy, diffusion, and epitaxial
Epitaxial construction
The epitaxial growth process involves growing silicon crystaline layers on a substrate. A
wafer of silicon is placed in an oven and exposed
to a mixture of silicon tetrachloride gas and a gas
containing the needed N or P impurity. The silicon tetrachloride breaks down in the heat and
free silicon atoms are deposited on the crystal
substrate in a thin layer. The process resembles
the way snow flakes grow in clouds by adding
new water molecules to existing ice crystals. In
the same way that snow can trap smog particles,
N
and P impurities can be trapped
in the silicon
crystal in the desired concentrations.
After
the
epitaxial
masked and
process. The planar
the
process,
epitaxial
N
diffused using the diffusion
layer is
four
fabrication
The
P+
regions are diffused around the tran-
it from other transistors which
might be on the same chip. The idea is that the
P+ to N junction makes a permanent back biased
diode which prevents one transistor from interfering with another. If the transistor were discrete,
i.e., alone on the silicon chip,
the P-l- regions
would not be needed.
sistor to isolate
silicon.
mm
C
1
Several of these fabrication methods can be
combined
to produce diffused alloy devices, alloy-
emitter-epitaxial-base transistors, and so on. All
N
mm
the
material.
in-
tegrated circuits where arrays of components are
diffused and deposited onto a single sheet of
3
illustrates
diffusion and epitaxial pro-
cess are the basic technology for producing
1
4-18
Fig.
methods. The diffused planar epitaxial transistor
is the newest and most complex technique. The
P+ and N+ regions are areas where the semiconductor is very heavily doped with impurity. The
N+ regions are added because the aluminum contact metal has a valence of +3 (3 holes) and tends
to make P-N junction diodes wherever it touches
25
P
N
these methods produce transistors that perform
equally well at low frequencies and low voltages.
The usual reason for exotic fabrication processes
^m
t
to improve high frequency performance and
high voltage ratings.
is
T
ALLOY TRANSISTOR
M.
Testing Transistors
GROWN TRANSISTOR
OB
EO
GLASS
test them, both in
transistor
5
fail and you need methods to
and out of the circuit. If the
out of the circuit, lying free in your
Transistors can
SI0 2
is
hand, there are three basic ways to test it. 95% of
the time you simply need a crude check to see if
the transistor still has its two P-N junctions intact. This is easily done with an ordinary ohm
meter.
^m;
ALUMINUM
METAL
DIFFUSED PLANAR TRANSISTOR
1.
T
/////
|n+|
1
N+
1
p+
1n+|
P+
N-EPITAXIAL
Ohm
meter
c
Set the ohm meter to a high ohms range and
attach one test lead the base. Attach the other
lead to first the emitter and then the collector. If
the P-N junctions are forward biased by this, you
> EPITAXIAL
VGROWN
J
LAYER
will
see a relatively low resistance indicated,
somewhere between 10 and 100 ohms. Now, reverse the leads and if the two P-N junctions are
back biased, they should show a very high resist-
P-SUBSTRATE
DIFFUSED PLANAR EPITAXIAL TRANSISTOR
SUITABLE FOR INTEGRATED CIRCUITS
ance, at least 100,000
ohms
sistor has a serious defect,
Fig. 4-18
junctions will have lost
Transistor fabrication techniques
71
its
or more. If the tranone or both of the two
diode characteristics.
In small transistors the emitter wire
is
often
marked with a tiny metal flange that protrudes
from the case or a
case. In
many
flat
spot in the side of a plastic
transistors the leads are labeled
with the letters E, B, and C. If you still aren't
sure which lead is which, you will have to look
them up in the manufacturer's specifications.
2.
Transistor curve tracer
A
better
way
to test a transistor out of the
circuit is to use a transistor curve tracer. This in-
strument resembles an oscilloscope. It plots voltampere characteristics on its screen and can be
used to measure other parameters such as breakdown voltages and switching speed. The data obtained can be compared with the manufacturer's
specifications for the transistor. Some curve
tracers, such as the one in Fig. 4-21 can store the
characteristic curve for a transistor known to be
good so that it can be directly compared with the
transistor being tested.
Testing transistors out of the circuit
is like
testing pilots out of an airplane. You can give
pilots written and oral exams and put them
through simulators, but you are never completely
Fig. 4-19
Testing transistors with an
certain of their performance until they fly the
ohm meter
airplane.
Identifying Leads
Curve tracers are rarely used
Transistors are mounted in a variety of cases
and it is often difficult to figure out which of the
three leads
the base.
is
Some
transistors have
and
in large
power
They
do not
specifically test
sistor will
way
metal cases or metal tabs which serve as heat
sinks. These metal tabs are usually connected
directly to the collector
shops.
in avionic repair
are not only expensive, $5000, they
work
whether or not the
in a particular circuit.
to test a transistor is to substitute
tran-
sistor.
tion.
METAL CASE
JS COLLECTOR
C
EMITTER
£_FLAT SPOT
EMITTER
SHOWS EMITTER
TAB
Fig. 4-20
it
into a
working piece of equipment which is identical to
the equipment in which you plan to use the tran-
sistors the metal case is the only collector connec-
.
tran-
The best
Identifying transistor leads
72
FLAT SPOT
by the failure of some other part in the circuit,
so don't assume that replacing a defective transistor is the end of your problem.
fail
QUESTIONS:
1.
What
a transistor or a
is
vacuum tube
sup-
posed to do? What properties would be
desirable for a bipolar transistor?
2.
3.
Why isn't amplifying an electronic signal
comparable to inflating a balloon?
What
are the three kinds of gain? If the cur-
rent gain
is
less
than one, how can there be a
power gain?
4.
can an amplifier be made to oscillate?
5.
What does fast switching speed have to do
with the amount of power (heat) dissipated
in a transistor?
6.
If a transistor amplifier is amplifying a continuously varying signal, like voice or
music, the transistor is dissipating power
Tektronix model 577 curve tracer. This
instrument plots volt-ampere characteristics of
transistors and other semiconductor devices.
Fig. 4-21
3.
How
almost the entire time that the signal
sent.
7.
Maintenance manuals
Testing a transistor while it is installed in the
means careful checking to be sure it is doing the job it was designed to do. The equipment
maintenance manuals help you do this in several
ways. There is usually a description of the circuit
and what the transistor is supposed to accomcircuit
8.
Why
is pre-
this?
What
is meant by saying that the output of
a transistor or vacuum tube behaves like a
current source? Why is this an advantage?
The gain bandwidth product
is
100,000,000.
you expect
9.
is
at 50
Referring to Fig.
of a transistor
current gain would
What
MHz
and 100
4-4, it
can be seen that
MHz?
plish.
V cc =
There are often photographs of oscilloscope
waveforms of currents and voltages that should
be seen on the collector or emitter of a transistor.
If the waveforms on your oscilloscope don't re-
Vce
+
load voltage.
the circuit leading up to the transistor must also
be checked. Many maintenance manuals show DC
voltages that may be expected at hundreds of dif-
Suppose this transistor is amplifying a sine
wave. The amplified sine wave output is a
voltage taken between the collector and the
emitter. The sine wave "zero" point is set to
be one half of the supply voltage, V cc Is the
phase of the voltage across the load resistor
different from the phase of the sine wave
voltage across the transistor? Is the phase
of the sine wave current through the transistor different from the phase of the voltage
ferent places in the circuit.
sine
semble those illustrated in the manual, the transistor may be to blame.
.
Of course, other parts
may
be at
fault rather than the transistor, so everything in
of the circuit
wave across the
transistor?
—
across
the load?
If
the voltages on a particular transistor are
different, it's possible that the transistor is to
blame. Quite often a failed transistor
is
made
10.
to
73
Why
can't a transistor be made by wiring
two P-N junction diodes together?
11.
Why
aren't the collector and emitter inter-
changeable?
12.
Referring to Fig. 4-15, the emitter current
equals the sum of the collector current plus
the base current. Using this relationship and
the equations defining a and (1, show that
1-
13.
How
a
could an ideal transistor dissipate zero
power while passing large currents?
14.
What
is
15.
What
is
a saturated transistor?
the safe operating area of a tran-
sistor?
16.
17.
How can an unmounted transistor be tested
with an ohm meter? What does this test fail
to tell you about the transistor?
What
is
the best
way
to test a transistor?
74
SECTION V
AC
Power Control Devices
In this section we are going to talk primarily
about thyristors. These are electronic devices
used for controlling AC power. They can be
thought of as faucets for alternating current.
They can vary large AC currents without overheating or wasting energy.
You may be wondering why we need special
devices to control AC power. Why can't large
bipolar transistors do the job? Circuits designed
around bipolar transistors could be used, but the
AC power control devices do it with so few parts
that transistors are rarely seen in AC power cir-
If
you
try to use a transistor as the variable
your control system,
it will be devoltage switches to the
polarity opposite to what the transistor was
designed for. The transistor could be pro-
resistor in
stroyed
when
the
AC
tected by a diode in series with it. However,
one transistor could only turn on during one
half of the sine
transistor
wave
cycle.
A second,
other half of the sine wave.
AC
separate
would be needed to control the
A
transistorized
control circuit would be complex and ex-
pensive.
cuits.
A.
The problem
Controlling Alternating Current
much power
of a resistance dissipating too
while
it is
come by having the
Attenuation-by-resistance
attenuating can be over-
control device
work
as a
Rather than have the control device
spend all of its time turned part way on, like a
resistor, it can spend part of its time all the way
ON and the rest of its time turned all the way
OFF. If the switching on and off is rapid enough,
the average power that passes to the lights or
heater will be less, but the control device will not
dissipate any significant power. Remember we
switch.
Suppose you wish to build a variable light
for the lights in your living room, or you
dimmer
need a variable heat control for a soldering iron, a
hair dryer, or an electric frying pan. This could be
done by placing some sort of variable resistor in
series with the lights or the heating element that
you want
to
control.
Unfortunately,
this
ap-
proach has two basic problems:
showed
in the last section that a perfect switch
does not dissipate any energy because
1.
the attenuator behaves like a simple resistor, the attenuator is going to run very hot
sistance
If
because large amounts of energy will be dissipated in it. If you cut the voltage across the
is
its
re-
seem
to
either zero or infinite.
The power delivered
to the load will
be continuous. After all, with AC power the current is already switching on and off 1 20 times per
lights to half, the other half of the voltage will
heat the resistor. If a control like this were
mounted in the handle of a soldering gun or a
hair dryer, the handle might become as hot as
the end you wish to heat!
second. Chopping the current into shorter increments will not matter, especially if the basic frequency of the AC voltage can be preserved.
75
matically trigger at a certain angle of the sine
will look at the devices designed to
Attenuation-by-switching
wave. Now we
do this.
Let's apply this principle of attenuation-byswitching to the light dimmer. Figs. 5-1 and 5-2
show two basic designs of light dimmer. Fig. 5-1
dissipates
load
original,
(the
but
it is
SWITCH
CLOSES
<
TIMES
PER SECOND V L
120
i
<LOAD RESISTANCE
> (LIGHTS)
EXCESS VOLTAGE
S
"^
_ /HEATS RESISTANCE
.
©
©,.Vs
decreased in amplitude.
X,_^
AC
SUPPLY
X.
unwanted power in a variable
hot. The voltage across
lights) is a sine wave like the
the
which becomes
resistor
the
HIGH SPEED
ELECTRONIC SWITCH
7
\
t
vs -
vL
VS
180°
V
A
LOAD
/
,
(LIGHTS)
1
360°
ih l
J
LOAD VOLTAGE
SUPPLY VOLTAGE
Fig. 5-2
+~
SUPPLY VOLTAGE
The Thyratron
semiconductor AC power
Thyratrons resemble
triode vacuum tubes but they contain a small
amount of ionizable gas such as argon, or mercury vapor. Like the triode vacuum tube, the
thyratron has a heated cathode, a control grid,
and a plate anode. When a trigger voltage is applied to the control grid, a current flows from the
cathode to the anode across the partial vacuum.
The ancestor
LOAD VOLTAGE
devices
Attenuation by resistance
In Fig. 5-2, the average voltage
is
attenuated
by switching. The switch admits only a fraction
each sine wave half cycle into the
This attenuates the average voltage because the area under the voltage curve is greatly
(about
Attenuation by switching
t
B.
Fig. 5-1
D
1/2) of
lights.
is
of the
the thyratron.
decreased.
A complete sine wave is 360° per cycle, so
each half cycle is 180°. The picture is drawn so
that the switch is conducting the second half of
each sine wave half cycle, about 90° out of a
possible 180°. This "angle" is known as the conduction angle, cp. Icp and 4 are both ways of
writing the Greek small letter phi.) The angle between when the voltage rises and the switch begins to conduct is called the delay angle.
Therefore, the delay angle equals 180° — tp.
ELECTRONS STREAM FROM
-CATHODE TO PLATE
GLASS ENVELOPE
FILAMENT
HEATS
1
CATHODE
PLATE +
(ANODE)
CATHODE
EMITS
Notice
how
the switching rate
ELECTRONS
synchronized to the AC voltage so that the basic frequency
of the waveform is still 60 Hz. This means that
the
AC
work
is
DOT INDICATES
IONIZABLE GAS
GRID
transformers designed
for 60 Hz. This synchronized switching can be acwill still
in
complished by having the switching device auto-
Fig. 5-3
76
IN
TUBE
Thyratron circuit symbol and diagram
However, unlike ordinary vacuum tubes,
once the electrons start to flow through the grid,
the mercury vapor becomes ionized. The tube
glows a pretty purplish blue, and the grid loses
control of the electron stream. The electrons will
continue to stream from the cathode to plate until
the voltage on the anode is reduced to the point
where the ionization will be "extinguished" and
the current path through the ionized gas disappears. Once the ionization is gone, the grid again
acquires the ability to turn on the cathode to
D.
The P-N-P-N Diode
The P-N-P-N diode is a four layer sandwich of
N-type and P-type semiconductor. It is called a
diode because it has onlv two terminals.
HOLDING CURRENT
ON
V.
plate current.
OFF
To summarize, thyratrons
IS
TU RNS
QN
-
.75
REVERSE
CHARACTERISTIC
are a switch that
can be turned full on by a small signal on the grid,
but cannot be turned off until the current flow
stops of its own accord. In other words, the plate
voltage must drop to the point where the vapor
ionization is extinguished.
C.
-
VOLTS
-Vfsv
FORWARD
LIKE A
SWITCHING
VOLTAGE
NORMAL
PN
DIODE
-I
— Vfbv
FORWARD
BLOCKING
VOLTAGE
Thyristors
Fig. 5-5
P-N-P-N diode
characteristics
P-N-P-N diode volt-ampere
honor of the thyratron, semiconductor
devices that behave like it are called thyristors.
characteristics
In
They
The volt-ampere characteristic of this diode is
seen in Fig 5-6 The construction resembles two
P-N diodes in series so it is not surprising that the
volt- ampere characteristic is similar. In fact, the
reverse characteristic is identical to what you
are functionally like the thyratron in that
once they are turned on by a control signal, they
can't be turned off by the control signal. The
main stream current must return to zero before
the gate or trigger can be reset.
would expect from two P-N diodes
p
Thyristors have four or more N-type and
P-type semiconductor layers. Their construction
and operation is related to bipolar transistors,
but
can also be argued that their circuit symbols and volt-ampere characteristics strongly
resemble ordinary silicon diodes.
N
P
N
P
in series.
N
it
ANODE
?*
There are four different types of thyristors.
They
are
silicon
controlled
rectifiers
P-N-P-N diode. They are all
based on the P-N-P-N diode, therefore we will
triacs,
^
(SCRs),
diacs and the
discuss this type
The P-N-P-N diode is equivalent
complementary transistors wired together.
Fig. 5-6
first.
The forward
CATHODE
Fig. 5-4
€>
characteristic
is
to
two
also similar, but
the P-N-P-N diode has difficulty starting to conduct. As the forward voltage is increased, the
ANODE
CATHODE
CATHODE
diode does not conduct until a large voltage, called the forward switching voltage, is reached. This
voltage is typically 10 to 15 volts. After this
point the diode abruptly turns on. It passes a
large current and the voltage across it abruptly
ANODE
P-N-P-N diode symbol
drops.
77
leakage current will begin to turn on at least one
of the transistors. As soon as one begins to turn
on, its large collector current will surely turn on
We
saw that bipolar transistors did not
two P-N diodes wired together because the thin base layer can be overrun by electrons (or holes) from the emitter. In the same
behave
like
At this point the two transistors will
turn each other full on almost instantly. This explains the abrupt drop between the forward
switching voltage and the on state voltage.
the other.
way, the P-N-P-N diode behaves unexpectedly because the two center layers are very thin. This
device can be thought of as a P-N-P transistor
wired to an N-P-N transistor so that each base is
connected to the collector of the other.
P-N-P-N
CAPACITOR
AND DIODE
what happens when you try to turn one of
these devices off by decreasing the mainstream
current? As the current through the two tranSo,
sistors is decreased, eventually a point will be
DIODE
reached where there is not enough collector current to keep the other transistor turned full on.
As soon as one transistor is not getting enough
base current to stay full on, it will decrease the
base current of the other. Both transistors turn
each other OFF as abruptly as they were turned
on because this is a runaway situation. When the
P-N-P-N diode turns off, the voltage jumps up to
the forward blocking voltage which is usually
slightly lower than the forward switching volt-
CAPACITOR CHARGES TOWARD
BATTERY VOLTAGE, BUT P-N-P-N
.DIODE FIRES FIRST
age.
VOLTAGE
Relaxation oscillator
FORWARD SWITCHING
VOLTAGE
A
P-N-P-N diode can be wired across a
pacitor to build a relaxation oscillator.
Fig. 5-7, a simple
RC
circuit is
As seen
ca-
in
charged with a bat-
tery.
The diode does not conduct until the forward
switching voltage is reached. Until this happens,
the diode is effectively out of the circuit. At this
point, the diode suddenly turns on and effectively
shorts out the capacitor. The charge stored in the
capacitor leaves through the diode so the voltage
across it falls. The capacitor soon discharges so
far that there is not enough current passing
through the diode to keep it turned on. The dioce
abruptly turns off and the voltage'' across the
capacitor begins to rise again toward the forward
switching voltage. The larger the resistance and
CURRENT PULSES THROUGH
P-N-P-N
Fig. 5-7
DIODE
the larger the capacitance, the slower the oscillation cycle because more time is needed for the
capacitor to charge on each cycle.
Relaxation oscillator
Each of these transistors is forward biased
when collector current flows from the other tranturned off, it
easy to see why neither one wants to turn on.
There is nothing but leakage current from each
collector entering the two bases and this is quite
sistor. If the transistors are initially
3.
Storing information
is
P-N-P-N diodes can also be used as memory
elements. In other words, these diodes can store
information. If the voltage across the diode has
not been higher than the forward switching voltage, the P-N-P-N diode can "remember" this fact,
by not being turned on.
small.
However, as the voltage across the tranbecomes higher and higher, eventually the
sistors
78
+ ok
DC
SUPPLY
s:
s:
ON
-
s:
OFF
51
OFF
s:
ON
s:
ON
OFF
O
>---
f
r
r
STORED.
BINARY
NUMBER
"V
TO READ/WRITE
CIRCUITRY
An
Fig. 5-8
P-N-P-N diode "memory. " Each P-N-P-N diode can "remember" one
On the other hand, if the voltage has been
higher than the forward switching voltage, the
on after
is
of information.
turned on, but not so
much
current
unduly burden the power supply. In
summary, each P-N-P-N diode can "remember"
one bit of information and a number of them can
record a large binary number.
that
diode will remain turned on for weeks or months
provided that there is enough current flowing
through it to keep it turned on. This minimum
current
it is
bit
it will
called the holding current.
if you had 24 diode memory ciryou could remember a 7 digit telephone
number. Computer memories have been built this
way, but transistor memory circuits are the most
common. As we shall see shortly, the most common use for P-N-P-N diodes is for turning on
For example,
A
resistor in series with each diode insures
that the diode will
draw enough current
cuits,
to stay
NEON
silicon controlled rectifiers.
BULB
45
E.
VOLTS
Inert Gas Lights and Voltage Regulators
Neon
X
bulbs
Just so you don't think we are completely off
the subject, neon gas light bulbs are often used to
turn on SCRs and triacs. They behave very much
NEON FLASHER
like
P-N-P-N diodes. The neon
light is simply
two
electrodes inside an evacuated glass envelope.
A
amount of neon provides the electrical
pathway once the voltage across the electrodes
small
+
>
INVERTER
—
10
VOLTS
-
ionizes the
R
neon gas and lights the bulb.
CONVERTS
VOLTS DC
TO
500 VOLTS DC
12
The neon bulb resembles the P-N-P-N diode
must reach a high voltage before the gas
ionizes and begins to conduct. Once the gas
//
i
in that it
~c L_
ionizes, the voltage across
it
lights a lovely orange color.
drops and the bulb
are used
Neon bulbs
on everything from coffee makers
little power and
when used with a series resistor, they can be
operated directly off the 120 volt AC line.
for pilot lights
STROBE LIGHT
Fig. 5-9
Neon
flasher
and an
SSTfUBE
to stereos.
aircraft strobe light
79
They consume very
The
Electronic flashes for cameras generally use
an audio frequency for the AC voltage and this
accounts for the "squeal" that you hear while the
strobe light is charging its capacitor. In summary, neon and xenon lights can be thought of as
5-9) can be
neon bulb in place of the P-N-P-N
diode. Of course, the neon bulb lights up every
relaxation oscillator (Fig.
built using a
time it discharges the capacitor, so this circuit is
an easy way to build a flashing light. Flashers
like this have been used to mark road construc-
self-triggering versions of the thyratron.
tion sites.
Neon
flashers
and strobe
With neon and xenon gases there is a pronounced difference between the voltage needed to
ionize the gas and the voltage at which the gas is
extinguished and stops conducting. With combinations of inert gases and special electrode
design, it is possible to build devices that have
very little difference between the ionizing voltage
(ignition voltage) and the extinguishing voltage.
lights
is another inert gas and makes a
blue-white light when ionized. Xenon
flashtubes are built like neon bulbs and are used
Xenon
brilliant
for aircraft strobe lights
and electronic flashes
for
cameras.
A drawback to the neon
and xenon flashers is
power source must have quite a high
voltage, 45 volts and much more for larger bulbs.
Since an aircraft or a portable camera flash have
that the
DC
Tubes like this were used for voltage reguand are still available in a variety of
voltages from 75 up to about 250 volts. These
lators
battery voltages far less than this, an electronic
inverter is used to convert the low DC voltage to
a high DC voltage to charge the capacitor.
The
voltage regulator tubes were used exactly the
way that zener diode voltage regulators are used
You may
today.
find
them
occasionally in old
equipment. They resemble vacuum tubes but can
be recognized by their beautiful pink, blue, or pur-
inverter consists of a transistor oscillator
which generates an AC voltage. The AC voltage
is passed through a voltage step up transformer
to produce AC voltage. The high AC voltage is
then rectified and filtered to make high DC
ple glow.
+ ANODE
P
N
a
N
CATHODE -
voltage.
6 GATE (TRIGGER)
>
+
CATHODE -
ANODE
GATE
UNREGULATED
DC VOLTAGE
REGULATED
DC VOLTAGE
*
2ENER
DIODE
+ANODE
>
">
CATHODE
A
GATE
TRANSISTOR EQUIVALENT OF AN SCR
>
Fig. 5-11
VOLTAGE
DROPPING
F.
RESISTOR
The
silicon controlled rectifier
Silicon Controlled Rectifiers (SCRs)
controlled rectifiers are very much
thyratron in terms of what they do. They
Silicon
like the
VOLTAGE
REGULATOR
TUBE
>
REGULATED
DC VOLTAGE
75 TO 250 VOLTS
The difference is
that one of the two "bases" of the internal tranare built like P-N-P-N diodes.
sistors is given a lead to the outside world. This
^
lead or control gate introduces a small control
current into the base of one of the transistors and
makes the device turn on
Voltage regulator tubes are used like
Fig. 5-10
zener diode voltage regulators.
it
80
ordinarily would.
at a lower voltage than
X
1
SCR VOLT-AMPERE
ON STATE
CHARACTERISTIC
HOLDING CURRENT
ON
h
OFF
-V
ZERO GATE
CURRENT
-I
THE SCR CHARACTERISTIC IS LIKE THE
P-N-P-N DIODE EXCEPT THAT THE
FORWARD SWITCHING VOLTAGE MAY BE
REDUCED BY ADJUSTING THE GATE CURRENT
ENLARGED FORWARD BREAKOVER CHARACTERISTIC
Fig. 5-12
SCR
Silicon controlled rectifier volt-ampere characteristic
would not fully illuminate a light which was
designed for both halves of the AC cycle. No matter how high the gate current is, the SCR will still
conduct only the positive half of the AC cycle.
volt-ampere characteristics
The volt-ampere
characteristic for an
SCR
is
seen in Fig. 5-12. When the control gate current is
zero, the characteristic is just like the P-N-P-N
However, as more and more current is introduced into the gate, the forward switching
voltage becomes lower and lower. When the gate
current is very high, the entire volt-ampere characteristic is not very different from a silicon P-N
diode.
G.
The
we showed an
SCR
which provides the gate current
can be simple or complicated depending on how much control is needed over the conduction angle. The circuit shown in Fig. 5-13 can
turn the SCR completely off or completely on.
for the
diode.
In Fig. 5-2
Controlling the
circuitry
SCR
electronic switch
table.
However, because the triggering current is
derived from a sine wave, the SCR must trigger
before or while the sine wave reaches its positive
peak. The trigger current will never get any
higher than the positive peak, so if the SCR is going to turn on, it had better do it by then. Since
the SCR is turned on before the 90° point if it
turns on at all, the SCR will remain on from at
However, if the lights were designed for
household AC, the rectified AC from a single SCR
least 90° to 180°. This means that the smallest
conduction angle available from this simple circuit is 90°. This is 25% of the complete AC cycle.
controlling both the positive
of the sine
wave
cycle. It
and negative halves
should be clear from the
volt-ampere curve for the SCR that it can only
turn on during positive half cycles. This means
that if an SCR were being used to control lights,
the lights would see pulsed DC current and not
AC
current. For
some
applications this
is
accep-
81
Vload A
cp
HIGH R
= 90° ANGLE
L\
AC SUPPLY
-t
LOAD
RESISTOR
LOWR
cp
= 180° ANGLE
-»»t
THE VARIABLE RESISTOR DETERMINES
WHETHER THE SCR FIRES AND WHAT
CONDUCTION ANGLE IT WILL HAVE
BETWEEN 90° and 180°. VERY HIGH
R KEEPS THE SCR OFF. (op = 0°)
Fig. 5-13
For
many
The simplest
devices, such as soldering irons,
SCR
control circuit
pletely off)
motors, or light bulbs, 25% or even 50% of the
complete AC cycle is not enough to make it work
properly. For example, a light bulb will barely
up
to 180°
positive half of the
R-C
glow with 50% of the voltage, a motor may not
even turn over, and a soldering iron may be too
cool to melt solder. So 25% (or 50%) may be just
as good as full off in these applications.
AC
which
is full
on
for the
cycle.
integrator circuit
An SCR
triggered with an R-C integrator
cir-
Because the voltage
across a capacitor can't change quickly, this circuit will fire later than the one shown in Fig. 5-13.
cuit is seen in Fig.
5-14.
The voltage across the capacitor is a sine wave,
but it lags the sine wave voltage across the SCR.
For other applications it is desirable to vary
the conduction angle continuously from 0° (com-
LINE
VOLTAGE
CAPACITOR VOLTAGE
LAGS BEHIND LINE
VOLTAGE AND TRIGGERS
SCR VERY LATE
WHEN
R
IS
LOW,
CONDUCTION ANGLE
APPROACHES 180°
NARROW MINIMUM
CONDUCTION ANGLE,
ABOUT 45° WITH
HIGH R
Fig. 6-14
R-C
triggering network for an
82
SCR.
This means that the sine wave on the gate will
have its peak later and therefore it can trigger the
SCR after the SCR sine wave has passed its peak.
This delayed sine wave is said to be phase shifted.
Another way to look
at this is that the
needed to fire the SCR. The
is
temperature sensitive
because the hotter the SCR becomes, the earlier it
fires and the larger the conduction angle becomes. If the SCR is controlling a heater, like a
hair dryer, the heat from the dryer could make the
gate current that
circuit
R-C
SCR
an integrator. The capacitor is still
charging and its voltage is still increasing after
the sine wave has passed its peak voltage. By proper selection of values of R-C, the SCR can be
made to fire after the sine wave has passed its
circuit
is
Now
it
is
practical to vary the conduction
full
This
is still
we would
like.
The capacitor has the
that
it
make
up to nearly the
short of the 0° to 180° that
SCR
180°.
additional advantage
serves as a low pass filter which helps
the SCR immune to triggering from short
5-7
In the circuit of Fig. 5-14 the gate current
up gradually as the sine wave becomes
positive.
When
the sine
wave
As we
we studied in Figs.
make good pulse generators for this
Whenever the P-N-P-N diode or neon
The
relaxation oscillators
5-9
shown
in Fig. 5-15.
Ingition system
A common
primarily dependent on
the temperature of the transistor.
The hotter
it is,
the more leakage current there
is
and the
less
application
for
the
SCR
is
in
automobile ignition systems. As you probably
know, the ordinary automobile ignition system
consists of breaker points, a high voltage transformer, and a rotary switch (the distributor)
which fires each spark plug in turn. See Fig. 5-16.
learned in the last section, the leakage
is
fire.
a combina-
is
tion of the gate current plus the leakage current.
current in a transistor
need to
and
circuit is
voltage reaches the forward switching voltage
determined by the gate current, the SCR "fires."
The current that triggers the SCR
hotter!
bulb conduct, they pass a short, intense current
pulse as they discharge the capacitor. If the gate
of the SCR is placed in series with the P-N-P-N
diode, whenever the P-N-P-N diode fires it will
turn on the SCR. A relaxation oscillator triggered
builds
more and more
will
purpose.
may
be on the line due to
noises from the brushes of electric motors.
voltage spikes that
still
is
5-14
Fig.
This problem can be greatly improved by
turning the SCR on with a circuit that generates
a precise, short current pulse when the desired
conduction angle is reached. This is preferable to
having the gate current build up gradually since
you can't predict exactly how much current the
peak.
angle from about 45°
in
P-N-P-N
DIODE
FIRES SCR
LINE
.VOLTAGE
VOLTAGE ACROSS
LINE
SCR
P-N-P-N
AND
DIODE
CAPACITOR
VOLTAGE
<St
AC SUPPLY
THIS-
VOLTAGE ACROSS
RESISTOR'
LIMITS"
GATE
CURRENT
P-N-P-N
Rload
A^V
P-N-P-N
X
DIODE
K
SCR FIRES
Fig. 5-15
SCR
half-wave relaxation oscillator control circuit
83
DIODE
AND CAPACITOR
O
HIGH VOLTAGE
SECONDARY
+ 12V
BREAKER
POINTS
VOLTS
10,000
rr
.
o^
o
I!
I
SPARK ^
.
W SPARK PLUGS
/
DISTRIBUTOR ASSEMBLY
Fig. 5-16
Ordinary automobile ignition system
vent a sudden rise of voltage across the breaker
points because voltage can't change suddenly
across a capacitor. This prevents excessive sparking at the breaker points so that they will not
burn out quickly.
Breaker points
To make
tery
is
DC
from the batprimary of the
that does this, the
a spark, 12 volts
briefly switched to the
transformer.
The switch
breaker points,
is
part of the distributor.
Even with the
The
distributor has a central shaft which
capacitor, the sparking across
the breaker points erodes
is
them and ordinary
igni-
need of a tune-up. The
capacitive discharge ignition system was developed to prevent sparks at the breaker points and
to produce a bigger voltage across the spark
turned by the engine and closes and opens the
breaker points for each spark. When the switch
closes, a current flows through the primary winding to ground. When the points open, the sudden
change in current through the transformer inductance causes a huge voltage to appear across the
secondary winding of the transformer.
tion
systems are usually
in
plugs.
Capacitive discharge system
The capacitive discharge system substitutes
for the breaker points. Wear and tear on
the breaker points is eliminated by not having
This large voltage, typically 10,000 volts,
causes a miniature lightning bolt, the spark, to
jump across the tip of a spark plug. A rotary
switch inside the distributor directs the spark
current to the correct spark plug. The capacitor
on the primary side of the transformer helps pre-
an
SCR
sparks
of the
jump
SCR
across the breaker contacts. Instead
operating directly off the 12 volt bat-
tery supply, the
SCR
discharges a capacitor. This
DISTRIBUTOR ASSEMBLY
+ 12V
^vWV
^K
BREAKER
POINTS
f\.
^i
INVERTER
CONVERTS
Q
VOLTS DC
TO
400 VOLTS DC
12
o
V
y
Fig. 5-17
o-
SPARK
1
Simplified capacitive discharge ignition system.
M
M
1
ROUTE OF POSITIVE
CURRENT DURING
POSITIVE
HALF CYCLE
ROUTE OF POSITIVE
CURRENT DURING
NEGATIVE
HALF CYCLE
V
j
/
*5
TRIGGER
CIRCUIT
Fig. 5-18
is
done because the
SCR must
Full-wave
turn on a current
turn off by itself. Remember, the
has no way to turn OFF a current.
will
SCR
circuits
positive half cycle.
"Z" shaped path
that will eventually return to zero so that the
SCR
LOAD
5
AC SUPPLY
SCR
from right to
left
Another way
from
as shown.
makes a
left to right,
then
to control both halves of the
sine
12 volt supply, but very high voltage sparks can
be achieved by charging it with a high DC voltage. Therefore, the capacitor is charged with an
inverter just like the ones used in camera flashes
and strobe lights. The capacitor is charged to 200
or more volts. This gives a very large surge of current through the transformer primary and results
in two or three times more spark voltage than ordinary ignitions systems. See Fig. 5-17
Controlling Full-Wave
positive current
wave cycle is to use two SCRs in inverse
parallel. The two SCRs are wired so that one of
them will always be in a position to conduct. The
trigger circuit must be able to drive both SCRs
which means that separate trigger signals of the
proper polarities must be generated during both
halves of the AC waveform. The two gates can
The capacitor could be charged with just the
H.
The
first
not be connected together because the
SCRs
would be damaged.
AC Power
/.
A limitation of the SCR and thyratron is that
they only conduct in one direction. Fig. 5-18
shows two ways of conducting both halves of the
sine wave with SCRs. The upper one is the most
clever. In order for the SCR to conduct, the anode
must be positive. A bridge rectifier can accomplish this by routing positive current to the
anode on the negative half cycle as well as the
TRIA CS and DIA CS
The TRIAC is a device which can be thought
two SCRs wired in inverse parallel and
mounted in the same package. Actually, they are
made as a single semiconductor device with 5
of as
SCRs, both halves of the
wave cycle can be controlled with a single
layers. Unlike separate
sine
gate signal.
85
/
1
\
1
,'
1
N
\
/
\
o- TERMINAL
N
N
P
#2
- -
"
1
1
1
?
n
°
GATE
•
\
/
\
/
P
m^tiv-i'
/
TERMINAL
\
:1
O
#1
/
_•>"
^.
DASHED LINES SHOW SCRS BUILT INTO TRIAC
GATE
TERMINAL
#2
TERMINAL
CIRCUIT
Fig. 5-19
The
TRIAC
SCRs
in inverse parallel.
Earlier we developed an SCR firing circuit
which used a neon bulb or a P-N-P-N diode relaxation oscillator. This same circuit can be used to
The dashed lines surrounding the top and
bottom of the TRIAC construction diagram point
out that the device is essentially two SCRs in inverse parallel. The single gate contact manages to
by means
SYMBOL
can be thought of as two
inject current into the center layers of both
drive the
TRIAC.
SCRs
One
P-N-P-N diode only
one direction so only one half
of the sine wave would be triggered. This problem
can be solved by using a neon bulb which can be
ionized in both directions and therefore will drive
of its strategic location.
difficulty is that the
conducts current
The TRIAC volt-ampere characteristic is
seen in Fig. 5-20. It is really just two forward
SCR characteristics back-to-back.
the
TERMINAL
#1
TRIAC
in
properly.
#2
POSITIVE
HIGH GATE
CURRENT
TRIAC
OFF
^xz
TERMINAL
NEGATIVE
#2
-I
Fig.
Fig. 5-20
TRIAC
volt-ampere characteristic
5-21
A
circuit for a
86
relaxation oscillator phase control
TRIAC.
TRIAC
UM
AC SUPPLY
DIAC CONSTRUCTION
DIAC CIRCUIT
Fig.
5-22
The
DIAC
SYMBOLS
bi-directional
DOUBLE R-C PHASE SHIFT NETWORKS
GIVE A VERY SMALL CONDUCTION ANGLE
switching
diode
Fig. 5-24
The neon bulb does not have a large voltage
difference between when it is turned on and turned off. So, better turn-on pulses can be generated
using a P-N-P-N diode to discharge the capacitor.
P-N-P-N diodes can be used to drive a TRIAC if
two are used in inverse parallel. Naturally it
wasn't long before the need for double P-N-P-N
diodes was recognized and this creation was
ed the DIAC.
TRIAC
full-wave proportional
shows a typical full wave AC conthe kind you would find in a hair
dryer or an electric drill. This full wave system acFig. 5-24
trol circuit of
complishes all the goals we outlined at the beginning of the section. It is relatively cheap and simple and it is not temperature sensitive. A very
wide conduction angle range, 15° to nearly 180°
call-
on each half cycle, is achieved by using two R-C
phase shifting networks in series.
The advantage of the DIAC over the neon
bulb can be seen by comparing their volt-ampere
characteristics. The neon bulb does not switch so
In theory, each R-C network could shift a sine
90°, but in practice, as the phase shift approaches 90 °, the amplitude of the phase shifted
completely from ON to OFF and the voltage drop
is not as great. As a result, the neon bulb does not
produce clean, abrupt turn-on pulses.
wave
sine
wave
will
approach
zero.
SUDDEN
90% DROP
IN VOLTAGE
DIAC
Because a certain
GRADUAL
50% DROP
IN VOLTAGE
NEON
BULB
-V
-V
+V
-I
Fig. 5-23
-I
DIAC and
power
control circuit
neon bulb volt-ampere characteristics.
ST
minimum amplitude is needed to trigger an SCR
or TRIAC, the minimum conduction angle that
10.
can be achieved with a single R-C network is
about 45°. With two R-C networks in series, this
conduction angle can be as low as 15°.
11.
Why
is
high speed switching superior to remeans of attenuating AC cur-
sistance as a
rent?
2.
Why
are thyristors preferred over bipolar
transistors for controlling
3.
power?
What
all
4.
AC
property do tunnel diodes share with
the devices discussed in this chapter?
Why
can't an
SCR
be used as a Hi-Fi am-
plifier?
5.
6.
7.
What
is
an inverter?
In order to use an SCR in a
requirement must be met?
DC circuit,
Referring to Fig. 5-11, the gate of an
what
SCR
is
generally connected to the center layer closis called a cathode
have an anode gate
est to the cathode. This
Some SCRs
gate.
also
connected to the type
N
center layer.
How
would a trigger signal designed for the anode gate differ from one intended for the
cathode gate?
8.
Suppose the anode
of an
the cathode gate of the
ance so that there
is
SCR is connected to
SCR by a low resist-
a large gate current
whenever positive voltage
is on the anode.
device will the volt-ampere characterof this circuit resemble?
What
istic
9.
the advantage of using relaxation
is
Why
is
a
TRIAC
SCRs wired
QUESTIONS:
1.
What
oscillators to trigger thyristors?
A
simple control circuit for a TRIAC is
What conduction angle(s)
would you expect from high, medium and
low control resistance levels?
shown below.
TRIAC
ft
R LOAD
CONTROL
RESISTANCE
88
easier to trigger than
in inverse parallel?
two
SECTION VI
FIELD EFFECT
TRANSISTORS
A.
Introduction
put resistance
Field effect transistors (FETs) have
so that essentially zero current is required
to turn an FET on or off. When turned off, the
is
extremely high, virtually
in-
finite,
many
im-
resistance they present to the current flow they
advantages over bipolar transistors.
When produced as integrated circuits they are
cheaper than bipolar transistors because they use
far less silicon chip area and the circuits require
fewer parts to do the same job. Field effect transistors make it possible to power a digital watch
for a year with a battery the size of a pea. If your
digital watch used bipolar transistors, you might
need a knapsack to carry the battery.
portant
are controlling can be extremely high, 10 billion
ohms. When turned on, this output resistance can
be very low. When controlling small currents, the
voltage across the output can approach zero when
the transistor is turned on. We can summarize
these features by saying that the FET approaches a perfect voltage controlled switch and
wastes very little power.
FETs are relatively immune to changes in
temperature and are easy to use in analog circuits
with a minimum of biasing resistors. FETs
generate less radio noise during operation than
vacuum tubes. As a result,
they are preferred for sensitive radio preamplifiers and mixers.
bipolar transistors or
In digital circuits they may be used without
resistors at all. This property makes them
perfect for integrated circuits where resistors are
awkward to "print." In FET integrated circuits,
FETs themselves are used in place of load
any
and extremely complex circuits are
or thousands of FETs and
practically no resistors, capacitors, diodes or
other kinds of components.
resistors
made from hundreds
Fig. 6-1
and
An assortment
of field effect transistors
FETs have one
ICs.
disadvantage that has kept
between
them from making bipolar transistors obsolete.
They are slow and generally have much smaller
bipolar transistors is that they are
voltage controlled not current controlled. The in-
gain-bandwidth products than bipolar transistors. This means that calculators, computer
The
major
functional
difference
FETs and
89
memories, microprocessors, and a whole host of
products made from FETs run more slowly than
equivalent devices made from bipolar transistors.
•
For example, a typical FET logic circuit can
perform about 1 million operations per second, although the latest models can exceed 4 million. In
contrast,
some
SOURCE
bipolar transistor digital circuits
Another disadvantage
FETs
of
O
(ELECTRONS
ENTER AT
(ELECTRONS
LEAVE AT
THIS END)
THIS END)
applies only
FET (MOSFETsi
VOLTAGE ON GATE
CONTROLS ELECTRON
and
concerns the technician more than the engineer.
MOSFETs are very easily damaged by static
electricity. This means that when MOSFETs are
installed or removed from circuits, they must be
handled carefully to avoid damaging them.
B.
DRAIN
O
can exceed 300 million operations per second.
to the metal-oxide-silicon
N-CHANNEL
SOURCE
Junction Field Effect Transistors
ductor. This piece
is
N
or type
6-2 Construction and symbol for an Nchannel junction FET iJFET).
Fig.
P semicon-
called the channel.
The
cur-
being controlled by the transistor
travels from one end of the channel to the other
without passing through any P-X junctions.
Technically, there are FET designs that do have
modified P-N" junctions in the channel. However,
they don't act like P-X junctions because the electrical carriers that pass through a FET channel
are all electrons if it is a type X channel or all
holes if it is a type P channel. Since only holes or
rent that
is
electrons are involved, these transistors are
way toward the positive side of
power supply. For a P-channel FET, the
source is where the holes enter the channel and
the drain is where the holes leave the channel on
their way toward the negative side of the power
channel on their
the
DC
supply.
Gate
call-
ed unipolar transistors.
The
Source and Drain
of the
FET
at
control lead for a
FET
is
called a gate. In
one design of the X-channel junction FET. the
gate is actually two P-+- regions which surround
the main channel piece of semiconductor. The
gate forms two equal P-X junction diodes with
the channel, but channel current never passes
through the junctions. The gate is almost always
operated back biased so that the gate current,
which is analogous to base current, is just the
leakage current through a back biased P-X diode.
That is why the gate current is so low.
Because there is no official P-X junction
along the path of the channel, there is no obligatory 0.6 volt silicon P-X junction voltage drop.
This is why the voltage across a turned-on FET
can approach zero. The lack of P-X junctions also
allows current to flow through the channel in
both directions.
The two terminals
DRAIN
GATE
All field effect transistors are basically built
around a single piece of type
GATE
FLOW
each end of
The more strongly the P-X junction between
is back biased, the more com-
the channel are called the source and drain. The
source is where the majority carriers enter the
channel. The drain is where they leave the chan-
the gate and source
the FET is turned off. When a large
voltage back biases the gate-to-channel junction,
the region in between the two gates becomes completely
nel
pletely depleted of carriers.
an X-channel FET the conduction band electrons are the majority carriers. So
the source is where electrons enter the semiconductor and the drain is where electrons leave the
For example,
In the case of an X-channel device, a large
negative gate voltage will completely strip the
X-channel of conduction band electrons and the
channel will cease to be a conductor. This is called
in
90
pinching off the channel When the FET is turned
off, the leakage current is surprisingly small, a
few nanoamperes or less. This is roughly one hundredth of the leakage current of a silicon bipolar
transistor. Junction FETs are also made with a
single gate-to-channel junction design that is
suitable for production in integrated circuits.
N.CHANNEL
OUTPUT CHARACTERISTIC
I
1
<
I
Vgs = 0.2V
5
E
1
-0.5V
\
UJ
oc
oc
NEGATIVE TO
Rl
DOES NOT
CONDUCT
-P
-1.0V
o
z
<
oc
o
X
I
-1.5V
J
I
I
-2.0V
- 2.5V
-3.0V
/
<
J
,.
*
.•
1
10
ZZT"
POWER SUPPLY
20
30
40
50
Vds DRAIN-SOURCE VOLTAGE (VOLTS)
P-CHANNEL
OUTPUT CHARACTERISTIC
i
-0.2V
BIASING AN
-0.7
NCHANNEL
<
%
-0.6
Vgs =
-0.5
oc
oc
O
z
?
< > Rl
PLUS TO N DOES
<
o
Q
NOT CONDUCT
^V 4 X
I
*
-0.4
+ 0.4V
-0.2
POM/PR
J
I
+ 0.6V
-0.1
p
+ 0.2V
-0.3
'+1.2V ~ +1.0V" ^+0.8V
r=±= =4= =h=
SIIPPI Y
-5-10-15-20-25-30-35-40
Vds DRAIN-SOURCE VOLTAGE (VOLTS)
+
+
"*
Fig. 6-4
nel
Volt-ampere characteristics for P-chan-
and N-channel JFET's.
that the junction
BIASING AN P-CHANNEL
FET
is
operated with the
junction back biased The curves
acteristic family are quite flat
Fig. 6-3
Back biasing
junction of a
the gate-to-channel
JFET keeps
it
P-N
of the char-
showing that the
FET output a good current source that depends
far more on the control voltage than it does on the
P-N
is
turned off
supply voltage.
Turning the
JFET On
Looking again at the volt-ampere curves for
the JFETs you can see that there is a dramatic
Turning the JFET on can be done by merely
disconnecting the gate from any voltage or by
connecting it to the source. As can be seen in Fig.
6-4, a little more source-to-drain current can be
made to flow by placing a small amount (=0.2
volt) of forward bias on the gate as shown. Actually, this is still less than the 0.6 volts needed
to make the P-N junction conduct. So, we can say
limit to
how much
drain-to-source voltage the
transistors will tolerate before they break
down.
Each curve in the family takes off straight up as
soon as the maximum drain-to-source voltage is
exceeded. This breakdown is comparable to a
zener breakdown in a diode in that it doesn't hurt
the JFET as long as it doesn't overheat.
91
Wiring
FETs
Current regulator diodes are made from junc-
in Circuits
tion field effect transistors wired internally to
hold the current constant.
Since the channel is a single "piece" of
semiconductor, you might be wondering if there
any difference between the drain and the
Some FETs are built symmetrically so
that drain and the source are interchangeable. In
other words, you can wire it either way and it will
is
source.
turns the
JFET
work just fine.
However, some FETs are designed so that
most of the voltage drop across the transistor occurs at the drain end so this end is made physical-
way
ly larger. In this
more
power
than
resistance in series
off.
+
i
DESTRUCTIVE
TYPICAL VOLT-AMPERE
BREAKDOWN
CHARACTERISTIC
the transistor can dissipate
a symmetrical design.
this is indicated by showing the gate
arrow closer to the source end of the symbol in-
Sometimes
stead of centered between the source and
If
A
with the source produces a voltage proportional
to the current flowing through the JFET. This
voltage is connected to the gate so that, the
higher the current through the diode, the more it
you have trouble remembering how
Ireg
2
mA
CURRENT REGULATING
RANGE
-V
drain.
REVERSE
to wire
FETs, remember that N-channel FETs are wired
with polarities like an N-P-N transistor and the
75
50
25
IS
PRACTICALLY
A SHORT CIRCUIT
+V
100
VOLTS DC
P-channel FETs are wired like a P-N-P transistor.
For you old timers, N-channel FETs are wired
like triode
C.
vacuum
tubes.
Field Effect Current Regulator Diodes
Field effect current regulator diodes are
made
Generally, the constant current that these
from junction FETs. They are the functional opposite of zener diodes. As you recall, zener diodes
by holding the voltage across
them constant, while the current through them
varies widely. Current regulator diodes hold the
across
them
them constant while the voltage
some of them will
varies. In fact,
hold the current rock steady for over a 100 to
re-
very small, a few milliamperes.
However, several diodes can be put in parallel if
larger currents are needed. Because they are so
accurate, they are preferred over zener and
stabistor diodes as calibration reference standards for digital voltmeters. In a typical digital
multimeter a regulator diode is made to pass a
fixed current through a precision resistor. This
provides a standard voltage which is compared
gulators pass
regulate voltage
current through
Field effect current regulator diode
Fig. 6-6
1
voltage variation.
against the
is
unknown
voltage.
9 ANODE
D.
+
The Metal-Oxide-Semiconductor
FET
(MOSFET)
N-CHANNEL
Metal-oxide-semiconductor
JFET
FETs) were introduced
in
1967.
FETs (MOSThe name
de-
scribes the gate construction of these transistors.
The gate
is a thin layer of aluminum metal deposited on an insulating layer of silicon dioxide,
The gate is designed like a tiny capacitor
with the semiconducting channel acting as one
side of the capacitor and metal gate as the other.
glass.
CATHODE
CATHODE
SYMBOL
The glass
of course serves as the dielectric in-
There is no such thing as the perfect insulator, but glass comes pretty close. The glass
sulator.
Fig. 6-5
cuit
Current regulator diode equivalent
cir-
and symbol
insulation
92
means
that the gate-to-channel resist-
ance
is
current.
virtually infinite
and the gate draws no
The gate does have some capacitance,
N-type semiconductor in the chanThis process depletes the N-type semiconductor of conduction band electrons and turns the
will leave the
a
nel.
few picofarads, so it is necessary to charge and
discharge the capacitance to change the voltage
transistor
off.
on the gate.
The gate
insulator
is
very thin, one ten
is
subject to damage by
voltages above 20 volts for small MOSFETs and
millionth of a meter, and
above 80 volts
for large
posite to
FETs. This doesn't sound
the opposite of
will
sistor.
1.
Enhancement
MOSFETs
There are two basic kinds of MOS tranenhancement mode MOSFETs and
Channel cut off
sistors, the
the depletion
JFET,
is
which has a polarity opthe one which caused depletion is put on
the gate, the charges that gather in the channel
increase the number of majority carriers
available for conduction. This increases the conductivity of the channel and turns on the tran-
very serious until you realize that when the current is virtually zero, very little power is needed
to produce 80 volts or even 80,000 volts. If you
don't believe this, scuff your rubber soled shoes
In the
MOSFET operation
depletion. If a voltage
is
on a wool rug and go touch a brass door knob. The
spark that jumps to the knob is propelled by several thousand volts. If the door knob had been the
gate of a MOSFET, the gate insulator would have
been punctured instantly.
The other mechanism for
enhancement and
called
the channel
is
turned off by the
charge depletion that occurs when the gate-tochannel P-N junction is back biased. In a
MOSFET the conductivity of the channel is increased or decreased by charging the capacitance
between the gate and channel. In the depletion
mechanism, majority carriers are removed from
the channel in the same way they are in a junction
FET. As the gate-to-channel capacitance is charged, an opposite charge will gather in the channel
on the opposite side of the insulator.
For example, if the MOSFET is an N-channel
type and electrons are forced onto the gate by a
negative voltage, an equal number of electrons
mode MOSFETs. As
the
name
im-
enhancement mode MOSFETs use only the
enhancement mode operation. The gate voltage
plies,
carriers
into the channel
In fact, the enhancement
MOSFET channels are induced into semiconductor of the opposite type to the kind of channel
induces
majority
semiconductor.
desired.
For example, conduction band electrons are
induced into P-type semiconductor to make the
channel into an N-channel MOSFET. As positive
voltage is applied to the gate of an N-channel
device, the first few electrons induced into the
channel semiconductor must fill in the holes
already there in the
P
material.
Once these
holes
GLASS INSULATOR-
SOURCE
ALUMINUM CONTACTS
GATE
0.1m
P-CHANNEL
INDUCED IN
N-TYPE
SILICON SUBSTRATE
Fig. 6-7
P-channel enhancement type,
93
IS
N-TYPE SEMICONDUCTOR
MOSFET
METER
are "filled,'' the induced electrons can begin to
enter the conduction
band
The importance
channel.
to establish an N-type
scheme
of this
that
is
turned
are
shown
.
an N-channel, they are
were P-channel, they
positive. If the transistor
which straddle the zero voltage
gate to source voltage, Vg S A
P-channel depletion mode MOSFET is just like
an N-channel, but all the pluses and minuses are
be negative.
Depletion
We
characteristics
MOSFETs
are belaboring
in figure 6-9.
for
point
2.
with a
without redesigning
Once again the gate voltage, Vg S is labeled
each curve in the family of output characteristics. Note that when the gate voltage is zero, the
depletion mode MOSFET is half turned on. This
is more easily seen in the typical C*typ") transfer
,
all
MOSFET
mode
and you cannot
MOSFET
is a little more complex,
the
construction
is similar to the
otherwise
but
enhancement type. The volt-ampere characteristics for a depletion mode N-channel MOSFET
the family, a gate voltage, Vg S is given. It is important to notice that every gate voltage, from
fully turned on to fully turned off, is the same
would
different
The depletion type
Volt-ampere and transfer characteristics for
an N-channel enhancement MOSFET are shown
in Fig. 6-8. For each output volt-ampere curve in
all
enhancement mode
the circuit.
off.
polarity. In this case, with
MOSFET is
depletion
it
makes the transistor '"fail safe." In other words,
whenever the gate voltage is zero, the transistor
is
Hon mode
replace an
all
this
because the deple-
the
of
.
reversed.
OUTPUT CHARACTERISTIC
OUTPUT CHARACTERISTICS
16
20
VBS =
VGS = 10V
v Bs = o
+ 4V
<
£
E
16
z
12
qw
+ 2V
LU
HI
CC
cc
=>
CC
DC
=3
12
8V
o
z
<
cc
Q
V
1
1
o
z
-
8
VGS =
<
d
Q
1
)
1
6V
—
3V
4
I
a
-2V
5V
|4V
*—
_4V
1
1
I
2
4
10
8
6
4
TRANSFER CHARACTERISTIC
20
I
16
VDS = = 10V
Vbs = =
- Unr
Vbs =
o
/
/
LU
DC
DC
20
1
16
cc
EC
12
2
8
I
Mt
/
D
U
z
<
(X
o
12
rYP_^
3
O
<
cc
Q
S
6
4
8
-6-4-2
10
Vds DRAIN-SOURCE VOLTAGE (VOLTS)
Volt-ampere
6-8
teristics
/ M IN
4
2
Fig.
16
TRANSFER CHARACTERISTICS
20
E
12
vds drain-source voltage <volts>
Vds DRAIN-SOURCE VOLTAGE (VOLTS)
<
8
for an
unci
N-channel,
transfer
2
4
6
Vqs GATE-SOURCE VOLTAGE (VOLTS)
eharac-
Fig.
enhancement mode
6-9
teristics
MOSFET.
MOSFET.
94
Volt-ampere and
a depletion
for
transfer
charac-
mode N-channel
SOURCE
GATE
DRAIN
O
i
m
1
m
">/////?////
N+
•
—
-r- J--i.
+ + +
NTYPE
_ +
N+
NCHANNEL TURNED OFF
PSUBSTRATE
"BY INDUCED
N-channel depletion type,
Fig. 6-10
Depletion
the depletion
When
fully
mode MOSFETs operate in both
mode and the enhancement mode.
principle. In order to turn all the
enhancement
way
off, they
take carriers
out of the channel. Having the depletion MOS-
operate in the depletion
mode and
FET
gate voltage swing centered on zero volts
ideal for amplifying small AC or
radio frequency signals. There is no need to bias
the signal to center the "zero" on some DC
voltage other than true zero volts. In the next section we will show how these two transistors are
actually wired in circuits.
makes them
substrate
there
It
is
internally connected to the source so
no need to worry about where
for
shows the common circuit symbols
MOSFETs. The symbols on the right are
only used for enhancement types, while the ones
on the left can be used for either. Sometimes the
gates are shown as "hooks" and other times they
look like capacitor plates. No matter how they are
wouldn't be necessary to make a big deal
symbols if they weren't so con-
MOSFET
DEPLETION OR ENHANCEMENT
ENHANCEMENT ONLY
A
A
r
-—
GATE (
\
DRAIN
DRAIN
GATE / ««"" "~\
~x
IT
I
LU BSTRATEl
7\
GATE /
SOURCE
u
DRAIN
\ to
I
\Jv
sourceI
SOURCE
P-CHANNEL MOSFET
DRAIN
DRAIN
GATE
«K
SUBSTRATE.
SOURCE
DRAIN
GATE
SOURCE'
NCHANNEL MOSFET
Fig. 6-11
to connect
Fig. 6-11
MOSFET Symbols
about
is
the substrate.
used
E.
MOSFET
fusing. The confusing part is the arrowhead
which sometimes points in the "wrong" direction.
And, of course, some symbols apply only to
enhancement MOSFETs while others can be used
for both depletion and enhancement types. The
semiconductor substrate that the MOSFET is
made on is occasionally brought out as a separate
lead but it is rarely attached anywhere other than
the source lead. In most MOS transistors the
turned on, they are adding majority
carriers to the channel using the
HOLES
MOSFET symbols
95
are confusing.
SOURCE
A MOSFET can be used as a load resistor by
simply wiring the gate to its own drain. The
voltage drop from drain to source is enough to
turn an enhancement MOSFET half on. This
gives the FET "resistor" about 4000 ohms
resistance when most of the supply voltage is
across it from drain to source.
drawn, they at least give the idea that the gate
makes a capacitor with the channel. When the arrowhead is shown on the source lead, it means
that positive charge flows in the direction of the
arrowhead. This is logical since this makes it like
a bipolar transistor emitter.
Unfortunately, most diagrams of
MOSFETs
have the arrowhead drawn on the
substrate, that is, on the lead in the middle. This
shows the direction of the P-N junction between
the channel and the sub-strate. This P-N direction
is true enough. However, the substrate is wired to
the source which makes the arrowhead point
toward the positive side of the power supply, not
the direction of flow of positive current. In summary, if the arrowhead is on the center substrate
lead, follow the arrowhead as if current flowed in
that direction and it will lead toward the positive
power supply.
you
F.
NMOS
called
PMOS
FETs
cuits the
sistors
because
made
N
or type P, but
and
or
all
the
is
it
way
en-
are
a transistor
shows two inverter
Fig. 6-13
use enhancement
it
Each
to its
"resistor"
own
substrate
chip,
each
MOSFETs
MOSFET
has
drain to turn
common
is
own
shown wired
its
gate connected
Since the
on the
cannot be in-
source. Therefore,
to ground.
VOLTAGE
OUTPUT
VOUT
+ SUPPLY
Vdd
Vin
1
VOLTAGE INPUT
LOAD RESISTOR
1
OUTPUT
INPUT
>
f
Nl
>
>
Fig.
&12 Basic
binary inverter
96
which
half-on.
substrate
dividually wired to its
substrates are
it
circuits
for load resistors.
to all transistors
transistor
Vdd
'
is
make a binary "0."
way off, it makes a
it is turned all the
binary "1." Notice the high current level required
to maintain a "zero."
can not be both. It is possible to dope wells or
islands of the opposite kind of semiconductor into
the substrate, but it is easiest to make all one
kind of channel MOSFETs on the same sheet of
substrate. Therefore the "resistors" should also
be made of the same kind of channel FETs.
1
called
NMOS
When
off.
usually to
When
digital cir-
themselves are used as load recheaper and easier than try-
either type
are
use transistors in place of resistors, we are going to look at
the simplest digital circuit, the binary inverter.
This circuit converts the binary number 1 into the
binary number zero, or vice versa. In digital circuits the active transistors are always used as
switches. That is, they are either turned all the
it is
is
PMOS
made
made
integrated circutis. ICs
To show how
ing to integrate resistors into the circuit. The
semiconductor substrate on which an integrated
circuit is
are
MOSFETs
P-channel MOSFET's
integrated circuits.
turned on,
MOSFET
that
from
tirely
way on
said earlier that in
circuits
almost entirely from N-channel
NMOS and PMOS
We
MOSFET
Digital
will see
D CURRENT
THROUGH RESISTOR
l
.
1
all
The majority of the microprocessors and
microcomputers on the market are made from
NMOS and PMOS circuits. The circuitry consists
transistors themselves.
numbers of single kinds of MOSFETs arranged to make inverters, counters, memories,
and logic circuits for decoding computer instructions.
Complementary
digital
watch on your
made
almost entirely from CMOS. They beat other
types of logic circuits in almost every respect except high speed. The advantage that makes a
digital watch practical is their low power consumption.
of vast
G.
The
wrist and the calculator in your pocket are
As long
MOSFETs (CMOS)
as a transistor
is
turned
off, it is
not
drawing any current and power consumption
insignificant.
N-channel and P-channel forms gave rise to com-
plementary
(CMOS).
MOSFET
CMOS
integrated circuits
ICs are as remarkable as MOS
Q
is
But when a switching transistor
turns on, it draws at least a milliampere or so. If
thousands of transistors turn on at once to make
binary zeros, the power supply is going to have to
deliver amperes to keep them all turned on.
The almost non-existent DC gate current of a
MOSFET and the ability to build them in both
The conventional way
- SUPPLY
Fig. 6-13.
A
of doing this is seen in
positive signal, a "1," turned on a
The drain-to-source voltage dropped
from high to low, making the output voltage drop
and the load current rise. While the transistor is
on, energy is being wasted in the load resistance,
even if the resistance happens to be a MOSFET.
transistor.
Complementary
MOSFET
circuits solve this
problem by replacing the load resistor with a
complementary MOSFET which is the opposite
channel type as the original MOSFET being switched. This second MOSFET is turned on and off
by the same input signal. Since it is the complement, it responds to the input polarity the opposite way; it turns on whenever its complement
turns off.
PMOS INVERTER
The output
O
of the inverter is
tied
to
the
drains of both transistors so the output is either
connected to ground or the high side of the power
+ SUPPLY
supply. In either case one transistor
is
always
turned off and
never flows. A small "spike" of current flows
significant source-to-drain current
whenever the inverter makes a transition between high and low, but whenever the circuit is
quiet or "quiescent," the current through the inverter is just the leakage current. This leakage
current can be very small. An inverter as seen in
draws less than 1 nanoampere
Fig. 6-14
ampere) when it is not actively
switching between high and low states. This explains how a battery the size of a pea can power
(0.000,000,001
your watch
Just to reinforce the idea behind CMOS, we
can compare a complementary pair of CMOS
transistors to a double pole, single throw relay.
The relay consists of two simple switches ac-
NMOS INVERTER
Fig. 6-13
for a year.
PMOS and NMOS inverter circuits
97
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V
SOURCE
P-CHANNEL
I
1
1
1
I
'
VlN
*~
DRAIN
VOUT
t
/-\
i
I
"
"
vJ
>
DRAIN
i
i
I
4
N-CHANNEL
i
I
i
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SOURCE
A
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Id
t
V
'
t
JL
k
\£- TINY CURREN
SPIKES
-QUIESCENT CURRENT
1 NANO AMPERE
6-14
Fig.
Complementary
MOSFET
digital
inverter
tivated by the magnetic field from a voltage
The
cond gate can control the gain of the amplifier
DC voltage on the second control gate. Because MOSFETs make very little
coil.
an iron lever arm which opens
and closes the two switches. The switches either
connect the output to ground or to the positive
supply. There is never an opportunity for current
to travel from the positive supply to ground
through the switches.
just by varying a
field attracts
radio noise, they are preferred over bipolar transistors for sensitive radio frequency amplifiers.
Dual gate
complementary binary inverter could also
be built with bipolar transistors. However,
bipolar transistors need 20 times more area to
"print" on a silicon chip and they need resistors
and extra components to bias them properly.
These economic reasons further explain the
H.
Dual Gate
CMOS
/.
with
two
control
automatic gain control is a circuit that
keep the signal level coming out of a
receiver constant, even
though the signal
strength may be rising or fading. The output
audio signal from the receiver is rectified and
filtered to make a slowly varying DC signal which
is used to bias the second gate of a dual gate
MOSFET. The first gate is used to amplify the
signal while the second gate holds the average
signal level constant. Depending on the polarities, it may be necessary to invert the control
signal before returning it to the second gate. By
"invert" we mean turn a high voltage into a small
tries
gates
allow two signals to control the source-to-drain
current
For example, if the
being used as an amplifier, the se-
simultaneously.
MOSFET
is
IRON LEVER
OPERATES SWITCHES
WHEN
COIL
voltage or vice versa.
HMMi^i
INPUT
RELAY VOLTAGE
COIL
OUTPUT
ONE SWITCH
OPENS WHEN
EVER THE OTHER
SWITCH CLOSES
2.
Mixer
cuits.
CMOS
inverters can be
compared
Circuits
Another common application is in mixer cirIn superhetrodynes the local oscillator
signal is mixed with the incoming RF signal to
generate a new difference frequency which is the
intermediate frequency. The incoming RF signal
is fed to one gate and the local oscillator signal is
>
&15
to
IS
ENERGIZED
Fig.
Automatic Gain Control
An
digital ICs.
MOSFETs
MOSFETs made
are especially attractive for
has many situations where it is desirable to
change the gain of an amplifier stage or multiply
one signal times another.
A
widespread use of
MOSFETs
radio receiver circuits because a superhetrodyne
to a
pairs of switches operated by a relay.
98
fed to the second gate.
rent
is
now
The
SOURCE
source-to-drain cur-
the combination of the
RF
signal
o
and
1
c
-GLASS
the local oscillator frequency and contains the
SI
new intermediate frequency component.
3.
Local Oscillators
Dual gate
MOSFETs
are also
commonly used
For good superhetrodyne
important to keep the local oscillator
signal a constant level which does not vary over
the frequency tuning range of the receiver. To
keep the oscillator level constant, one gate serves
as part of the oscillator feedback loop, while the
other is biased by a DC automatic gain control
signal derived from the local oscillator signal
for
local
mixing,
oscillators.
V
it is
/// H+
VN+
I
\
P
P
///
/
N-
J
1
J
(LIGHTLY DOPED)
N+
^ DRAIN
itself.
Dual gate
MOSFETs
Fig. 6-16
becoming universal
are
A VMOS power transistor
low noise radio receivers. Since
most modern avionics receivers use them, you
will become very familiar with these versatile
on.
transistors.
verted to an N-channel when the device is turned
on. But when it is turned off, it behaves like a
in sophisticated,
/.
VMOS Power
VMOS
stand for? The
current flow."
Most
It also
VMOS power tran-
make up
devices
we discussed above
are not only very small in area, they
depth on the
silicon chip that
have so
VMOS
power transistors still have a
breakdown voltage, only
volts,
more
MOSFET
enhancement mode
but large ones can dissipate 80 watts or
Others can tolerate large drain-tosource voltages, 350 volts or more. Because the
gate draws no DC current, they can be driven
directly by very low power circuits.
80
sistors.
The
are
limited gate-to-source
describes the shape of a notch cut in the plane of
the silicon chips that
transistors
reverse biased silicon diode which can tolerate
very high voltages without breaking down.
Transistors
What does the "V" in
"V" stands for "vertical
VMOS
transistors so the layer(s) in the center are con-
little
they are practically
safely.
the bottom of the chip so that the source-to-drain
For example, a large VMOS transistor can be
turned on and off with the voltage output from a
low power CMOS inverter. The catch is that the
VMOS gate in a large transistor has a high capacitance, 650 pf is typical. So for switching the
VMOS rapidly, large currents must be jammed
into and pulled out of this capacitance. A single
CMOS inverter cannot provide this much current
and even several in parallel may not provide
enough. So, in practice, a VMOS transistor used
for high frequency, high power switching takes
about 1/10 the AC driving current that a bipolar
power transistor would need to get the same performance.
current flows vertically instead of horizontally.
This greatly increases the volume of semiconduc-
J.
two dimensional.
The
source-to-drain
flows horizontally through so
tor material,
little
current
semiconduc-
that they cannot dissipate
heat without being damaged.
A
typical
much
MOSFET
can dissipate about 0.2 watts or less. If the area
were greater, more heat dissipation could be
achieved but a larger area of control gate would
be needed. The transistors would be even slower,
and there are technical
in making
enough so that the
difficulties
large gate insulators uniform
current does not concentrate at "hot spots."
The problem
tor in
the
V
is
solved by putting the drain on
which to dissipate the heat. The purpose of
notch is to propagate the effect of the gate
How
to
Protect
Integrated Circuits
The part
into the center of the silicon layers so that the
path between the source and drain can be turned
of a
MOSFET
dielectric insulation
99
MOS
Transistors
that
is fragile is
and
the
between the gate and the
lowing large voltages to be applied across the
transistors are supplied with a small metal band
crimped around the leads. If the gate is shorted to
the drain and source, obviously the gate-to-source
gate-to-source or gate-to-drain.
voltage can't
channel. Since excess voltage can puncture this
insulation,
When
there
damage can be prevented by not
the
MOSFET
nearly always
is
resistance (conduc-
tive path) between the gate and the channel. This
prevents a voltage build-up across the gate insu-
So a MOSFET installed in a circuit is nearalways safe. The "nearly" can happen when
lightning strikes your building or the local electric lines. This is more common than you might
think and is a good reason to keep expensive
oscilloscopes and other equipment unplugged
when not in use, especially during the summer
thunderstorm season. In the author's experience,
lightning damage to test gear happens, and when
lation.
ly
it
occurs,
it is
usually the
rise.
MOSFET integrated circuits are sometimes
supplied with the pins shorted with a thin metal
clip. More commonly they are supplied plugged
into a small piece of conductive black rubber
foam. The black color comes from carbon black
which is impregnated into the plastic or rubber
a circuit,
is installed in
some
al-
foam to make it conductive. As a general rule, if
the foam isn't black, it isn't conductive! Storing
ICs by sticking them in white styrofoam is probably worse than leaving them loose in a parts
drawer because the act of pressing them into insulating foam may generate enough static electricity to
damage them.
MOSFET components
that need replacing.
Lightning damage to ICs
is rare,
but static
damage is common and is most likely
occur when transistors or ICs are being han-
electricity
to
dled or installed in equipment. Static electricity is
usually generated by rubbing an object against
an insulator such as a rug or cloth. When two obrubbed together, and one or both of them
are insulators, charge is transferred from one to
jects are
the other.
The
direction
movement
of
of
the
Fig. 6-17
charge depends on the particular materials.
For example,
if
you rub a piece
a piece of fur, the rubber will
MOSFET
shirt is
A
an easy
of rubber with
person
across a
in
waxed
acquire as
body has
L.
much
to ruin
rubber soled
shoes
MOSFETs
in Circuits
you are already seated at a bench, you are
up a charge than when you are
walking around. However, if the MOSFETs have
the opportunity to be rubbed against insulating
surfaces, you may inadvertently charge them. To
avoid this possiblity, it is desirable to have the
surface of the bench conductive and grounded. In
some IC factories, the workers are even grounded
by conductive straps worn around their wrists!
If
it.
floor over to his
Installing
less likely to build
transistor's leads against a nylon
way
ICs and transistors should
become negatively
charged. If you rub a glass rod with a piece of silk,
the glass rod will become positively charged. In
either case the moving object acquires a large
voltage with respect to ground. Carelessly stroking a
MOSFET
be stored with their leads shorted by metal clips
or conductive plastic foam.
walking
work bench car
as 15 kilovolts on his body. His
a capacitance of about 250 pf with re-
spect to ground. This capacitance can store only
Before installing an IC or MOSFET in a
make sure that the circuit is turned
off. Even without static electricity, it is too easy
to have a pin touch the wrong socket hole and
damage the IC. Before removing the IC from its
package or foam protector, make an effort to
deliberately touch something grounded, like the
metal bench or a metal chassis, then remove the
IC from its conductive foam or metal clip. While
0.03 joules of energy even at 15 kv, but the
voltage is what is dangerous to MOSFETs. When
socket, first
he reaches over to touch an IC, a tiny spark may
destroy the gate without him ever feeling a shock.
K
Storing Loose
When
storing
MOSFET Devices
loose
MOSFET
ways keep the leads shorted
devices,
together.
al-
MOSFET
100
holding the device with one hand, use the other
hand to touch the circuit board or wiring of the
circuit in which you are installing the part. This
should discharge any static voltage difference
between you, the IC, and the circuit. Then insert
the device in its socket just as you would any IC.
Another form of built-in protection is an array of diodes and a resistance which does not
allow the gate-to-source voltage to go below
ground or above the positive supply. This system
is used for inputs of CMOS integrated circuits.
The
resistance indicated in the circuit is not a
discrete resistance as drawn, but is actually part
of the two diodes on either side of it. In spite of
In a wet, humid climate, damage due to static
may be so unusual that you may decide
that all this is unnecessary. But if you are working in the Arizona desert, it can be a very expenelectricity
sive
problem unless these precautions are taken.
M.
Built-in
these protection circuits, these integrated circuits
are still fragile and the protective steps outlined
n
earlier
should be followed.
ONE OF THE INVERTERS
THE CD4049 CMOS HEX
MOSFET Protection
INVERTER
MOSFET gates are frequently protected by
diode and resistance networks built right into the
IC or transistor.
IN
+ VDD
IC.
D2
OUTPUT
> —vWSA*
A
simple built-in protection is a zener diode
connected from the gate to the substrate or
source. Whenever the gate voltage approaches its
breakdown voltage, the zener diode will conduct
and prevent the gate from being harmed. Many
individually packaged
MOS
RESISTOR IS
ACTUALLY PART
OF DIODES D-(
>AND D
i
2
6 -vss
transistors are avail-
able with or without built-in zener protection.
Fig.
6-19
Diode/resistor input protection in a
CMOS inverter IC.
DRAIN
QUESTIONS:
GATE
—
>
V
ZENER
SUBSTRATE
1.
In what
way
is
the input gate of an
FET
functionally different from the base of a
S
bipolar transistor?
DIODE
2.
BUILT INTO
N-CHANNEL MOSFET
SOURCE
In bipolar transistors the current that is being controlled by the transistor passes from
collector to emitter and passes through two
P-N junctions. In an FET the current that is
being controlled does not pass through any
functional P-N junctions and is therefore
"unipolar."
DRAIN
GATE
?
this
just interesting or
is
side of a transistor unipolar?
i
1
1
i
3.
v\
P-CHANNEL
L
1
.
1
'
\
MOSFET
WITH
Is
there any advantage in having the output
transistor?
COMMON
SOURCE AND
SUBSTRATE
can a junction FET be called
"unipolar" if it has a rectifying P-N junction
in it? Doesn't this imply that both holes and
conduction band electrons are at work in the
How
SOURCE
4.
Suppose you needed a very accurate
DC
Fig. 6-18
Built-in zener protection for
2 volt
voltage for calibrating a
digital voltmeter. Using a poorly regulated
10 volt power supply and a 2 milliampere
MOS tran-
sistors.
101
reference
current regulator diode, design a simple
voltage source. You may assume that the
voltmeter will not draw any appreciable cur-
14.
Why
does a
MOSFET
have higher input
resistance than a junction
6.
When
FET?
repairing a radio circuit,
frequent-
it is
MOSFET
with
another similar MOSFET. Does it matter if
one is a depletion type and the other is an
enhancement type as long as they both have
the same current and voltage ratings and
the same kind of channel, N or P?
ly possible to replace
7.
In
an
NMOS
or
one
PMOS
logic
circuit,
enhancement MOSFETs are used as resistors by connecting the gate to the drain. If
you were designing an integrated circuit
with depletion mode MOSFETs, would you
still
connect the gate to the drain to use the
depletion
8.
Why
mode
MOSFET
are dual gate
as a resistor?
MOSFETs
especially
useful in superhetrodyne receivers?
9.
You are stranded on Mars because the fuel
management computer in your lander module has a faulty digital inverter made from
an N-channel enhancement mode MOSFET
transistor. You proceed to fix the inverter
using an N-P-N bipolar transistor salvaged
from an old Viking lander. What changes
will be needed in the control gate circuit to
make the bipolar transistor compatible with
an inverter circuit like the one shown in Fig.
6-12?
10.
11.
12.
In a complementary MOSFET inverter,
which of the two transistors is the inverter
and which is the load resistance?
When does a CMOS inverter draw
cant amounts of current?
signifi-
What is there about the VMOS transistor
design that enables it to dissipate high
power safely. Do you think the VMOS transistors could be easily "printed" in with
other components in a complex integrated
circuit?
13.
List four or five basic steps for installing a
MOSFET
component that
damage from
will help
is
MOSFET
from
rent.
5.
What disadvantage
diodes built into a
prevent
static electricity.
102
static electricity?
there in having zener
to protect
it
SECTION VII
Transistor Amplifiers
A.
Amplifiers
B.
In this section
transistors
are
"amplifier"
is
know,
it
we
used
are going to describe
in
amplifiers.
pretty general, but as
means
how
Before
The word
you already
Amplifiers are used for a number of purposes.
They can amplify voltage, current, power, or all
three. Sometimes they are used in place of a
what
a junction
ternal
impedance of the power
to the load.
most
is
is
The
efficient
equal to the
in-
source.
VOLTAGE
Rload
OUTPUT
POWER
FET
The
FET
MAXIMUM
"amplified" its own drain-to-source current
to hold that current constant. Sometimes large
OUTPUT
bipolar transistors are used as series regulators or
POWER
parallel regulators to hold
power
transfer of power to the load
when the impedance of the load
transformer where a larger voltage or larger curis desired but more power is not important. Sometimes amplifiers just serve as switches or relays. For instance, you might want to
switch large numbers of powerful lights on and
off for a disco light show or perhaps a price quotation display board in a stock exchange.
Sometimes an "amplifier" is just a current or
voltage regulator stage in a power supply.
we saw how
role the resistance of the load plays in the ef-
ficiency of the transfer of
rent signal
to build a current regulator diode.
discuss the three basic con-
understand impedance matching. Up until now
we have talked about transistor amplifiers in
terms of the transistor controlling the current
through the load. Now we are going to look at
controlling a large signal with a
In the last section
we can
figurations for transistor amplifiers, you need to
small signal.
was used
Impedance Matching
POSSIBLE
ZERO POWER
power supply voltage
WHEN
RL =
constant.
In a radio transmitter a powerful radio freis generated and delivered to a
transmitting antenna. It is usually not practical
to generate the entire signal in one single electronic operation. In most transmitters an oscillator generates a very stable, low power RF signal which is then amplified a number of times to
reach the desired output power level. In this case,
only the frequency of the original signal is
preserved by the amplifier.
quency signal
Rl = Rsource
LOAD RESISTANCE.
RL
The optimum load resistance for
transferring power to a load is reached when load
Fig.
7-1
resistance equals source resistance.
103
°°
1.
Power Calculations
review how power is
calculated: Power equals voltage times current.
To
we
2.
begin,
In Section 4
shall
P = V
Resistance
we
talked about "perfect voltage
sources." If you could build such a thing, it would
have zero internal resistance so that it could sup-
ply unlimited current to low resistance loads.
I
Real power sources, such as amplifiers, always
Ohm's law
V =
is:
R
I
have some internal resistance, even if it is just a
few ohms. Whenever the amplifier supplies current to a load, the current is also passing through
If we substitute the Ohm's law expression for
voltage into the power equation, we get:
P =
(
I
R
)
I
=
I
2
the resistance inside the amplifier. This resist
ance will dissipate power and produce heat. If the
R
resistance inside the amplifier
load resistance, more
or,
Power
=
(current) 2
X
power
is
larger than the
will be
wasted mak-
ing hot transistors than will be delivered to the
resistance
load.
important because it
shows that power is more dependent on the size of
the current than the size of the resistance. For example, if the resistance doubles, the power only
This last formula
But
doubles.
is
Whenever the
We
already
know
work
the
that the extremes in load
power to a
ohms, a large
flow find their best compromise when the load
resistance equals the internal resistance.
for transferring
load. If the load resistance is zero
current will pass through the load, but nothing
will slow it down and it will do no useful work. We
know
of
is no more advantage to be gained by using a smaller load
resistance in hopes of getting more current to
flow. So the two separate effects of load re
sistar.ce and internal resistance limiting current
goes up 4 times.
resistance do not
resistance
limiting current to the load, there
the current doubles, the power
if
internal
voltage source becomes the dominant factor in
+ 150 VOLTS
that the voltage across the load will be zero
is zero and you can't have
voltage across zero resistance. The power output
because the resistance
will be:
Power =
(zero volts)(large current)
At the other extreme,
if
infinitely large, the current
=
zero watts.
the load resistance
through the load
approach zero, so the transfer of power
be zero.
will
is
will
again
Power =
(large voltageKnearly zero current)
£ zero watts
8Q
LOUDSPEAKER
we can reason that the optimum
resistance for a load must be somewhere "halfway" between these extremes, but why must it
From
this
equal the voltage source resistance?
Since power varies as the square of the curit follows that the load resistance should be
rent,
very
low
so
that
a
large
current
will
rrn
flow.
However the resistance must not be so low that
the power delivered is approaching zero along
with the resistance. The question is, what limits
Fig.
the current that the amplifier can deliver?
KM
7-2
A vacuum
tube has high output im-
pedence and needs an impedance matching transformer to drive a low impedance speaker. Transistors usually do not.
In
most applications amplifiers must be
matched to the impedance of their loads.
C.
Basic Transistor Amplifiers
carefully
Common
Loudspeakers generally have about eight ohms
impedance. We call it impedance because there is
a lot of inductance in a loudspeaker. Transistor
stereo amplifiers can be designed to have about
that much output impedance. This makes it very
convenient to use loudspeakers directly as load
impedances.
In
all
the examples of transistor amplifiers
so far, the emitter has been con-
we have used
nected to both the input and the output. As you
might guess, an amplifier like this is called a common emitter amplifier. If a comparable amplifier
from a
would be
source amplifier. This configuration seems the most straightforward and it is the
most common way to use any of the amplifying
is
built
called a
3.
Emitter Amplifiers
Impedance Matching Transformer
field effect transistor, it
common
transistors.
do not always tailor
themselves to match the impedances of the am
plifiers. In the bad old days, vacuum tubes had
MEDIUM TO HIGH INPUT IMPEDANCE
MEDIUM TO HIGH OUTPUT IMPEDANCE
much
INVERTED VOLTAGE OUTPUT
Unfortunately,
loads
higher internal impedances, often thou
sands of ohms. For a vacuum tube to drive an
eight ohm speaker, it was necessary to match the
speaker to the amplifier by means of an impedance matching transformer.
HIGH VOLTAGE GAIN
HIGH CURRENT GAIN
HIGH POWER GAIN
A transformer can convert a small AC volt
age to a large AC voltage at the expense of the
current, or it can convert a small AC current into
a large AC current at the expense of the voltage.
The latter was what was needed for the tube to
drive a loudspeaker. The power leaving a trans
former is the same or perhaps a little less than the
power entering a transformer. Unlike transistors,
transformers do not have power gain. All they
"transform" is the ratio of voltage to current.
VouT
rm
EMITTER COMMON TO
BOTH INPUT AND OUTPUT'
By altering the ratio of current to voltage,
the transformer can appear to change the im-
The common emitter amplifier
pedance of a load. In other words, it can make a
3000 ohm tube "think" that an 8 ohm loudspeaker is a 3000 ohm loudspeaker. From the
point of view of the loudspeaker, an impedance
matching transformer can make the 3000 ohm
tube transfer power as though it were an "8 ohm
current gain, and because the current gain is high,
it doesn't require very much base voltage to produce a current large enough to control a very
vacuum
large current from collector-to-emitter. Since the
Fig. 7-3
The common emitter
tube."
amplifier has a large
load resistance can be high and the power supply
voltage can be high, a common emitter amplifier
Impedance
several
matching
dollars
so
this
can have a large voltage gain. And, of course,
transfor-mers cost
usually
is
and voltage gains mean that the
emitter amplifier can have a large power
gain. It is important to keep in mind that the
common emitter amplifier is a voltage inverting
expense
large current
common
eliminated in a transistor stereo amplifier. Impedance matching of transmitter outputs to an
tennas is still a problem with transistors,
transformers, and L-C networks are used to do
this.
You
will see
examples of
this at the
end
amplifier.
The bigger the input
voltage, the
more
turned on and the smaller the
output voltage across the transistor becomes.
the transistor
of
this section.
105
is
a.
high resistance, then I ou t will be small and this
output power will be small. Nonetheless, it is this
small output current that is the whole purpose of
Output Resistance
An amplifier parameter that is important is
output resistance. This is the internal voltage
source resistance we talked about earlier. In the
case of the common emitter amplifier, output resistance is the resistance between the output lead
and ground. So this is the resistance of the transistor from collector to emitter. The output resistance of this amplifier can be high or low, depending on how much the transistor is turned on. If
the transistor is full on, the output resistance will
be quite low. On the other hand, if the transistor
is turned nearly off, the resistance can be quite
high. This is of course determined by the level of
the input current. High resistance outputs are
very good for producing high voltage signals, but
poor for producing high currrent signals.
b.
Common
the first amplifier.
The current going through the load
RL
resistor,
does nothing useful except to change the
voltage which drives the second amplifier, so the
second amplifier is a bonafide load on the first
amplifier. Since this new load is being driven by a
voltage source, the first amplifier must have an
output impedance. This new output impedance
must include the load resistor, Rl,i- because it is
part of the voltage source and not part of the new
load. From the point of view of the second
lt
amplifier, the load resistor, RLi- is in parallel
with the resistance of the transistor. In summary,
when we talk about the output impedance of an
amplifier, we must think about where the power
is actually being delivered if we want to optimize
the transfer of power.
Emitter Amplifiers In Series
demonstrates how two common emitcan be put in series to increase the
current or voltage gain, but the main point we
want to illustrate is that the output power of a
transistor is not always exclusively delivered to
Fig. 7-4
c.
Input Impedance
ter amplifiers
Another important characteristic
fiers is
the input impedance.
amplifier
is
of ampli-
The common emitter
often built so that the base current
is
limited by a high value base resistor, Rj. Because
the load resistance.
this resistor usually has a high value, the input to
a common emitter amplifier can have quite a high
input resistance. The advantage of this is that the
input to this amplifier will draw very little current and will not put much load on preceeding
In Fig. 7-2 the load was a loudspeaker connected between the collector and the power suppso there was no doubt where the power was being delivered, but in Fig. 7-4, some power is
diverted from the collector to drive the next
ly,
amplifier stage. It
is
true that
if
R3 and
R4
have
y—yvw
means that very
stages.
needed to drive the amplifier and
is
little
if
<L2
«L1
N-P-N
This
amplifier
power
»
-
R3
>
'out
VOUT
V.N
>
^>
J
1st
Fig. 7-4
R-i Rj,
Two common
and the second
»
^
AMPLIFIER
2nd AMPLIFIER
emitter amplifiers in series. The load on the
transistor.
106
first
amplifier
is
not really Rlj,
it is
several different amplifiers can be
driven by one preceding stage. This last situation
is unusual in audio or radio circuits, but common
necessary,
1.
An
important to notice that a
high input resistance causes the amplifier to have
a high output impedance! The transistor can't be
turned full-on when the current into the base is
limited by very large impedances.
in logic circuits. It is
is
that
important feature of the emitter follower
has built in negative feedback that tends
it
whenever it trys to turn
on a transistor, the voltage
from the base to emitter must exceed some thres
hold so that base current will flow.
to turn off the transistor
on. In order to turn
The Emitter Follower (Common Collector
D.
Negative Feedback
The input
Amplifier)
differs
from
because,
This amplifier configuration is similar to the
emitter but the load resistance is moved
from the collector side of the transistor to the
emitter side.
As
seen on a circuit diagram, the collector
to both the input and the output, so
common
most people prefer to call this the emitter
The output of this amplifier follows the
follower.
emitter voltage because, as the transistor turns
on, the voltage across the load rises.
Emitter
follower amplifiers do not invert the input signal
Common
emitter amplifiers invert the
in-
put voltage because a turned on transistor has
lit-
voltage.
tle
voltage across
it.
But
common
addition
to
emitter
the
amplifier
base-to-emitter
threshold voltage, the input voltage must also exceed the voltage across the load resistance. This
means that for a given level of input voltage, the
transistor is much less likely to turn on. And,
when the transistor does turn on and current
flows through the load resistor, this increases the
threshold voltage that the ba*se-to-ground voltage
must exceed. In other words, it tends to turn the
transistor right back off again. This is the same
principle as the current regulator diode which is
made out of a JFET and a resistor in series with
the source. Whenever current increases through
the transistor, the extra voltage across the
resistor tends to turn the transistor back off
common
isn't
in
to the emitter follower amplifier
the
in the emitter follower
again.
amplifier, the output voltage is across the load.
Therefore,
when
the transistor is really turned on,
make large voltages when they
In the emitter follower, the result of this feedis that the voltage across the output can
never exceed the voltage across the input. In
other words, the voltage gain of an emitter
follower amplifier is always less than one. On the
other hand, the current gain of an emitter
follower is as good or better than that of a com-
large currents will
back
flow through the load resistor. So with the emitter follower, the output voltage
input voltage is large.
HIGHEST INPUT IMPEDANCE
LOWEST OUTPUT IMPEDANCE
NON-INVERTING OUTPUT
NO VOLTAGE GAIN
HIGH CURRENT GAIN
+
HIGH POWER GAIN
is
large
when
the
mon emitter amplifier. Since the output signal
voltage is about the same as the input voltage,
and the current gain can be very large, 50 or
more, it is still possible to have a large power gain
E
gUPPLY
approaching 50.
2.
/777
The emitter follower (common
And Output Impedance
Emitter followers have the highest input impedance of any of the three basic amplifier configurations. The input current must not only go
through the base current limiting resistor, Ri,
and the transistor base-to-emitter junction, but it
must also go through the load resistor in order to
arrive at the common lead or "ground." These
three impedances add together to produce a very
VOUT
Fig. 7-5
Input
collector
high input impedance.
amplifier)
107
because the base-to-col lector voltage drop can be
very large. Remember that it is the collector-tobase junction where most of the voltage drop occurs. Of all the amplifier configurations the common base configuration has the lowest input impedance and the highest output impedance.
Because the output current and the input current
are essentially the same current, they are obviously in phase with each other.
Finally, emitter followers have the lowest
output impedance of the three basic amplifier
configurations. Like the common emitter ampli
fier, this output impedance depends a great deal
on the resistance in series with the base, Ri- The
smaller this resistance
is,
the more easily the
turned on and the lower the transistor resistance will be. This low output resistance explains why this amplifier is often used
in place of an impedance matching transformer; it
is very good for converting a high voltage, low
current signal into a low voltage, high current
transistor
is
LOWEST INPUT IMPEDANCE
HIGHEST OUTPUT IMPEDANCE
signal.
NON-INVERTING OUTPUT
To summarize, emitter follower amplifiers
have three big advantages over common emitter
amplifiers. They have the highest input impedance and the lowest output impedance of the
three basic amplifier configurations and they do
The
not invert the voltage signal.
VOLTAGE
SUPPLY
+
HIGH VOLTAGE GAIN
NO CURRENT GAIN
HIGH POWER GAIN
chief disadvan-
' '
Rl
>
*\
k
_
/
/ nTn
tage is that they have no voltage gain, only current gain. The output voltage is nearly, but not
quite as large as the input voltage.
VOUT
E.
Common Base
The
Amplifier
This configuration is weird and you may find
become used to However, it is commonly used as the final amplifier stage in modern
radio transmitters, so there is no way to avoid
learning about it.
it
difficult to
We
»
.
rrn
MUST PASS THROUGH THE
INPUT SIGNAL VOLTAGE SOURCE RESISTANCE.
LOAD CURRENT
Fig.
have always talked about the base as be-
ing the control lead that accepts the input signal.
In Fig. 7-6 it looks as though the emitter is serving as the control lead! Actually the base current
is still
determining how
much
The
current flows from
collector to emitter, so the transistor
is
plifier is
that
it
is
behaving
at this
am
the opposite of the emitter
follower in terms of what it does. This amplifier
can be thought of as a voltage step-up transformer with high voltage gain but no current
gain. It also has good power gain because even
though the current gain is less than one, the
voltage gain can be very large.
no way that the
rent can be amplified. Actually,
it
easier
make
to
In spite of the fact that the base
is
is
grounded,
responsible for producing a
small base current which turns the collector-toemitter current on and off. Let's figure out what
kind of input signal would be necessary to make
In this amplifier, the current that flows into
is the same current that flows through
is
circuit in Fig. 7-6 is simplified to
circuit
the emitter current
the input
the load resistor, so there
The common base amplifier (grounded
understand. The load
resistance is connected between the power supply
and collector, just like the common emitter circuit. The base is "common" to both the input and
output and this gives the amplifier its name. This
amplifier is also known as the grounded base
amplifier, which assumes that the base is actually
connected to ground.
the
as before.
Perhaps the simplest way to look
7-6
base amplifier)
the transistor in Fig. 7-6 turn on. Since
N-P-N
cur-
loses a little
transistor, the base
must be
it is
an
positive with
respect to the emitter in order for a base current
current to the base so the current gain is always
less than one. The voltage gain can be very large
we put a big positive input signal on
the emitter, positive to N will not conduct and the
to flow. If
L08
transistor will remain off. However, if the signal
on the emitter is negative with respect to ground,
then the base will be positive with respect to the
transistor emitter.
Base current
will
turn on the transistor. So as the circuit
a negative, or below ground signal
turn on the transistor.
Now
is
base voltage several volts above ground, the transistor will turn on when the input voltage is below
that of the base, but still above the level of
ground. If an input signal varies between the new
base voltage and ground, it can control the transistor completely from full on to full off, provided
of course, that the new base voltage is high
enough. The new base voltage is established by
flow and
is drawn,
needed to
the voltage division across the resistors Ri and
R 2 The capacitor Ci holds the voltage across R 2
look at the output signal voltage
taken off the collector. When the transistor is turned off, the output voltage will be
equal to the supply voltage. But when the transistor begins to turn on, the output voltage will
drop downward. Notice that while the output
voltage is dropping downward, the input voltage
on the emitter is also dropping downward, dropping down below ground, that is. So, even though
which
let's
is
the polarities are disturbing, the voltage signal
not inverted.
.
constant so that the base voltage
tant even
when
input signal voltage,
Now
will
stay cons-
there are sudden changes in the
Vs
.
look at the input and output impedances
common
base amplifier. Like any of the
must complete the
pathway from the supply voltage all the way
down to ground. In this case the current flows
through the load resistor, through the transistor,
and finally must flow through the input signal
of the
amplifiers, the output current
is
SUPPLY VOLTAGE
9 +V CC
source.
In
order
for
a
large
current to flow
through the load resistance, there can not be very
much resistance in the input source, R s because
the current must also pass through that barrier.
This is why the input driving source must have a
low impedance for this circuit to work well. If the
"source" must have a low impedance to transfer
power, then we can reason that the amplifier
must have a low input impedance to receive
power efficiently. Since the source impedance is
added onto the impedance of the transistor and
load resistor, this explains why this circuit has
the highest output impedance of any of the three
,
VOUT
basic amplifier circuits.
F.
rrn
THE CAPACITOR CHARGES
UP TO THE VOLTAGE
ACROSS
R2
ON WHEN
B ELOW
AND
A
practical
V| N
GOES
Often two or more of the basic amplifiers are
to produce a single amplifier with a different combination of characteristics than can be
obtained from one of the three basic amplifiers.
Suppose a common emitter amplifier is needed
with ten times more gain than a single amplifier
can produce. One way to accomplish this is to put
combined
f—
SERVES AS A BATTERY
TO HOLD THE BASE
VOLTAGE ABOVE GROUND
Fig. 7-7
Direct Coupled Transistor Amplifiers
transistor will turn
common
base amplifier
the power supply voltage would require a second,
two common emitter stages in series, like Fig.
7-6. But often the same goal can be achieved with
fewer parts by using a Darlington transistor.
below ground, power supply. The common base
amplifier can be modified so that the transistor
will turn on when the input signal is above
ground. A method of doing this is shown in Fig.
7-7. The network R lf R 2 and Ci can be thought of
as a rechargeable battery which is connected be
tween the base and ground. By establishing the
Two or three transistors can be wired together so that the base of one transistor is driven
by the collector of another. The emitters are wired
together and the result is a device that can be
used like a normal transistor, but has extremely
high gain. Since the collector current of one tran-
Having the input
signal outside the range of
,
109
sistor
becomes the base current
of another,
two or
even three transistors can produce current gains
a high as 10,000 or more. Transistors wired this
way by the factory resemble ordinary transistors
but are called Darlington transistors.
-V cc
*
Q
PN-Ps
>
VWVA
R2
Vout
Vout
J
Y
>
>
Fig. 7-9
COMMON
EMITTER AMPLIFIER
USING A DARLINGTON TRANSISTOR
A
\
Y
N-P-N
P-N-P
COMMON
COMMON
EMITTER
EMITTER
direct coupled amplifier stage using
P-N-P and N-P-N
transistors.
mon
in switching applications where the output
does not have to be high fidelity or linear. The
distortion of an amplifier, its speed as a switch,
the power required to run it, and its frequency
response depend, not just on which transistor
lead is "common," but on where the transistor is
operating on its volt-ampere characteristics.
<2
G.
Basic Field Effect Transistor Amplifiers
Before
we
look
TRIPLE TRANSISTOR
specific purposes,
DARLINGTON
FET
Darlington
transistor
Fig.
7-8
make
a super high gain transistor.
we
at
biasing transistors for
will look at the three basic
amplifiers.
combinations
Fig. 7-10 shows MOSFET transistors, but
they could just as well be junction FETs. We can
summarize by saying that they are like the
bipolar versions, except that the names match
the FET terminals: common source, common
drain, and common gate amplifiers. The characteristics of these amplifiers are also about the
Fig. 7-9 shows an amplifier stage that is a
combination of two common emitter stages in
which one transistor is N-P-N and the other is
P-N-P. This amplifier stage has much higher gain
than a single stage could have and it does not invert the voltage signal. Moreover, it uses fewer
parts than two conventional common emitter
stages would. Many hybrid combinations like
this are possible between any of the three basic
amplifier designs and the use of complementary
transistors. We won't attempt to cover all the
possible combinations, but at least you won't be
surprised when you see a circuit like this in a
diagram. As a rule these hybrids are more com-
same
H.
as the bipolar versions.
Alphabet Classification of Amplifiers
The "alphabet classification" of amplifier
designs is hard to remember, but it is widely used
and does give an indication of the amplifier
linearity and the intended use for the amplifier.
We have already classified transistor amplifiers
110
terms of which lead is common to both the input and output terminals. These common classifications tell us about input and output impedances and current and voltage gains. Another
way to describe amplifiers is by the frequency
range they are designed to amplify. For example,
audio amplifiers generally cover a band of frequencies from about 20 to 20,000 Hz. Video
amplifiers cover a wide band from about 60 Hz up
to about 5 MHz. Radio frequency amplifiers are
designed to amplify frequencies anywhere from a
few kHz up to thousands of MHz. Usually RF
amplifiers just amplify a single frequency, but
sometimes they are designed to amplify wide
bands. Amplifiers which can amplify wide bands
of frequencies are quite linear. That is, if an
amplifier is capable of amplifying many different
frequencies and the relative amplitude of each is
faithfully reproduced at the output, then the
amplifier has low distortion. On the other hand, a
tuned amplifier is designed to amplify just one
frequency and is very non-linear because any
other frequency will be greatly attenuated.
in
rm
COMMON-SOURCE CONFIGURATION FOR MOSFETs
/.
Linearity and Distortion
we go
Before
farther, let's look
rrn
SOURCE FOLLOWER (COMMON DRAIN) CONFIGURATION
Id
SUBSTRATE
Rl
sN\W
more
closely
A
pure sine wave has only a
single frequency. It can be shown by lots of math
and arm waving that any other alternating waveform is composed of more than one frequency. For
example, a violin playing a single note sounds like
a "pure" tone. But if the sound is displayed on a
frequency spectrum analyzer, the sound is actually a combination of several frequencies, some of
which are just as loud as the basic note the
violinist is playing! It is these overtones or harmonics which make a violin sound different from
at signal waveforms.
O +
a piano.
Fourier, a Frenchman, showed that any alternating waveform (AC) can be duplicated by the
sum of a number of pure sinewaves even though it
Vout
take an unlimited number of them to produce a perfect copy of the original. In Fig. 7-11
the note "A" on a violin has 6 vertical bars. The
stereo amplifier must be able to amplify each of
may
\
rm
\_ DRAINTO-SOURCE CURRENT MUST ALSO
FLOW THROUGH INPUT VOLTAGE RESISTANCE,
these six sine
Rs
COMMON GATE CONFIGURATION
Fig.
7-10
Enhancement
MOSFET
wave frequencies accurately
with-
out changing the frequency or relative sizes of
any of them. If the amplifier fails to make a
perfect copy of any of these six signals, the
amplifier is non-linear and is guilty of distortion.
basic ampli-
Notice
fiers.
that
sound
Ill
like
if
the
distortion
were
carefully
would be possible to make
a violin and vice versa!
engineered,
it
a piano
VIOLIN
FREQUENCY SPECTRUM
If
LU
O
o
z
o
s
cc
<
I
_i
a
<
>
<
2O
HI —
SZ
<o
1.0-
a2
z cc
=> <
U. X
,s-
UJ
cc
i
2
cc
<
X
1
CN
1
-1
n
1
440H Z
"A"
IS
35
S
1000
880
tion
of 100.
w
1500
2000
2500
1320
1760
2200
line as is
shown
for a gain
slope of this line (the degree of
equal to the transistor current gain. Distorthe amplified signal will occur to the
that
However,
^*
The
in
degree
1
i
)
tilt) is
s
cc
<
X
i
i
500
o
z
o
.c
;
\
.
o
z
o
2
cc
<
I
1
"O
c
u
z
o
O
z
o
2
CC
<
X
(hfe
the characteristic were perfectly linear, the
would be a straight
plot
if
line is not a straight line.
confine the amplification to the
the
we
range 0.25 to 1.5 milliamperes of base current,
then the collector current will be a good reproduction of the base current, but it will be 120 times
larger.
FREQUENCY-HERTZ
NOTE
How
BEING PLAYED
can the input signal be confined to a
tain range of base current?
PIANO FREQUENCY SPECTRUM
nating current
is
By
symmetrical about the zero
rent axis. It looks as though
we could
alternating current waveform.
<
1.0
The input
wave
signal
can be moved to the center of the operating range
by adding a bias current to the input signal. This
>
is
—
cc
J_
I
A 500
440H Z
"A"
IS
1
1000
1
_L
2000
1500
shown graphically
Fig. 7-11
Class
2500
I
FREQUENCY-HERTZ
NOTE
COLLECTOR WAVEFORM
Frequency spectra of a piano and
A
in Fig. 7-13.
1
BEING PLAYED
J.
cur-
only am-
plify part of the positive half of the sine
a.
cer-
definition alter-
violin
Amplifier
The class A amplifier is an excellent design
high fidelity applications. We saw earlier that
many transistors have volt-ampere characteristics that are quite linear over a certain operating
range. This can be demonstrated by plotting base
current versus collector current. A graph like this
for
is
called the transfer characteristic.
2N3724
Fig. 7-13
NPN SILICON
Class
A
amplifier operation
TRANSISTOR
The bias is added by providing a fixed
amount of bias current that is always present
even when the amplifier is not processing a
signal. In the drawing we have added 0.75
milliampere to the base current. This means that
even when the amplifier is not actually amplifying a signal, there will be a collector current of
about 90 milliamperes. This point is called the
quiescent current. The maximum input signal
BASE CURRENT —
MILLIAMPERES
would be limited to about 0.6 mA peak so that the
signal will stay within the most linear part of the
b
operating range.
Fig. 7-12
Collector current rcrsus base current
112
The disadvantage
sistor
always acts
of Class
A
is
like a resistor
Another feature of capacitor coupling
that the tran-
and always
sipates power. Theoretically, Class A amplifiers
could be as much as 25% efficient when amplifying sine waves. In practice, the maximum useful
power that comes out of the amplifier is rarely
more than 10 or 20% of the power consumed.
With the waveforms shown in Fig. 7-13, the effi-
ciency
is
that
DC
voltage levels. The capacitors will charge to
those levels without changing the average DC
voltages on either side of the capacitor. An
amplifier that couples the AC signal in and out of
the transistor with load resistor and coupling
capacitor
is
called an
RC coupled amplifier.
about 10%.
Class
K.
A
is
the capacitors can be coupled between any two
dis-
class
A common
emitter audio amplifier
always there and
is
made from one
transistor and have a linear re
sponse characteristic (high fidelity) are class A
amplifiers. The disadvantage of class A is that
the transistor is on all the time and wastes power
and generates heat. Class B amplifiers are designed to be more efficient than class A amplifiers.
They are almost always made from pairs of transistors which are biased so that each transistor
only amplifies one half of the AC signal wave
form. The two transistor outputs are then combined again to produce the complete amplified
output. In this way zero collector current can
represent zero current in the original waveform.
Since the base and collector currents are zero
when the input signal is zero, power is not wasted
when the amplifier is not actually passing current. For example, a battery powered radio consumes more battery energy in its audio output
stage than all the rest of the receiver circuitry
combined. A class A amplifier in a battery
powered radio would be high fidelity, but would
drain the batteries very quickly. In practice class
B amplifiers can deliver power to the loudspeaker
(or other load) with up to 50% efficiency.
always turn-
ing the transistor at least part way on. The second resistor, R2, insures that the transistor can
turn off once it has turned on. The audio signal is
coupled into the base by means of the capacitor,
The output signal leaves the amplifier by
means of the capacitor, C2. These capacitors cou-
Ci.
AC current in and out of the amplifier, but be
cause DC cannot pass through capacitors, the
average DC current that biases the amplifier is
not changed by the input signal. The AC coupled
through the capacitor temporarily raises and
lowers the input current as the AC alternates
ple
polarity.
10
VOLTS
LOW RESISTANCE
IS
NEEDED TO PASS THE
MAXIMUM COLLECTOR
CURRENT, =170MA
50Q = Rl
1
2N3724
SILICON
M fd
>
Amplifiers
All bipolar transistor amplifiers which are
is
seen in Fig. 7-14. The bias current (0.75 mA) is
added to the base by means of the resistor R^.
Since Ri is always connected to the positive sup
ply, this bias is
B
Vout
BIG COUPLING
CAPACITORS
ARE NEEDED
TO PASS LOW
AUDIO FREQUENCIES
Each
transistor
in
a
class
B
amplifier
is
operating in the region of low base currents where
linearity can be fair, but is never excellent.
However, both halves of the amplifier are distorting their respective halves of the signal the same
way, but with opposite polarities. It turns out
that most of the distortion is cancelled out when
the two half outputs are recombined to make a
single output signal. The two transistors resem-
*
two men sawing a log with a whip saw. One
pushes while the other pulls. For this reason
ble
these
BASE-TO-EMITTER DC VOLTAGE
DETERMINED PRIMARILY BY
THE SILICON P-N JUNCTION POTENTIAL.
Fig.
7-14
A common
emitter, class A,
amplifiers
are
often
called
push-pull
amplifiers.
IS
RC
common ways to build a class
The transformer design is the way it
used to be done with vacuum tubes and is still
found in transistor output stages. The signal to
There are two
cou-
B
pled audio amplifier
113
amplifier.
be amplified must be split into signals which will
turn on the two transistors during opposite polarity halves of the cycles. This is easily done with
a transformer. The signal to be amplified is fed into the primary. The transformer secondary is center tapped and grounded. Since the polarity of the
signals at the opposite ends of the secondary are
always opposite, the two transistors will be turned on during alternate half cycles. Since the bases
are grounded through the transformer winding,
the average DC current through the bases will
average out to zero. The outputs of the two transistors are again recombined in another transformer. This time the load resistor has been
replaced by the two inductances of the two halves
of the output transformer primary.
The supply
voltage goes to the center of the transformer
primary so that it can supply both transistors
from the same point. Each transistor output current produces a magnetic field in the output
transformer and the two fields combine to produce a single AC waveform which is induced into
the secondary.
A
more modern way of building class B amis to use complimentary transistors to
make a complementary transistor amplifier. This
scheme resembles the CMOS FET logic inverter
plifiers
we
When a signal goes
turns on one transistor but turns the
studied in the last section.
positive,
it
BASES ARE GROUNDED
THROUGH TRANSFORMER
WINDING
t
"PUSH-PULL" TRANSFORMER COUPLED CLASS B AMPLIFIER
BASES ARE GROUNDED
THROUGH RESISTORS
N-PN
T
P-N-P
"PUSH-PULL" COMPLEMENTARY CLASS B AMPLIFIER
Fig. 7-15
Practical class
114
B
amplifiers
COLLECTOR CURRENT WAVEFORM
EXACTLY
Vi
OF THE
—
ORIGINAL WAVEFORM
IS AMPLIFIED
Fig. 7-16
Class
B
amplifier operation
L.
other
off.
When
reverse happens.
med
at one point to
make
C Amplifier
The class C amplifier is very non-linear and is
intended for efficient power amplification where a
powerful signal is needed at a single frequency.
the signal goes negative, the
The two output
Class
signals are sum-
the single output wave-
The two complementary transistors must
be carefully matched in their parameters so that
class
they amplify equally but with opposite polarities.
Even their distortion and non-linearity must be
equal! Manufacturers make special mated pairs
of transistors designed for this purpose.
one where the collector current is zero
for most of the input sine wave cycle. The output
from the transistor is a "train" or series of short,
rounded, current pulses which have a repetition
Final amplifiers in radio transmitters are often
form.
C.
amplifier
The
official
definition
of
a
class
C
is
rate equal to the desired frequency. Since these
pulses are not a perfect sine wave, they contain
many harmonics which are filtered out by the
Push-pull amplifiers can also be designed to
run class A by biasing the bases. Class A pushpulls have the distortion cancelling advantages of
the class B and can exceed a single transistor
Class A amplifier in linearity. Unfortunately they
still waste power because both transistors are
always biased on.
tuned circuit (LC resonant
are so prominent, that
it is
output
harmonics
circuit) in the
circuit of the amplifier. In face, these
possible to
filter
out
the basic frequency and produce useful power at
twice or three times the frequency of the fun-
damental pulse
115
rate!
AMPLIFIER
QUIESCENT
POINT
(CANT BE
LOWER
THAN
ZERO)
TRANSISTOR IS ON
LESS THAN 50 %
OF THE TIME.
Fig. 7-1 7
Class
C
amplifier operation
SUPPLY VOLTAGE
DC AMMETER
(ADJUST Ci FOR
A
SUPPLY
BYPASS
CAPACITOR
^C
3
)
MINIMUM
CURRENT)
\f
V in
IMPEDANCE
MATCHING
TRANSFORMER
Rl
EMITTER
RESISTOR
RF BYPASS
CAPACITOR
Fig. 7-18
A
class
C RF amplifier
116
ANTENNA
LOAD
shows how the
Fig. 7-17
C
class
The
transistor
strongly biased so that the transistor
conducts only during the high positive peaks of
the input RF sine wave. So much for the usual
amplifier
C
class
amplifier expanation.
the transistor
better
way
to look at tuned amplifiers is
tuned circuit in which an oscillation has already been
established. The LC tuned circuit can be compared to a child's swing. The swing moves back
and forth with a natural oscillation frequency
which depends on the length of the rope and the
acceleration
"pump,"
damped out
circuit
of
the
the
gravity.
If
oscillation
the
will
(attenuated)
by
"friction"
is
In the
resistance
dissipates the energy in the oscillation
ting
it
to heat.
An
is
LC
tuned
just like this.
The
must be
sus-
circuit
even
is
turned
off,
in the circuit.
so most of the time
In the
collector is continually varying
large sine
wave even when
up and down
the transistor
LC
which
by conver-
sustain the oscillation with light pushes
whenever the swing reaches the extreme end of
its travel. The pushes on the swing must be
carefully timed. If the adult tries to push when
the swing is coming toward him, the oscillation
be decreased instead of increased. If the
pushes are somewhat out of phase, a large oscillation can still be sustained, but only at the expense
of a great deal of extra energy because much of it
is being wasted.
it is
the input frequency isn't matched to the
If
natural oscillation frequency of the tuned circuit,
the collector voltage and collector current will be
high simultaneously and the power consumed in
the transistor will be very high. When this hap-
COLLECTOR VOLTAGE COLLECTOR VOLTAGE AND CURRENT
CLASS C AMPLIFIER
SINE
IN
A
WAVE
OSCILLATION
IN
LC CIRCUIT
TRANSISTOR TURNS ON
111
-
ONLY WHEN COLLECTOR
VOLTAGE IS VERY LOW
a.
z>
o
cc
O
HO
TIME
LU
^r
]
O
o
_ CURRENT
PULSE
PUSHES LC
OSCILLATOR "SWING"
TIME
AVERAGE OF VOLTAGE
=
AVERAGE OF CURRENT
CONDUCTION ANGLE = 25% OF
Fig. 7-19
in a class
in a
turn-
amplifiers.
will
POWER
is
Whenever the
circuit.
adult pushing the swing can
INPUT
it
same way the
connected to the oscillating LC
collector voltage drops
toward the low side of the sine wave, the transistor turns on briefly and lets a short pulse of
current into the tuned circuit. Because the transistor is on only when the collector-to-emitter
voltage is low, the current flowing through the
dynamic resistance of the transistor is low in accordance with Ohm's law. Since current and voltage are low, the power that is dissipated in the
transistor as waste heat is low. The efficiency of
these amplifiers can be high, 65% or better in real
ed off because
child doesn't
eventually be
friction.
amplifier
adult only contacts the child and swing for a
small part of the swing travel. The voltage on the
LC
that the output circuit contains an
C
tained by carefully timed, short current pulses
flowing through the transistor. Most of the time
isn't
A
class
oscillation in the
is
[
j
E
=j
TIMES
DURING CONDUCTION
360° = 90° IN THIS
EXAMPLE
Graphs of collector voltage and current
C amplifier.
117
pens, the output will
matches the output impedance of the
still be the driving frequency of the pulses but a great deal of energy will be
expended to stifle the natural resonant frequency
of the tuned circuit. In vacuum tube amplifiers
you can actually tune the LC circuit to the driving frequency by watching the anode in the tube
go from red hot when out of resonance to a cool
black when resonance is reached! Transistors are
much more easily destroyed by overheating and
this is why vacuum tube amplifiers are still found
in transmitter output stages, especially in amateur radio transmitters and large broadcast sta-
All the basic features of a common emitter
Class C amplifier are illustrated in Fig. 7-18. In
the output circuit the LC resonant circuit consists of Li and Ci. Ci is tunable so that the reso-
tions.
mum
The proper indicator for tuning a class C
is a DC ammeter in series with the collector power supply. The resonant frequency of
the L-C circuit is adjusted for minimum average
current flowing into the collector. Even though
Like any amplifier, a transmitter output
stage should match the impedance of its load. In
Fig. 7-18 the load is a transmitting antenna and
these generally have an impedance between 50
and 300 ohms. This impedance is usually entirely
3.
is
actually a series of short
the average of these pulses
DC pulses,
and the
Our drawings of voltage waveforms on the
C amplifiers has probably given
AC
you the idea that it is practical to look at these
waveforms with an oscilloscope with RF amplifiers. First, large transmitters often have over
500 volts on the plates of vacuum tubes and your
voltage.
was
altered.
Now
is
also tuned to the
is matched to the transistor impedance by means of an impedance matching transformer. The inductor Li
is part of the L-C resonant circuit but is also the
primary of the impedance matching transformer.
It is important to realize that any change in the
impedance of the antenna will reflect back into
the resonant circuit and change the resonant frequency. For example, suppose part of the antenna
were broken off or damaged by a windstorm. The
antenna resonant frequency will change and it
will behave more like an inductor or capacitor
than it did before, depending on how the antenna
may not be able to tolerate that. The
second problem is that the probe often has a
significant capacitance with respect to ground.
This can range from 10 pf to 100 pf. If you put
this on the collector of a VHF RF amplifier or a
mixer in a high frequency receiver, this capacitance will add to the capacitance in the LC filter
and detune the circuit. If you do manage to get
the amplifier tuned with the scope probe in place,
it will no longer be tuned as soon as you remove
the probe. A low frequency receiver, such as an
ADF, may not be significantly affected by this.
amplifier
is
In Fig. 7-18 the antenna impedance
oscilloscope
C
antenna
way, the antenna AC current does not lag or lead
the antenna AC voltage, but behaves like a resistor in which the current is in phase with the
collectors of class
Class
circuit is properly tuned.
operating frequency. The antenna is resistive
because energy sent out into a properly tuned
antenna leaves the antenna permanently. The
antenna energy does not reflect back into the collector circuit as it would if the antenna behaved
like an inductor or a capacitor. Saying it another
final."
in
LC
resistive because the
is
the collector circuitry
often an
nant circuit can be tuned to the repetition rate of
the current pulses from the collector. The ampere
meter in between the LC circuit and the power
supply indicates when the DC current is mini-
seen as a steady
drain of DC current from the power supply. In
order to tune the amplifier, the capacitor Ci is adjusted for minimum DC collector current. The
meter needle dips sharply when resonance is
reached so this process is called "dipping the
In summary'
is
antenna.
amplifier
the current
It
amplifier with the load which
the
let's
look at the input side of the circuit
why the amplifier operates in
in Fig. 7-18 to see
designed to meet three goals:
the class
C mode. The
input circuit design has at
least three goals:
1.
It
filters or
means
of
an
resonates the output signal by
resonant circuit.
LC
1.
Even though
the input signal to the tranbe a sine wave, the sine wave
signal must turn on the transistor only during the highest, above ground voltage peaks
sistor
2.
It
indicates
when
ed by means of a
the resonant circuit
DC
is
tun-
current meter.
118
may
of the sine wave. For an
N-P-N transistor
Goal number three, not sacrificing the potenoutput power of the transistor, is achieved by
capacitor C 2 If C 2 were omitted, the amplifier
would have two "loads;" the antenna, which is
where we want the power to go, and the emitter
resistor which gets hot and wastes useful power.
The capacitor holds the average emitter to
ground voltage down so that more of the supply
voltage can appear across the load and less appears across the resistor during the current
these are the positive peaks. For a P-N-P transistor these are the negative peaks.
tial
.
2.
The input circuit should not waste a large
amount of power to drive it. That is, the input
current peaks should go into the base and not
heat up resistors.
3.
The biasing system should decrease the
potential output power of the amplifier stage
as
little
pulses.
as possible.
In Fig. 7-18 the base is connected to ground
by a radio frequency choke inductor (RFC). From
the point of view of the
RFC
RF
The power supply by-pass
capacitor, C3, has a
holds the voltage across
the power supply and meter constant so that
large pulses of current will not decrease the
voltage across the transistor and LC circuit. The
similar function to
input driving signal,
a very high impedance, so the base
looks like it is isolated from ground. In other
the
is
words, the
sine
wave
RFC
doesn't attenuate the input
RF
goal.
current, the base
ground.
Goal number one, turning on the transistor
is accomplished by the
emitter resistor, R^ The current pulses going
through Rx are all in one direction. The voltage
across Ri is a sort of pulsating DC that resembles
the output of an unfiltered DC half wave power
supply of the kind we studied in Section 2. Just
like the half-wave power supplies, the DC can be
made more continuous by putting a large capacitor across the load. In this case the "load" is R\.
Now we have a fixed, DC voltage difference between the emitter and ground. The input voltage
must be greater than this voltage in order to turn
on the transistor. By using the correct resistance
for Rj, the transistor will turn on only during the
voltage peaks. And, because the emitter to
ground voltage depends on the collector current,
just during voltage peaks,
Bear
But
will
mind that
C
Fig. 7-18
amplifier.
Now
is just one examthat we have you
checked out on emitter resistor biasing, I hate to
mention that sometimes the emitter resistor is
omitted. In some amplifiers the base-toemitteremitter junction voltage provides enough
voltage offset to make the transistor operate in
class C without the emitter resistor. Often the
emitter resistance
no need to bypass
C2. Class
C
VMOS FETs
is
so small, 0.05 ohms, there is
with the emitter capacitor,
it
amplifiers can also be built from
in
common
gate or
common
source
They can have "pi" or "T" output
impedance matching networks. They can also be
very large input signal will try to turn
on the transistor over most of the input sine wave
which
in
ple of a class
A
cycle.
It
wires.
this "bias" voltage will adjust itself to the input
signal.
.
meter and power supply have inductance and resistance in the wires that connect the circuit
together. This is part of the voltage source impedance which limits the maximum current
pulses that the supply can deliver. Putting a bypass capacitor across the supply averages the
current from the power supply. Now much of the
peak current in the pulses can come from the
capacitor which recharges during the time intervals when the transistor is turned off. The bypass capacitor must be as physically close to the
LC circuit as possible, otherwise it will not
"bypass" the resistance and inductance in the
signal to the base. This satisfies the se-
However, from the point of view of DC
is grounded, through the radio
frequency choke. After all, chokes are just a coil
of copper wire. This means that any voltage on
the base which drives current into the base must
be greater than whatever DC voltage difference
there happens to be between the emitter and
cond
C2
configuration.
biased by a separate power supply, like grandfather's "C batteries" in his 1920 vacuum tube
radio. The only feature that all class C amplifiers
have in common is that the transistors or tubes
turn on less than 50% of the sine wave cycle.
this will increase the collector current
make
a bigger emitter-to-ground voltage that the input sine wave voltage will have to
overcome. This is negative feedback just as we
discussed earlier for emitter follower amplifiers.
119
M.
Class
We
AB Amplifiers
could have covered this before class C,
we
if
AB
amplifiers are useful. Class
tuned amplifiers,
much
C
O.
Class
E Amplifiers
Class
E
amplifier.
between class
A
and
when driven by a sine wave, the
conducts somewhere between 50% and
so that
transistor
100%
class
AB amplifiers are
like the class
are biased halfway
B
class
why
had, you wouldn't understand
but
They
tremely non-linear and can't be used directly for
generating RF power or linear amplification of
analog signals.
amplifiers are a relatively
new
inven-
and are not widely used. They can be
most of the applications where class C
tion (1970)
used
of the cycle.
in
amplifiers
are
used.
Class
E
amplifiers
are
a
tuned, non-linear, amplifier with most of features
AB amplifiers are more efficient than
but less efficient than class B or C. Depending on the percentage of time the transistor
Class
class
is
on, a typical class
25%
of a class
A
efficiency.
AB
amplifier might achieve
The advantage
of class
AB
is
that
can operate over a fairly wide frequency range
with fair efficiency without being retuned every
time the frequency is shifted. Generally a radio
transmitter has several stages of amplification
between the oscillator which generates the basic
frequency and the final amplifier which drives the
antenna. Most of the power consumed by the
transmitter is consumed by the final amplifier, so
it is most important that this stage be as efficient
as possible. The low power "buffer" stages can be
class AB or even class A with little loss in overall
efficiency and a great gain in engineering and tuning convenience.
it
AB
amplifier
75%,
it
collector-to-emitter voltage
is
when
the
On
the
Unlike the class
if the tuned output is off resonance, it
not greatly reduce the efficiency because the
transistor was turned on most of the time
will
anyway.
Class
D Amplifiers
You have
already met the class
D
amplifier.
simply the use of a power transistor for a
switch. In other words, the transistor is either
turned full off or full on. Since the output is a
square wave, very little time is spent with current
flowing through the transistor, while significant
It is
voltage
is
across the transistor.
The
efficiency
can approach 100';, but this "amplifier"
is
for a class
a square
E am-
wave so
C
amplifier, the voltage across
not just "low" when the transistor is on, the transistor is turned full on and is
saturated, or on the verge of saturation, the entire time that current is passing through the transistor. The transistor current is used to "charge
up" the inductor, Li. In this respect the transistor resembles the breaker points in an automobile ignition system. Since the current passing
through an inductor cannot change instantly, the
current slowly increases over time as the transistor switch is kept closed. When the transistor
switch opens, the inductor will try to maintain
the current flow by producing a huge voltage.
This voltage appears across C\ and the diode.
This capacitor prevents the voltage from damaging the transistor, just the way that a capacitor
prevents much of the sparking across the breaker
points in an automobile ignition. The diode
prevents a backward bias voltage from appearing
across the transistor from emitter to base.
Remember that the emitter to base junction is
easily broken down by reverse voltages. The
energy that was stored in the inductor, Lj. Li and
Ci form the tuned circuit and oscillate at the
desired frequency. The repetition rate (frequency)
of the square wave driving the transistor base
must be exactly tuned to the L[ — C\ resonance.
It it is not, the transistor will come out of saturation and dissipate large amounts of power.
the transistor
other hand,
N.
is
that the output transistor operates as a switch.
is
rather high.
However, the transistor
shows a circuit
The base is driven by
Fig. 7-20
plifier.
relies
of the cycle, including parts of the cycle
amplifier.
destroyed.
turned on over
on the tuned circuit to produce only 25% of the output sine wave.
If the output circuit is perfectly tuned to the input sine wave frequency, the efficiency will be
less than that of a class C stage because current is
still flowing through the transistor during most
Since the class
half the cycle, say
C
turned fully on and off like a class D amplifier.
Because the transistor turns on and off like a
switch, the efficiency can approach 100% in real
amplifiers. Tiny transistors can deliver hundreds
of watts with this design. Of course, the catch is,
that if the amplifier ever goes out of tune, the low
power dissipation transistor would be instantly
ex-
120
is
Vcc
>
+
SUPPLY
VOLTAGE
SUPPLY
BYPASS
COUPLING
CAPACITOR
Vout
SINE
WAVE OUTPUT
SQUARE WAVE
INPUT
IMPEDANCEMATCHING
TRANSFORMER
Fig. 7-20
The energy
cuit
the
is
transferred from the tuned
load
by means
Class
E RF amplifier
P.
of
the
CLASS
A
AB
Biasing transistors establishes the resting
state of the transistor when it is not amplifying a
signal. The following is not intended to be a complete course on biasing transistors. There are so
many kinds of transistors and so many classes
common
no
half turned
1/4
on
turned on
B
turned off
C
turned off so far that a large signal
to
overcome the bias and
turn
is
it
on.
D
turned
off,
like
Class B or C. Some-
times transistor rests
and configurations of amplifiers, it isn't practical
to give more than a general outline. However,
a
is
TRANSISTOR RESTING STATE
needed
is
there
AMPLIFIER
Biasing Transistor Amplifiers
there
when
state of the transistor should be
signal to amplify.
cir-
coupling
capacitor C2 and the transformer TV C2 is just a
coupling capacitor that prevents DC current from
the power supply from traveling directly to
ground. Without C2, you could not turn on the
amplifier without burning out Li, Tj, the power
supply, or all three! Ti serves the same purpose
as the transformer in our Class C amplifier circuit. It matches the impedance of the load, Rl,
with the collector circuit as a whole.
to
E
Like
B
on.
or C.
The second consideration
pattern to transistor biasing.
full
for
establishing
you can get the general idea, you will usually be
able to figure out what you need to know.
particular type are alike.
There are two considerations in setting up
the operating point of a transistor. First, the
class of the amplifier determines what the resting
manufactured, they are made in big batches of a
general class. Then they are sorted out into
groups of each particular type, say 2N2222,
2N2148, and so on. The type number is assigned
these resting points
If
121
is
that not
all
When
transistors of a
transistors are
Rl
*
>
TRANSISTOR
FULL ON
>
>-
>
COMMON
EMITTER
AMPLIFIER
Ij^Vc
VALUES OF BASE
CURRENT (OR GATE
VOLTAGE)
TRANSISTOR
FULL OFF
(COLLECTOR OR
DRAIN VOLTAGE)
Vcc
SUPPLY
VOLTAGE
CLASS B
BIAS POINT
Fig. 7-21
Load
line
drawn on
transistor voltampere characteristic
on the basis of current gain, leakage current and
so on. Within each type number there is still
variation, so a good biasing system must adapt to
the differences. A related problem is that a hot
transistor is easier to turn on than a cold transistor. Negative feedback is used to make the circuit behave the same even when transistors differ
or are overheated.
We
the quiescent point
first.
resistance,
out of the
Rl
and the transistor
circuit. Therefore,
is
essentially
the output voltage
across the transistor will be the supply voltage
and the current through the transistor will be viris plotted on the voltage
line where current is zero and is seen on the lower
right hand corner of the curves.
tually zero. This point
will look at establishing
When
the transistor turns
full
on,
its
sistance approaches zero and the current
Biasing transistors is most easily visualized
with a load line. The load line is just a straight
line drawn on a transistor volt-ampere family of
curves to show where the transistor will operate
in a particular circuit. Referring to Fig. 7-21, we
can plot the load line by looking at the extremes,
which are turned full on and turned full off. To
avoid confusion, let"s just think about a common
emitter amplifier. When the transistor is turned
off, the output of the common emitter amplifier is
connected to the power supply through the load
large as
it
transistor
can become.
How
large can
had zero resistance, the
it
be?
re-
is
as
If
the
maximum
cur-
rent that could flow would be the supply voltage
divided by the load resistance, Rl-
Maximum
transistor current
maximum
= V cc Rl
/
is plotted on the colwhere transistor voltage is
zero at the upper left. Now we have the ends of
our load line plotted and all the other possible
This
lector current
122
axis
current
Remember
points of transistor operation are located on this
line. If
you want
use a different load resistor,
ply voltage, or both.
Q.
Biasing Class
A
FET
you must
a different power sup-
to operate off this line,
A
that with depletion type MOSwhen the gate voltage is zero,
transistors,
approximately half on, so biasing
MOSFET amplifier can be
easy. Just connect the gate to the source with a
resistor. The resistance must be high enough so
that it is not too much load for the input voltage.
the transistor
a class
Amplifiers
A
is
depletion
NEGATIVE SUPPLY
have the transistor biased dead center on the load line. This
means that when no signal is on the amplifier, a
base current or gate voltage must be set up so
Class
amplifiers usually
9 -v dd
Vgale
turned half on. To figure out
what gate voltage or base current is needed, you
just read the base current or gate voltage that
corresponds to the center of the load line.
that the transistor
is
(DC)
2
VOLTS
>
(ENHANCEMENT
>
PCHANNEL
MOSFET)
Vout
NEGATIVE SUPPLY
>
Id
*
Rl
/777
Vgate(DC).
IS
ZERO
*
>
2 VOLTS FOR
HALF TURNED ON'
'out
DEPLETION^
PCHANNEL
MOSFET
J
GATE
>
>
VOLTAGE
/777
Fig.
7-23
class
A
Biasing an enhancement
MOSFET
amplifier
A enhancement type
not so easy because its
gate voltage characteristic requires that an above
ground voltage (negative in the illustration) be
placed on the gate to turn it half on. This voltage
can be provided by two resistors in a voltage
divider network.
Biasing
MOSFET
a
class
amplifier
is
-v d
QUIESCENT
POINT
a class A bipolar transistor amneeds extra base current to turn the
transistor half on when the transistor is resting.
This extra current can also be provided by a sim-
As you know,
V UT WILL TRAVEL UP AND DOWN ON
THE LOAD LINE, BUT WILL REST AT
THE QUIESCENT POINT.
Fig. 7-22
Biasing a depletion
plifier also
MOSFET class A
ple pair of resistors in a voltage divider.
amplifier
123
One
con-
increases, the gain of the transistor will increase
nected to the supply voltage turns the transistor
half on. the second makes sure the transistor has
a way to turn off.
will turn more "on." As the
moves up the load line to the left,
the transistor will draw more current and dissipate more heat. As the heat rises, so does the
and the transistor
quiescent point
Now that we have the proper bias current
entering the base, what will happen if the transistor temperature increases? As the temperature
temperature which will turn the transistor on
even more. If this process continues, it is possible
that the transistor will turn full on and may burn
NEGATIVE VOLTAGE
itself up.
This calamity
is
is
called thermal runaway. This
a very big deal with
germanium
BASE
because germanium semiconductor
CURRENT
perature sensitive. It
(DC)
much
is
less a
transistors
very tem-
is
problem with
is
silicon transistors,
-3M a
but
it is still
necessary to take
precautions. In Fig. 7-24 a relatively small emitter resistance is added to the class A amplifier in
order to make the quiescent point more stable.
Vou.
EMITTER RESISTOR
PROVIDES NEGATIVE
The emitter
resistor provides a small amount of
negative feedback to turn the transistor back
toward the center of the load line when rising
temperature makes it drift upward to the left.
FEEDBACK TO HOLD
QUIESCENT POINT
CONSTANT
EMITTER RESISTOR
COUPLING CAPACITOR
ADMITS AC SIGNAL
BUT DOES NOT CHANGE
DC BIAS VOLTAGE
>
The emitter
resistor by-pass capacitor
and holds the voltage across the emitter
BYPASS CAPACITOR
HOLDS EMITTER TO
GROUND VOLTAGE
is
large
resistor
quite constant. In this way the feedback voltage
does not respond to the AC signal that is being
amplified. The feedback voltage responds only to
slow changes that try to raise or lower the quiescent point. Some of the circuit gain is lost by this
CONSTANT RELATIVE
V, n WHICH
CHANGES RAPIDLY
TO
it makes the circuit behave
more consistently over temperature extremes.
When it is mass produced, the circuit gain will be
more consistent than the gains of the individual
feedback. However,
the circuit.
Small emitter
common when
transistors
two
or
help to
BASE
r CURRENTS
used
resistors like this are especially
in
more transistors
make
are run in parallel.
They
the transistors turn on equally even
though one transistor may have more gain than
another.
temperature compensated JFET class A
is seen in Fig. 7-25. Remember that
JFETs require large voltages below the source
voltage to turn them off. Zero volts between gate
and source will turn them almost full on. Gee
Whiz! This circuit is identical to Fig. 7-24! The
A
amplifier
QUIESCENT
differences between the
POINT
of the various resistors. Rj,
AND R 2 PROVIDE A
VOLTAGE WHICH PRODUCES A
BASE CURRENT OF - 3M a AFTER
THE FEEDBACK FROM R E IS
TAKEN INTO ACCOUNT
7-24
Biasing a bipolar transistor class
circuits are the sizes
RL
>.
R3.
large voltage below the source voltage
Ri
Fig.
two
the
source
resistor,
R3.
must
Because a
is
have
needed,
a
large
develop the large voltage. This
below-the-source voltage should be at least large
enough to turn off the transistor. The quiescent
gate voltage for class A operation is also below
resistance
A
amplifier with negative feedback.
L24
to
the source voltage, but
it
between the
still lies
R.
supply voltage and ground. It can be firmly established by a pair of voltage dividing resistors,
Ri and R2, where Ri is very much larger than R 2
Notice that if a bipolar transistor were plugged
into this circuit, it would turn off so completely
that it would be operating class C, not class A. In
shown
And Dynamic
Amplifier
in Figs. 7-24
and
DC
7-25 could be biased to operate in classes A,
AB,
Examples include the emitter (and
B
circuits
(one-half of a class
B
amplifier) or
C with any
currents that establish the quiescent point.
source) re-
by-pass capacitors and input coupling
capacitors seen in Figs. 7-24 and 7-25. The radio
frequency choke input and the LC resonant cirsistor
type of transistor. However, the values of Rj, R2
and R3 must be carefully selected in each case.
and the impedance matching and
cuit in Fig. 7-18
NEGATIVE SUPPLY
signal inverting transformers in Fig.
VOLTAGE
INPUT
Char-
Throughout this section there have been examples of capacitors and inductors in the circuits
that behave differently for the rapidly changing
AC signal than they do for the slowly changing
.
summary, the
Static
acteristics
COUPLING
CAPACITOR
7-15
other examples.
Even
transistor
itself
depend on whether you
discussing
DC
or
AC
are
the characteristics of a
are
parameters. For example,
static transistor gain and dynamic transistor
gain are usually listed separately in transistor
SOURCE
even though they are usually very
Because the components of an amplifier
appear differently to DC and AC currents, it is as
though there were two different amplifier circuits
RESISTOR
in one. It is all the
specifications,
Vout
similar.
>
BYPASS
CAPACITOR
considered separately.
>
circuit of course,
but
To
calculate the
DC
bias
sometimes redraw an amplifier
circuit without any inductors or capacitors.
Capacitors are simply left out and inductors
point, engineers
fjfy GROUND
GATE TO SOURCE DC
become ordinary wires. To consider the AC operation, DC components like the power supply are
conveniently left out and the transistor becomes
an imaginary AC generator. If the power supply
is "left out" of the circuit and becomes a wire, the
VOLTAGE IS +2 VOLTS.
IS "BELOW" THE
SOURCE VOLTAGE IN THE
THIS
NEGATIVE SENSE. E.G.,
IS EVEN MORE
"BELOW" THE SOURCE
VOLTAGE, SAY +6 VOLTS
GROUND
-Id
same
sometimes the behavior of an amplifier can be
better understood if the DC and AC aspects are
emitter follower configuration really does have
the collector common to input and output. How
useful these procedures are for technicians is an
open question, but it is important to know the dif-
PCHANNEL
JFET CHARACTERISTICS
ference between static
and dynamic amplifier
characteristics.
QUESTIONS:
GATE
VOLTAGE
1.
2.
In order to transfer power efficiently from
a voltage source into a resistor load, what
must be true about the voltage source and
the load resistance?
Suppose the resistance
of the load is not
given voltage source. Is
there anything that can be done to optimize the transfer of power? Will your
answer to this question work with all
appropriate for a
Fig.
7-25
Temperature compensated class
JFET amplifier.
A
125
voltage sources?
3.
In each of the three basic bipolar transistor
amplifiers, what is the relationship be-
8.
What does
the transfer characteristic of a transistor
have to do with the amplifier distortion
tween input resistance and output resistance? In other words, if you change one,
what will be the effect on the other?
a class
A
classes
AB, C and E?
the
in
What about
there any way to
amplifier?
Is
for distortion that is built into
transistor
used
B
or class
compensate
What
the degree of straightness of
when
characteristic
it
is
in a linear amplifier?
amplifier configuration can be used
in place of a voltage step-up transformer?
What
A
amplifier configuration can be used
B
push-pull class
amplifier
is
built
complementary P-N-P and N-P-N
in place of a current step-up-transformer?
tors.
What
sistors
from
transis-
properties should these tran-
have
in order to
minimize
distor-
tion?
In Fig. 7-4 one
common emitter amplifier is
shown driving
another. Since the load
re-
RLi. is not the load where we wish
to deliver power, it follows that the resistor
RLj should have the maximum resistance
so it will not waste power. What two factors determine this maximum resistance?
Why is a 200 Megohm resistor unlikely to
10.
sistor,
work
for
A class C
amplifier
DC ammeter
is
tuned by observing a
in the collector circuit.
What
adjusted to tune the amplifier? What
happens if the amplifier is not tuned?
is
RLi?
11.
Explain the analogy between the class C
amplifier and an adult pushing a child on a
swing.
A
large
power transistor when wired
in the
common
emitter configuration has an output impedance of 10 ohms. Now suppose
that it is desirable to use this transistor in
a class
C
amplifier to drive an antenna
12.
ohm
impedance. Would
another amplifier configuration be more
desirable for driving the antenna? Which
one and why? Does this change the input
requirements?
which has a 300
Suppose you
are building an amateur radio
transmitter with which you plan to change
frequencies frequently. Why are classes C
or E undesirable for this application? What
classes could
must you pay
class
C
it
preferable
A
to
in the
A class D amplifier is used as a switch to
turn on a powerful solenoid. The solenoid
in turn controls a hydraulic piston in automatic braking system for aircraft landing
Four large transistors in parallel
Each transistor has its
own resistor in the emitter circuit. Why do
you suppose that these resistors are necesgear.
drive the solenoid.
you use and what penalties
for your decision not to use
or class
use inefficient
low power
amplifier stages of a powerful transmitter?
is
amplifiers like class
13.
7.
Why
E?
sary?
126
HYDRAULIC
FLUID TO
BRAKES
PISTON
WHEN TRANSISTORS
FAIL THEY USUALLY BECOME
SHORT CIRCUITS. SO. FUSES IN SERIES WITH
COLLECTORS ALLOW A TRANSISTOR TO FAIL AND
TAKE ITSELF OUT OF THE CIRCUIT.
FOR CONVENIENCE
BASE LINES ARE
SOMETIMES DRAWN
PASSING THROUGH
THE TRANSISTOR
BRAKE ON
BRAKE
OFF
BRAKE OFF
EQUAL EMITTER
RESISTORS
4
14.
Referring to Fig. 7-21, where on the load
line is the bias point for class
C
17.
A
transistor
Suppose you wish
to bias a class
B am-
using a set of volt-ampere charand a load line. Outline the
steps you would follow using any transistor. Why is your amplifier likely to need extra power supplies if you use JFETs or
depletion type MOSFETs?
circuit in the
drawing
is
a class
A
made from an enhancement
MOSFET. What is the advantage of conamplifier
bad
necting Ri to the drain instead of to the
positive
power supply as
it
was
in Fig.
7-23?
Why?
thermal runaway? When is it likely to happen and what can be done in the
design of amplifiers to prevent it and
lessen its consequences?
19.
What
is
the difference between static and
dynamic amplifier
—MM
Vin
idea.
is
id
>
I
/ENHANCEMENT^
NCHANNEL
\MOSFET
J
Vout
20.
characteristics?
You have been asked
to build
a
power
amplifier to drive a heater to keep a bush
pilot's feet warm in darkest Alaska. A
block
diagram of the heater
shown. The
R2
four individual tran-
What
R1
>-He
made from
18.
+ SUPPLY
RL
save
This single Darlington transistor
has a gain of 10,000,000. They plan to build
a class A circuit like Fig. 7-24 but using the
Darlington transistor. Vj n will come directly from the needle cartridge (a kind of
microphone) on the tone arm of the record
turntable. The output of the transistor will
be a loudspeaker in place of Rl- Theoretically, the amplifier can be made to work
beautifully. However, this design is a very
acteristics
The
to
sistors.
plifier
16.
plans
building an entire monaural high
fidelity amplifier from a single Darlington
Explain your answer.
15.
manufacturer
stereo
money by
amplifiers?
off
circuit
is
resistive heater elements run
the aircraft battery and are tied to
at one end. The heater elements are
ground
>
>, 27
built into the pilot's boots.
Together they
have a very low resistance and require a
DC current to run them. The temperature is controlled by thermistors built
into the boots. The temperature signal
from the thermistors goes to a feedback
amplifier circuit that compares the thermistor voltage with a comparison voltage
that represents the proper temperature for
cozy feet, the output of this circuit is a
positive voltage signal which tells the
amplifier when to turn on the heater. Draw
a circuit diagram of your amplifier
(everything inside the square) and specify
the general type of transistor, the class of
large
the amplifier, the amplifier configuration,
and the kind of voltage input signal your
needs
amplifier
Remember
pilot-feet,
that
not
(AC,
we
DC
pulses,
etc.)
are interested in
warm
A
basic
warm
transistors.
principle from Section 5
may
be useful.
COMPARISON VOLTAGE
9 +12 VOLTS
FEED BACK
AMPLIFIER
CIRCUIT
TEMPERATURE
CONTROL
SIGNAL TURNS
ON HEATER
POWER
AMPLIFIER
YOU MUST
DESIGN
/777
VOLTAGE REPRESENTS
ACTUAL TEMPERATURE
"K
RESISTANCE HEATER
IN PILOT'S
THERMISTOR MEASURESTEMPERATURE OF PILOT'S
FOOT.
128
S
V
BOOT
SECTION VIII
Sine
Introduction
A.
wave
Sine
toward the center of the bottom of
the trough and will try to remain there. In other
oscillators
are
receivers as local oscillators.
basic
words, the center of the trough is stable. In contrast, if we roll the ball up onto the side of the
trough and release it, it will always roll back
toward the center because the ball is unstable on
radio
to
They
are used in
transmitters to generate the basic operating
the sides of the trough.
fre-
quency signal which is then amplified to produce
the high power signal which is sent to the antenna. Transistor (and
vacuum
variation of the amplifiers
trough,
we discussed
will
in the
Tunnel diode oscillators are a form of
which uses some of the same
principles we are going to cover in this chapter. It
may be useful for you to review that portion of
Section
we
If
tube) oscillators are a
not only
overshoot and
forth until
oscillator
the ball on the side of the
release
it will
trough. In fact,
last section.
wave
Oscillators
ball will roll
transmitters and superhetrodyne radio receivers.
Sine wave oscillators are used in superhetrodyne
sine
Wave
friction
all
will
down
up the
ball
The
comes
friction
called
it
is
roll
back and
dissipated by
to rest in the center of
that degrades or dis-
sipates the amplitude of a sine
3.
to the center,
far side of the
continue to
the original energy
and the
the trough.
it
roll
roll
wave
oscillation is
damping.
UNSTABLE
POINTS
h/ \5
A TENNIS BALL
PLACED ON THE
Now, instead of looking at a cross section of
the round-bottomed trough, we will look at a long
ROUND
BOTTOMED TROUGH
SIDES OF A
section of the trough as viewed from above. To
complete the picture we will incline the trough
slightly downhill to the right. Now when we release the ball up on the side of the trough, it not
only rolls back and forth through the stable point
WILL ROLL TO THE
BOTTOM OF THE TROUGH
V
STABLE
POINT
trough, downhill to the right.
Fig. 8-1
B all-in-trough
use a mechanical sine wave generator
you to sine wave oscillators. Picture
a large, round-bottomed concrete ditch or metal
trough. A tennis ball is placed on the edge or side
to introduce
of the trough
along the
of the ball
and allowed to
roll
down
into
it.
it
seen from above is a sine wave. After the ball has
traveled a distance down the trough, the amplitude of the sine wave cycles will begin to die out
due to friction. This happens in any practical
oscillation system. The only way to keep the sine
wave amplitude constant is to inject a little energy into each sine wave cycle to keep the energy of
the ball or other oscillating system constant.
analogy for a sine wave
oscillator.
We will
rolls
The path
in the center of the trough,
The
129
BOTTOM
TROUGH
LINE
BALL ROLLS DOWN TROUGH FROM
LEFT TO RIGHT WHILE IT OSCILLATES
OF
IS
BACK AND FORTH ACROSS THE TROUGH
QUIESCENT POINT
DOWN
HILL
END OF
TROUGH
BALL RELEASED HERE
A
Fig. 8-2
ball rolled
down
a trough can
Most
A
sine
the center of the load
is in
wave
oscillators are
amplifiers which amplify their
line.
based on class
own
seconds to complete a single cycle. The lesson
from this is that a sine wave oscillator must have
two basic parts: an amplifier with positive feedback and a resonator or filter which determines
the frequency of the sine waves that will be produced.
the amplifier, an electronic oscillation would be
wiring of the
wave as seen from above.
culvert 20 feet across, the ball will take several
outputs.
The amplifier adds a little energy to each sine
wave cycle to keep the oscillation energy constant
and the sine wave amplitude uniform. Without
damped out very quickly by
a sine
back and forth across the trough are controlled
by the acceleration of gravity and the physical
size of the trough. If the trough were only two
feet wide, the ball might roll back and forth twice
a second. However, if we roll a bowling ball into a
Hopefully, gravity pulling the ball back
toward the center of the bottom of the trough
reminds you of a class A amplifier in which the
quiescent point
make
the resistance in the
circuit.
An amplifier which amplifies its own output
an example of positive feedback. We have
already discussed examples of negative feedback
in which some of the output is inverted and fed
back into the input in a way that decreases the
amplifier gain. That is, negative feedback makes
the output signal smaller than it would be with-
The Phase Shift Oscillator
B.
is
the resonator or
let's
it,
amplifier
first
is
means that
out the feedback. In positive feedback, some of
the output is fed back into the input to make the
amplifier output larger instead of smaller. In the
case of sine wave oscillators, it is this increase in
gain that compensates for energy losses in the circuit and keeps the sine wave amplitude high.
filter
we just spoke
of.
look at the amplifier
a
it is
voltage
explain
The
type,
which
common
emitter
inverting
almost always a
To
itself.
common
source design. Since the amplifier inverts the input signal, the output of the amplifier
or
go down whenever the input goes up in voltwere used so that the output were directly coupled to the input, this would
be negative feedback. This is because the feedback is the opposite polarity of the input. In order
for this to be positive feedback and reinforce the
input, the output signal must be inverted before
it is fed back into the input. Now when the input
signal goes up, the feedback signal will go up too.
will
age. If direct feedback
wave generator has no
feedback to maintain the oscillations,
but it does have an oscillating system that is comparable to an LC tuned circuit or other electrical
resonator. The frequency of the ball's oscillations
The
shows a block diagram of a phase
The "tuned 180° phase shift" is
Fig. 8-3
shift oscillator.
ball-in-trough sine
positive
130
+ FEEDBACK
CLASS
A
AMPLIFIER
TUNED
180°
PHASE
THE TRIANGLE
SHIFT
IS
A GENERAL SYMBOL
FOR "AMPLIFIER."
THE LITTLE CIRCLE
MEANS THAT IT
INVERTS THE VOLTAGE
>
SINE
Fig. 8-3
Sine wave oscillator block diagram
wave by
Inverting the output signal is one of the purposes of the 180° phase shift. It takes sine waves
and "turns them upside down." What it really
does is delay the sine wave 1/2 cycle so that the input to the amplifier and the output of the phase
shift network can go up simultaneously and go
down simultaneously.
You
didn't
are probably saying to yourself, "If
want an inverted
signal,
why
didn't
steps,
must compensate
phase shift by re-inverting the signal.
180°,
it
must be done
RC
60° with each
shifting characteristic of an
in at least three
circuit.
RC
The phase
circuit is frequen-
cy dependent. In other words, the triple RC circuit won't shift any sine wave 180°, just a particular frequency of sine wave. Consequently, the
oscillator will oscillate at that certain frequency
where the total feedback phase shift, including
you
you
the amplifier,
is
360°.
An RC
use a non-inverting amplifier?" The reason is that
resonator or filter networks usually invert the
signal and we are stuck with the 180° phase shift.
Therefore, the amplifier
WAVE OUTPUT
phase shift oscillator is shown in Fig.
A amplifier happens to be a JFET,
but it could be any device with voltage gain. Instead of RC integrating phase shift circuits, it
8-4.
for this
The
class
-*
+
FEEDBACK
If a non-inverting amplifier were used and the
phase shift network were omitted, the circuit
might oscillate, but the oscillation frequency
would be determined by the stray inductances
and capacitances in the circuit. There would be no
deliberate control over the oscillation frequency
and the frequency usually turns out to be very
high, tens of megacycles. Moreover, the frequency of the oscillation would be very unstable and
would change with temperature, the supply voltage and even mechanical vibration.
Remember in Section 5 we talked about the
need to delay the triggering sine wave so that the
SCR could be made to fire after the voltage peak
of the AC sine wave? If you recall, we used one or
two RC phase shifting circuits for this purpose.
We said that in a practical circuit, each RC circuit
could shift the sine wave over about 60°. If you
try for 90°, the phase shifted signal becomes
CLASS A
AMPLIFIER
V_
180°
vanishingly small. Therefore, to delay the sine
Fig. 8-4
131
RC phase
PHASE SHIFT
shift oscillator
uses differentiating RC phase shift. In other
words, this circuit makes the voltage sine wave
advance in phase rather than retard in phase.
Either way you do it, it has the effect of inverting
the sine wave signal. This phase shifter is a high
pass filter, since low frequencies can't get
circuit.
plied across an
frequency, the impedance becomes very high,
finite, and the circuit is neither capacitive nor
ductive.
At resonance the LC
Resonant Circuit Oscillators
Resonant
circuit
oscillators
are a
to generate radio frequency sine
nals.
The
common
wave
when
the oscillator
sig-
is
is
working on the desired frequency.
principle of an inverting amplifier driv-
ing the 180° phase shift network
is the same.
unpopular for low, audio frequencies because they would require large, expensive transformers. But they are very good for
generating radio frequencies because they may be
tuned just by varying the single capacitor.
These
in-
operating at the resonant frequency, the LC circuit does not change the phase
of the transformer feedback. When the phase
shift of the transformer circuit is exactly 180°,
the oscillator
way
in-
circuit abruptly
loses its phase shifting ability. Therefore,
through the capacitors.
C.
LC parallel
When the voltage sine wave apLC circuit matches the resonant
Let's review the properties of an
resonant
REACTANCE
X
oscillators are
A
NDUCTOR
VOLTAGE
IS ADVANCED,
U
>z
uj
PHASE SHIFT
OCCURS ACROSS
TRANSFORMER.
IF YOU WIRE
IT CORRECTLY
I
ACTS LIKE AN
180°
FREQUENCY
w
> o
uj
L-C
CIRCUIT
II
<o
RESONANCE
DETERMINES
THE FREQUENCY
Q.
<
ACTS LIKE A CAPACITOR
VOLTAGE IS RETARDED
WITH RESPECT
TO CURRENT
<
UJ
<J 0C
Fig. 8-6
A
LC resonant
parallel
circuit
Now suppose that the oscillator frequency
begins to drop below the resonant frequency. As
you can see in Fig. 8-6. the LC circuit now acts inductive and will begin to advance the voltage
with respect to the current. This phase change
speeds up the oscillation by changing the overall
Fig. 8-5
A
phase shift from 360° to 270°. The entire feedback loop now has three phase shifts. The amplifier inverted the signal and accounts for 180°
phase shift. The transformer action inverts the
signal and shifts it another 180°. The inductance
of the off resonance LC circuits shifts it forward
90° for a total shift of 270°.
resonant circuit oscillator
As we saw in Section 7, the transformer is an
way to invert an AC signal and the induc-
easy
tance of one of the windings can be used as part of
an LC resonant circuit. Just by wiring the
transformer correctly, we can invert the signal
and provide the 180° phase shift. The LC resonant circuit determines the resonant circuit frequency by providing extra phase shift, either
retarding the feedback or advancing it. whenever
the oscillator frequency strays from the resonant
180°
+
180°
This forward shift
and soon returns
faster
-
90°
makes
it
=
270°
the oscillation go
to the resonant frequen-
cy which is the condition where the total shift
exactly 360°.
frequency.
132
is
Now
suppose the oscillator frequency drifts
LC circuit suddenly acts like a
capacitor and retards the voltage with respect to
the current. This gives a total shift of 360° plus
90° for a total of 450°. This "slow" feedback
forces the oscillation frequency back down again
until it matches the LC resonant frequency. As
you can see, this circuit locks the oscillator frequency into the LC resonant frequency and
makes the oscillation frequency very stable.
amplifier circuit and this phase shift
changes
too high, the
There
catch
is
is
The
may not
180° implies that the LC
circuit and transformer may make up the difference to produce a perfect 360° overall shift.
This implies that the LC resonant frequency may
not be exactly the same as the oscillator frequency, but is usually extremely close. The one fact
you may count on with all sine wave oscillator
designs is that the overall shift will be 360°.
There is
This resistance "blurs" the nice sharp resonant
point and may allow the oscillator frequency to
drift slightly. This idea is usually expressed with
the idea of "Q," or quality of an inductor (or
D.
where
X is
R
is
resistance
R
the resistance that
the impedance in
ohms
we
Colpitts
And
Hartley Oscillators
In the transformer resonant circuit the
transformer and capacitor provided two separate
functions: the transformer inverted the voltage
signal 180° and the LC combination resonant circuit modified this phase shift whenever the oscillator frequency drifted.
The Colpitts and
Hartley oscillators accomplish exactly the same
task using a "n" or pi network.
other reactance.)
X
fact that the transistor amplifier
shift the signal exactly
always a "catch." In this circuit the
reactance
subject to
can alter a
perfect 360° phase shift can contribute to an
unstable oscillator frequency.
that "pure" inductors can't be built.
always some resistance in the LC circuit.
Q =
is
too. All of the factors that
don't want and
of the reactance that
we do want.
X
can be the reactance of any component or
not just inductors. The higher the Q of the
LC circuit as a whole, the more sharp the resonant point will be and the more stable the oscillator frequency will be. Don't forget that the
other 180° of phase shift is in the transistor
Looking at the
pi
network
that these circuits do the
circuit,
>
THIS RESONANT TRANSFORMER
CIRCUIT IS EQUIVALENT TO
rr
(PI)
Fig. 8-7
NETWORKS
LC pi
not obvious
two functions can not be visualized separately.
Let's redraw the two pi networks as parallel resonant circuits in which "ground" is located midway between top and bottom of the LC circuits.
>
THESE
it is
same job because the
(n) networks can replace a tuned transformer.
133
Cp
PARALLEL
LC
[NO GROUND
I
Fig. 8-8
SPECIFIED
EQUIVALENT PI NETWORK
WITH GROUND SPECIFIED
Pi networks can be equivalent to a parallel
From
The location of ground isn't specified for the
simple LC circuit in Fig. 8-8, but is specified for
the two redrawn pi networks. Looking at these
diagrams we can conclude that:
The
pi
LC
This
how
the third conclusion you can see
is
a
is
little off
the subject, but this
is
as
is
formers are frequently replaced with pi LC networks. For example, Fig. 7-18 showed a transmitter class C RF amplifier driving an antenna. In
transmitters the tuned transformer is often replaced with a pi network. In fact, the values of L
and C do not have to be symmetrical and the pi
network can even match two different impedances, just like a transformer with different
numbers of turns on the transformer windings.
grounded.
pi circuits are symmetrical, so
when they
posite ends of the network.
FEEDBACK
+ FEEDBACK
LARGE
COUPLING
CAPACITOR
^
specified.
good a time as any to mention that tuned trans-
are resonating in the parallel mode, the opposite polarity of voltage will appear at op-
-+
is
accomplished. Also, when
the pi network is ringing at its reasonant frequency, the inductive and capactive phase shifts
abruptly disappear, just as we described earlier.
circuit.
Neither end of the pi networks
The
but ground
circuit,
the inverting action
networks must have a resonance point
like a parallel
2.
LC resonant
LARGE
COUPLING
CAPACITOR
J
V
v
^
J
AMPLIFIER
Colpitts
"n"
v
J
CLASS A
PHASE SHIFT
CLASS A
Fig. 8-9
K.
NETWORK
Fig. 8-10
134
v
PHASE SHIFT
AMPLIFIER
RF oscillator
k.
"n"
Hartley
NETWORK
RF oscillator
J
The Colpitts oscillator uses the pi network
which has two capacitors and one inductor. The
circuit resembles the RC phase shift oscillator,
except that the pi network replaces the three RC
circuits. To remember the difference between the
Colpitts and Hartley oscillators, the Colpitts
oscillator uses the pi network which is a low pass
filter. It passes low frequencies because low frequencies can pass through the inductor, but high
frequencies are shunted to ground through the
HALF OF COIL
SERVES AS LOA
-Vcc
CENTER TAP IS
"AC GROUNDED"
TO SUPPLY VOLTAGE
COUPLING
CAPACITOR
BLOCKS DC
FROM
V cc
capacitors.
The Hartley
oscillator is practically the
same
uses two inductors and one
(or more) capacitors to form a pi network which is
a high pass filter. It is a high pass filter because
low frequencies are shunted to ground through
the inductors, Li and L2, while high frequencies
as the Colpitts, but
it
are conducted through the capacitor C2. The main
purpose of the capacitor Ci is to keep the DC on
the collector from being shorted to ground
through the inductor L^. This coupling capacitor,
like the others indicated in the Hartley and Colpitts oscillators, are very large so that they pass
the oscillation frequency with negligible voltage
drop. Their function is to pass the AC signal
voltage without passing DC current which would
effect the quiescent point or burden the power
supply unnecessarily.
+ FEEDBACK
you might
realize that the oscillator is just an inverting amplifier driving a phase shift feedback
network!
E.
In the Hartley oscillator, one of the two
coupling capacitors can be eliminated by connecting the inductors to the power supply terminal
(V cc instead of ground. Remember we said that
from the dynamic, AC point of view, the DC
voltage source is an "AC ground" at both ends of
the supply voltage and can be thought of as a
short circuit for small AC currents. Moreover, the
function of the load resistor can be accomplished
with one of the inductors, thus saving another
component. A streamlined Hartley oscillator is
seen in Fig. 8-11.
Crystal Oscillators
Quartz rock is a clear, glass-like naturally occuring form of silicon dioxide. Unlike ordinary
glass, it is a precise crystalline structure where
each atom is arranged at specific angles to its
neighboring atoms. It is an excellent insulator
and electrons and holes are permanently trapped
in the crystal matrix. When thin wafers of this
crystal are subjected to radio frequency electric
fields (voltage), the trapped charge is attracted or
repelled. Because the charge is trapped in the
crystal, the charge can't flow toward the voltage.
)
Instead,
voltage.
If it is
"Streamlined" Hartley oscillator
Fig. 8-11
the
whole crystal bends toward the
The reverse
is
also
true— if you bend a
quartz crystal, a voltage potential appears across
it. This effect is called the piezo electric effect and
is more common than you think. Bones grow in
thickness in response to excercise and muscle
growth. This process is controlled by subtle
voltages that appear on the surface of the bone
desired to vary the frequency of a Col-
can be done by
varying the inductance or capacitance of the odd
component in the pi network. For example, in the
Colpitts oscillator, the single inductance could be
a slug tuned coil. By screwing the powdered iron
slug in and out of the coil, the inductance can be
varied. By varying the capacitor C lt the resonant
frequency can be varied over 2 to 1 range. Out in
the real world, Colpitts and Hartley oscillators
are very common. However, the circuits are almost never drawn as clearly as here for fear that
pitts or Hartley oscillator, this
when bones
are bent.
Getting back to radios, thin wafers of quartz
made to mechanically and electrically oscillate when a radio frequency sine
wave voltage is applied across the wafer. While
crystal can be
135
X
REACTANCE
INFINITE
METAL
LU
PLATES
QUARTZ
CRYSTAL
>
SERIES
CLAMPED
AROUND CRYSTAL
O
=
WITH SPRING
Q
Z
/
RESONANCE
IMPEDANCE
WITHOUT
INDUCTANCE
OR CAPACITANCE
ZERO
IMPEDANCE
PARALLEL
RESONANCE
FREQUENCY
-r c P
LU LU
II
<o
0.
<
o
RELATIVELY
<
LU
cc
SMALL
RESISTANCE
EQUIVALENT
CIRCUIT OF A
CRYSTAL MOUNTED
BETWEEN METAL
PLATES
Fig. 8-12
the crystal
crystal
is
Quartz crystal
where
vibrating, the charge trapped in the
moves back and
forth.
From an
communication and navigation
aircraft
radios operate. Without crystals to provide
electrical
point of view, the crystal behaves like a resonant
fre-
By grinding thinner and thinner wafers of
quartz crystal, higher and higher resonant frequencies are obtained.
quency standards, aircraft radio transmitters
would not stay on their assigned frequencies and
would be constantly be drifting off into other
nearbv channels.
Unfortunately for textbook writers, the
quartz crystal does not behave like a simple series
or parallel LC resonance. The quartz crystal
behaves like a combination of parallel and series
resonance. However, these two resonant points
are extremely close together. Between the two
resonant points is a very narrow region in which
Crystal oscillators can be built a variety of
ways. Crystal controlled versions of the Hartley
and Colpitts oscillators can be built by substituting the crystal for an inductor in either circuit. The capacitor(s) in the pi circuit can then be
given values that optimize the circuit for the resonant frequency of the crystal. Three common os-
circuit.
the crystal behaves like an inductor. Everywhere
else,
the crystal acts like a capacitor.
As you
know, inductors shift an AC sine wave voltage
90° ahead of the current. In contrast, a capacitor
shifts the voltage 90° behind the current. An inductance and a capacitance can work together to
produce the necessary 180° phase shift. The
crystal will only behave like an inductor over an
extremely narrow range of frequency. This means
that when crystals are used as part of the phase
shift element in a feedback circuit, they shift the
phase of the feedback correctly only at one
changes.
This
The second
stability
circuit
teresting because
it
has
The
first
been directly
which uses the JFET is inon the capacitance be-
relies
tween the drain and gate to provide the feedback
signal from output to input. This capacitance
sum
is
P-N junction capacitance plus the
capacitance due to the proximity of the gate and
the
of the
drain wires. Strange as
it
seems, the use of this
stray, unintentional capacitance is quite reliable
this same circuit is widely used with vacuum
tubes and bipolar transistors. This second circuit
can be shown to operate as a Hartley oscillator if
and
or the load on the
frequency
in Fig. 8-13.
because the inductor
replaced by the crystal.
frequency. This property gives the
oscillator very little tendency to change its freoscillator
shown
clearly based on the Colpitts
is
plan,
specific
quency as the transistor heats
cillator circuits are
oscillator circuit
is
especially critical at very high radio frequencies
you assume the crvstal
136
to be an inductor
and the
drain circuit inductor to be the second inductor.
The stray capacitance provides the capacitor to
complete the pi network.
The
and
third circuit
it is
oscillator.
and
is
called a Pierce oscillator
supposed to be related
The connection
isn't
to the Colpitts
obvious though
perhaps better to look at the circuit as a
circuit which provides positive feedback from collector to base.
it is
parallel
The Armstrong Oscillator
F.
The next two oscillator circuits are presented
with vacuum tubes because that is the way you
may see them on a radio-telephone license examination. Also, these are old circuits and you
will probably not find them in newly designed
equipment. The regenerative radio receiver is a
kind of early day radio receiver closely related to
the Armstrong oscillator. Up until 1979 the King
Radio Company made an aircraft marker beacon
receiver that used the regenerative receiver principle. The marker beacon is a 75 MHz radio beacon that is part of the instrument landing system
(ILS). When the marker beacon can be received,
the pilot knows that he is in the correct position
to continue his final approach to the runway.
A CRYSTAL CONTROLLED COLPITTS OSCILLATOR
C STRAY
VDD
+
C DRAIN GATE
UINIIN CIN IIUINHL
STRAY. UNINTENTIONAL
SI««I.
/p
I
CAPACITANCE IS
FEEDBACK PATH
THE^.
I
!"""
Voul
!~~v
»
PCHANNEL
LC
JFET
The Armstrong
oscillator is very similar to
the transformer coupled oscillator in Fig. 8-5.
f-i
MIM7K
HIGHLY MODIFIED
HARTLEY OSCILLATOR
/7?7
RFC
I
The
volt-ampere characteristic of a vacuum tube resembles that of a junction FET. When the grid is
biased to the same voltage as the cathode, this is
analogous to biasing a JFET so that the gate has
the same quiescent voltage as the source. One difference between a vacuum tube and a JFET is
that a small but significant positive current flows
into the grid during operation. In Fig. 8-14 the
bias for the grid is established by the parallel
resistor and capacitor, Rg and Cg. This parallel
RC circuit is used the same way we have been
putting parallel RC circuits in series with the
emitters in bipolar transistor amplifiers. A DC
voltage appears across the resistor to establish
the resting grid voltage. The capacitor Cg holds
this bias voltage constant.
Vout
>
N CHANNEL
JFET
The
positive feedback from the plate to the
grid (analogous to feedback from the drain to the
is accomplished with transformer coupling
between L2 and Li. Notice that the dot markings
are reversed in order to produce the 180° phase
shift. Instead of resonating the oscillation on the
output side, this circuit tunes the oscillation on
gate)
PIERCE
CRYSTAL OSCILLATOR
Fig. 8-13
fTYj
Crystal oscillator circuits
137
PHASE SHIFT
FEEDBACK BETWEEN
180°
COUPLING CAPACITOR
PREVENTS DC CURRENT
FROM GOING TO GROUND
L2 and Li
USED IN
PLACE OF LOAD
RESISTOR
R.F.C.
6
+VB
o -
PLATE VOLTAGE
TYPICALLY 45 to 150 VOLTS
/777
Fig. 8-14
Tuned grid Armstrong
the input or grid side. Another feature of this
functions in a single circuit. First,
cir-
detects an amplitude modulated
taken off a third winding
of the transformer on the input side. Of course
there is no reason why a voltage signal could not
be taken off the plate.
cuit is that the output
G.
oscillator
is
a crystal
set.
Second,
it
it
(AM)
rectifies or
signal like
amplifies the signal.
The Regenerative Detector
if it is
these functions are performed better by several,
separate transistor circuits all working together.
shows a regenerative detector radio
receiver which is very similar to the tuned grid
armstrong oscillator circuit we just looked at.
The regenerative detector accomplishes three
Fig. 8-15
However, in the early days of radio when a single
vacuum tube cost a week's pay, a circuit like this
was a boon to the budget. As late as 1960, if you
AMOUNT OF
FEEDBACK
IS
VARIABLE
BYPASS CAPACITOR
GROUNDS R.F.
COMPONENTS HERE
BUT PASSES AUDIO
FREQUENCY TO HEADPHONES
HEADPHONES
CONVERT AUDIO
'FREQUENCY DC
CURRENT VARIATIONS
INTO SOUND
rm
rrn
Fig. 8-15
And
adjusted so that it is self-oscillating,
it modulates morse code signals so that they
sound like the familiar musical dots and dashes.
In a modern superhetrodyne radio receiver, all
third,
Regenerative detector radio receiver.
138
wanted
to build a very small, sensitive, very high
frequency two-way radio (walkie-talkie), the regenerative detector built with a miniature
vacuum tube was still the best way to do it.
In
fact,
many
arrow signifies that the coupling between L^ and
is variable. This control is called the regeneration control. When the coupling between Lj and
L2 is very weak, the amplifier acts like an ordinary tuned amplifier serving as an active detector. As the feedback is increased by increasing
the coupling, the gain of the amplifier is increased. This makes the signal heard in the earphones
much louder than it would be without the feedback. When too much positive feedback is used,
the circuit begins to oscillate. When this happens
a voice signal becomes distorted and the background static becomes a high-pitched rushing
sound.
L2
of these old walkie-talkie cir-
used the same oscillator circuit to generate
the transmitted signal! Often a separate vacuum
cuits
tube audio amplifier amplified the received signal
when on "receive." When on "transmit," the
same audio amplifier amplified the microphone
AM
modulated the transmitted signal.
signal and
These crude walkie-talkies really worked. One of
the author's fondest memories is standing on top
of the chimney with a home-built regenerative
walkie-talkie and talking to another ham 2000
miles away. Today there is no reason to endure
When morse
code is received by a simple crysimple superhetrodyne, it does not
sound musical. If the morse code signal is strong,
it just makes a thumping noise in the receiver
that is very hard to decode. When the code signal
is weak, it makes the background static go on and
off with a rhythmic pattern that is also difficult
to follow. However, when the code is mixed with
an RF signal from a separate oscillator, such as
that caused by an oscillating regenerative receiver, suddenly the morse code dots and dashes
are musical tones and are very distinct to the ear.
Modern superhetrodyne receivers generate a separate local oscillator signal called a beat frequency oscillator in order to make code readable. Even
the newest automatic direction finders for aircraft are equipped with BFO circuits for reading
stal set or a
of a regenerative receiver.
difficulties
Miniaturized superhetrodynes and crystal controlled transmitters can be built even for high frequencies. The tiny aircraft emergency transmitters which have 2-way voice capability on 121.5
MHz and 243 MHz illustrate the current tech-
the
nology.
In Section 2
we
studied diode detectors and
crystal radio receivers.
rectifies a radio
A
crystal diode detects or
AC waveform because it only con-
ducts current in one direction. A transistor or
tube amplifier can also detect or rectify an RF
signal by operating the amplifier as if it were half
of a class B amplifier. In this way, only one polarity of the signal is amplified. An amplifier used as
a detector like this is called an active detector
morse code.
because it amplifies as well as detects. If it is really operating class B, it is appropriate to call it a
linear active detector. Referring to Fig. 8-15, if we
ignore the feedback coil, L2, the regenerative
detector is basically an amplifier with a tuned LC
circuit on the input to select the desired station.
Notice that the radio frequency choke in the Armstrong oscillator has been replaced with a pair of
headphones. Headphones are inductive devices
which behave much like RF chokes electrically,
but also make sounds proportional to the changes
in DC current passing through them.
Now
that you are checked out on sine wave
we regret to tell you that the inverting
oscillators,
amplifiers used in these oscillators are not always
class A. It is possible to
cillate
class
make an
even though the transistor
B
or C. It
is still
amplifier osis
biased like
true that the purest sine
waves are generated with class A amplifiers
because these have the least distortion. In the
last section we saw that a class C amplifier can
produce a sine wave output even though the transistor is turned on only to make short current
pulses to sustain the ringing in the parallel
circuit. The oscillating is happening in the LC
The
and the transistor just "strikes the bell." A
class C oscillator can be thought of as a selfexcited class C amplifier in which the input current pulses are derived from the sine wave oscilla-
added to this amplifier circuit by the coil L2. As you might guess,
this makes the circuit oscillate at about the same
frequency that the tuned LC circuit is tuned to.
Notice the arrow which connects Li and L2. This
positive feedback
LC
cir-
cuit
is
tion in the output
139
LC
circuit.
Sine wave oscillators, especially crystal conhave very stable frequencies compared to the oscillators we shall talk about next.
Even though sine waves are not needed for computers and other digital circuits, digital circuits
trolled ones,
usually
are
oscillators
controlled
because
of
with
the
sine
wave
superior
clock
frequency
stability.
QUESTIONS:
6.
Why
RC
are
phase shift oscillators pre-
wave
ferred for low frequency sine
1.
What
wave
are the
two basic elements
oscillator?
7.
2.
What two
What is
4.
Why
does
the
oscillation
specific frequency in an
properties does the phase shift
network have?
wave
oscil-
lators?
of a sine
the role of positive feedback in sine
8.
wave
oscillator?
Why
are three separate
cuits
oscillators?
In order to work, an amplifier in a sine
oscillator amplifies its own output.
9
wave
occur at one
RC phase shift sine
RC phase shift cirneeded to delay the sine wave 180°?
Suppose you wished to vary the frequency
the RC phase shift oscillator. What
would be required to do this?
of
This concept is easy enough to understand
once it is started, but where does the input
come from when it is first turned on? Why
doesn't the amplifier remain at its quiescent point indefinitely?
10.
What would be
the easiest
frequency
the
coupled,
of
way
Hartley,
to vary the
transformer
Armstrong, and Colpitts
oscil-
lators.
5.
Suppose an oscillator is observed to have
an extremely stable frequency. That is,
there is very little tendency for the frequency to drift with temperature or vibration. Suppose you could measure the exact
degrees of phase shift produced by the
phase shift network for frequencies immediately around the operating frequency
of the oscillator. What would you expect to
find about the relationship between degrees of phase shift and frequency?
11.
What
is
the advantage of a quartz crystal
oscillator?
What
property gives
it
this ad-
vantage?
12.
What
three functions does a regenerative
detector perform in one circuit?
What
are
regenerative detectors used for today?
13.
What
is
an "active detector?'*
"linear active detector?"
140
What
is
a
SECTION IX
Square
Wave
Generators
And
Bistable Circuits
A.
tennis ball on top of the log. If there are no
Introduction
ir-
on the surface of the log to keep the
ball from rolling, there is no reason to expect the
ball to stay on the top of the log. It is bound to
roll off in one direction or the other, but we have
no way to predict which direction it will go. The
position on top of the log is unstable because the
ball will not remain there. Once the ball has reached the ground we shall assume that the ground is
flat and sandy so that the ball will come to rest.
The positions on either side of the log are stable
because once the ball arrives, there is no tendency
for the ball to jump back up onto the log.
regularities
we
are going to study bistable
which have two stable operating points; fully turned on and fully turned off.
The multivibrator and other bistable circuits are
important in computer and digital circuits. The
multivibrator is a form of transistor oscillator
which can be made to switch rapidly between its
two stable states like the P-N-P-N diode and neon
In this section
transistor circuits
relaxation oscillators
The multivibrator
square waves and
is
is
tant form of digital
puter temporary
we described in Section 5.
a simple way to generate
presently the most impornumber counter and com-
memory
The tennis ball balanced on the log is an example of positive feedback. If you set it carefully
on top of the log, it slowly begins to roll off in one
direction or the other. The farther it rolls from the
center, the faster it goes and the steeper the slope
it is traveling over. It is as though the original
slight deviation from the center multiplies itself
again and again until the ball has gone as far as it
storage circuit.
A TENNIS BALL
PLACED ON TOP OF
UNSTABLE^
POINT
X*
A SMOOTH LOG CAN
BE EXPECTED TO
ROLL OFF IN ONE
DIRECTION OR THE
OTHER
can, into the sand.
In the last section
we discussed
sine
wave
oscillators which were based on amplifiers with a
single, stable quiescent point.
Bistable circuits
can also oscillate. Surely
you must be having misgivings about how a circuit with "stable operating points" can oscillate.
These operating points can't be very stable if the
circuit is incapable of remaining there. The
following observations may help you:
like the multivibrator
Fig. 9-1
Ball-on-log analogy for two-state tran-
sistor multivibrator
To introduce you
to the bistable circuit,
we
any circuit that is self-oscillating does
not remain at its "stable operating point(s)" for
any length of time, so perhaps it would be better
to call these "temporary stable points." Some
about on curved
surfaces. This time we will use the conceptual opposite of a trough which is a solid cylinder. Suppose a large, smooth log is lying on a sandy
beach. Now imagine that we attempt to balance a
will return to tennis balls rolling
First,
textbooks
141
call
these quasistable operating points.
A common name
Second, the positive feedback path in freerunning oscillators from output to input is always
capacitive or transformer coupled so that there is
no direct DC feedback from output to input. If
you look back through the oscillators in Section 8,
you will see that not one of the transistor amplifiers has a direct DC connection between output and input. Invariably the positive feedback
changes with time. A capacitor discharges, or an
inductor charges with current so that the feedback either disappears or changes its polarity. As
soon as the feedback ceases or changes, the sine
wave oscillator amplifier goes back toward its
flop.
we
V ou ti and V ou t2- Whenever V ou ti
goes high, the other output, V ou t2 goes low and
output,
vice versa. Instead of V ou ti and V ou t2, the outputs of flip-flops are usually labeled "Q" and
"Q." The bar over the Q means that Q is the op-
posite of the output Q. In other words,
Q
is
low
whenever Q is high and vice versa. For example,
you might find a lead on a circuit diagram marked
"Q of FF6." This means that the wire is the Q
output of the sixth
From
flip-flop.
the ball-on-log analogy,
have concluded that
The Multivibrator
if
I
hope you
these two transistors were
turned precisely half on, it would be a toss up as
to which output went high and which output went
low. The halfway point is unstable, and for most
applications should be as unstable as possible, so
that the circuit spends as little time as possible
Some
multivibrators do not actually oscillate, but
remain indefinitely in either of the two stable
states. These bistable multivibrators are usually
made from two direct coupled common emitter
amplifiers. The collector of each transistor is
directly coupled to the base of the opposite transistor. The result of this circuit is that whenever
one transistor turns on, this action turns the
will
other transistor
for this circuit is the flip-
transistor flips while the other flops,
presume. In any case, each of these two common
emitter amplifiers has a load resistance and an
quiescent point. In the case of a bistable oscillator circuit, the change in the feedback makes
the transistor go to its opposite "stable'" state.
B.
One
half-turned on. Fig. 9-3
shows a typical bistable
transistor flip-flop.
The half-turned on condition is made more
unstable by the addition of speed up capacitors,
C\ and C2. These are relatively small capacitors
which are placed across the resistors driving the
off.
transistor bases.
The capacitor across the base resistor of a
turned-on transistor is charged to a fairly high
voltage, nearly +V CC This is the difference be.
tween the opposite collector and the base. In contrast, the capacitor across the base resistor of a
turned off transistor discharges to a very low
voltage, nearly zero. This zero voltage
is
the
dif-
ference between the collector-to-ground voltage
which is about 0.6
and the base-to-ground voltage of the turntransistor, which is also about 0.6 volts.
of the turned on transistor,
volts,
ed off
When
ONLY ONE OF
THESE TRANSISTORSCAN BE ON (OR OFF)
AT ONE TIME.
WHENEVER ONE
SWITCHES ON. THE
OTHER SWITCHES
charge instantly. Therefore, when the collector
voltage rises, the opposite end of the capacitor
tries to "pull" the opposite transistor base
voltage upward. Large currents flow from the
base into the capacitor and the transistor turns
on. Once the transistor is turned on, the current
through the resistor is enough to keep it turned
on indefinitely.
OFF.
Fig. 9-2
off, the capacitor on
discharged to zero volts and can't
a transistor turns
its collector is
Bistable multivibrator
142
SPEEDUP
+ V CC = 6 VOLTS
O
CAPACITORS
INSURE ABRUPT
SWITCHING
NOTICE
WHERE
COLLECTOR
IS
PULLED
DOWN
BY
ZERO VOLTS
ON TRIGGER
WAVE FORM.
time
VERTICAL SCALE
IS 2 VOLTS PER
DIVISION.
HORIZONTAL SCALE
Q OUTPUT
IS 0.5 MILLI-
WAVEFORM
SECONDS PER
DIVISION.
WAVEFORMS PHOTOGRAPHED ON AN OSCILLOSCOPE
Fig. 9-3
A practical
The opposite can be said about the capacitor
across the base resistor of the turned on tranThis capacitor
the ground voltage. This turns that transistor off
was turned
Speed up capacitors make the switching between transistors states very quick and insure
that the voltage wave forms on the collectors, Q
and Q are square wave in form.
as abruptly as the opposite transistor
charged to a high voltage.
When the opposite transistor turns on, the base
capacitor can't discharge instantly. Therefore,
the voltage on the base is "pushed down" below
sistor.
bistable flip-flop
on.
is
143
low a voltage for positive current to flow from the
collector into the trigger pulse network. Current
will flow only when the diode is forward biased to
allow the flow. The result is that only the collector with high voltage on it is affected. Moreover,
the effect happens only when the input pulse is
going down. The pulse must be on the way down
in order for it to push the speed-up capacitor
down and turn off the turned-on transistor.
In the past we have always added resistors
between the base and emitter to be sure that the
bipolar transistor has a source of current to turn
it has been turned on. The
speed up capacitors make this resistor unnecessary because the charge stored in the capacitor
will momentarily put a very low voltage on the
base, below the base-to-emitter voltage and turn
the transistor off. Whether the turn off current is
a current entering or leaving the base depends on
the transistor off once
An
whether we are using P-N-P or X-P-X transistors.
So, since the capacitor does the turning off, we
may omit the base-to-emitter resistors.
In order for the bistable multivibrator to be
we need a way to trigger the flip-flop to
useful,
puts. First
The two diodes connected
to the collectors allow an external pulse to make
the flip-flop switch. As shown, a positive pulse is
make
it
change
Clever,
is free
down
pulled
the flip-flop change states, the input
1.
turn
it
down
to nearly zero volts.
enough
down
The output pulses have
half the frequency of
the input trigger pulses.
•2.
to zero. It just
speed-up capacitor to affect the base of the turned-on transistor.
The speed-up capacitor is charged up to nearly
the full collector voltage, Vcc- It can't discharge
instantly, so when the collector voltage is pulled
down slightly, the base end of the capac-itor will
try to push the base of the turned-on transistor
below zero volts. As soon as the base is driven
below its turn off voltage, about 0.6 volts, the
transistor turns off. When it turns off, the capacitor on that transistor's collector will pull up
the base of the formerly turned off transistor and
far
the trigger input
at zero volts
flops that
not pull the collector clear
down
is
The two basic lessons about triggered flipyou should learn from this example are:
to leave both collec-
pens whenever the trigger pulse waveform drops
back down to zero volts. The trigger pulse does
it
when
between the pulses. This
artifact is kept small by the 15K ohm resistor
which prevents large currents from flowing down
through the diode to zero volts. Without the
resistor, the turned-off collector voltage would be
voltage pulse pulls down the collector of whichever transistor is not already turned on. This hap-
pulls
you should notice that the frequency
a fraction of a volt
voltage
huh?
To make
is
output pulses. Also notice the small artifact or
"dent" in the collector waveform when it is high.
This dent is the collector voltage being pulled
introduced to both collectors simultaneously.
With the two diodes back-to-back, a current can't
travel from one collector to the other collector. Xo
matter what the polarity of one collector may be,
one of those two diodes will be facing the wrong
direction to let current flow from one collector to
the other. On the other hand, negative current
can flow into both collectors because the diodes
are pointing in that direction. To put another
tors.
waveforms
of the input pulses is twice as high as the longer
states.
way, positive current
oscilloscope picture of the
shown at the bottom of Fig. 9-3. The upper picture shows the trigger voltage pulses which are
applied to the diodes. The bottom picture shows
the waveform of either of the two collector out-
for the
trigger pulses make the flip-flop
switch only when the input trigger pulse is
going downward. It does not change state
while the trigger pulse is going up. As you
shall see shortly, responding to the end of the
pulse, rather than the beginning, is vital when
several flip-flops are put in series for use as
The input
counters.
It
may seem
strange for the input to go to the
collectors in this circuit. In fact, other textbooks
are full of flip-flops that have the input pulse go-
ing to the bases. However, sending a single input
trigger pulse to both bases simultaneously is not
practical.
This
is
because there
is
no significant
difference in voltage between a turned-on base
on.
and a turned-off base. They are both pretty close
to 0.6 volts. Since both bases are getting the
same message simultaneously, both bases will try
to turn on or turn off together. Using the collec-
Notice what is happening to the turned-on
collector while the turned-off collector is being
pulled down. The answer is that nothing is happening because the turned-on collector has too
tors insures that only the side that is vulnerable
111
PULSES TO BE COUNTED
time
\y
23456789
1
10 11 12
ON FALLING
RISES
EDGE OF TRIGGER PULSE
01
h
1's
1
1
f1
1
1
FF1
Q1
time
0123456789
10 11 12
"1"
LOGIC CIRCUIT
Q2
T2
+1
1
1
2's
RECOGNIZES THE
COMBINATION OF
PULSES THAT
MEAN 11 PULSES
HAVE OCCURRED.
FF2
I
I
Q2
time
I
*
123456789
'
'
'
1
'
1
'
•
'
1
*"
'
1
1
i
10 11 12
"1'
03
1
4's
A'T'ON THIS
LINE MEANS THERE
ARE NO 4's AT
THE MOMENT
1
FF3
03
o
|
1
234 56789
|
1
|
i
i
1
i
1
1
]
time
1
1
i
A ONE HERE
MEANS
1
^
(ONE) +
(TWO) +
1
NO FOURS +
10 11 12
11
TOTAL PULSES
1
(EIGHT)= ELEVEN.
BINARY NUMBERS.
ELEVEN = 1011
IN
1
8s
/
t
time
|
i
1
Fig. 9-4
•
•
1
234 567 89
i
i
i
i
i
i
i
i
i
i
i
10 11 12
"1"
Flip-flops wired in series count pulses. Logic circuits look at all flip-flop outputs to recognize
specific
number of pulses.
with two complete 40 transistor flip-flops costs
about 50c. Moveover, they switch faster and
there is no pull-down artifact. The details will
have to wait for your digital electronics course.
to being switched gets the message, while the
other side
The
is
not affected.
flip-flop circuit in Fig. 9-3 is relatively
simple and is made from about $1.50 worth of individual parts. However, the output waveforms
|
/
8's 4's 2's1's
FF4
04
any
M
BINARY
INVERTER
Q4
T4
are not very clean
due
C.
Uses For The Bistable Flip-Flop
to the collector pull-down
The
Today, bistable flip-flops are nearly
always built in the form of integrated circuits.
These integrated flip-flops may contain as many
as 40 transistors to accomplish little more than
what we have done here! On the other hand, an IC
artifact.
flip-flop
memory and
has two basic uses.
It
serves as a
as a counter or divider (Fig. 9-4).
The
relationship between the trigger input pulses and
what happens
flop so useful.
145
Q or Q is what makes the
Each input pulse consists
at
flip-
of a
and registers
voltage that rises suddenly, stops at a high point,
then comes back down. Since it goes up and
down, this
come out of
To use
it
the
flip-flop.
flip-flops as a
pulse counter or divider,
several flip-flops are wired in series as
Fig. 9-4.
The output
shown
Qj high.
flip-flops at this point are un-
changed. A high on Qi means we have counted
from zero to one. The second trigger pulse comes
in and causes Qi to return to zero (low) while the
output of the second flip-flop rises to one (Q2 = 1).
Notice how the flip-flop responds only to the falling edge of an input pulse. We now have an electrical representation of the binary number two or
"10." The third pulse comes in and sets Qi high.
Now we have the binary number for three, "11,"
in which Qi = 1 and Q2 = 1. The fourth pulse sets
the first and second flip-flops back to zero and the
third to 1. This gives us the binary number four,
"100." If you continue, you will see that four
two changes. Every time a trigger
causes one of the flip-flop outputs
to go up only: in other words, one change. But a
complete pulse consists of two changes of state.
Therefore, in order for a single up and down pulse
to emerge from the Q (or Q), output there must
have been two complete trigger pulses. In summary, twice as many pulses go into the flip-flop as
is
pulse occurs,
in the first flip-flop setting
The other three
in
of one flip-flop is fed into the
trigger input of another. Since the second flip-flop
counters can count up to 15, then they return to
receives pulses from the first flip-flop, its output
zero on the 16th pulse.
will
be one pulse out for every 4 original trigger
pulses, three flip-flops can count to
and so
8,
recognize specific
not powers of two?
Number recognition is done with logic circuits
that examine all the Q outputs and look for
specific combinations of highs and lows. For example, to count to eleven the first flip-flop will be
high meaning a "one." The second flip-flop will be
high meaning that there is a "two" at the moment. The third flip-flop will be low meaning that
there is no "four" at present. The fourth flip-flop
will be high meaning that there is an "eight." If
numbers
If we are counting pulses that represent
numbers, then zero is a legitimate number and
must be used to represent one of the 16 possible
states
of
4
through the
flip-flops.
follow
Let's
Fig. 9-4
in
flip-flops
the pulses
and see how
are counted and recognized. We start
with the number zero by setting all four flip-flops
numbers
to zero (Q
how can counters
So,
four to 16,
on.
=
0).
The
first trigger
pulse comes in
o
like eleven that are
+v cc
RL1
R L2
R2
R1
•
ii
vVW
AAAA-
«
ii
—
H(C2
Cl
S
SET
SET
INPUT
RESET
INPUT
n
/777
SET
>
RESET
RESET
>
Fig. 9-6
Set and Reset triggers for a bistable multivibrator.
l
16
we add up 1 plus two plus
the number of input pulses
eight,
so
we
get eleven,
far.
Circuits like Fig. 9-4 are used in practically
tuned aircraft radios to set the
all digitally
quency channel.
A
fre-
crystal controlled oscillator
provides standard pulses which are counted as a
frequency standard. The digital tuning knob on
the front panel sets up the logic circuit to
recognize when the proper count is reached for
vout
the desired receiving or transmitting frequency,
just as
is
we
did for the
number eleven. This number
compared with pulses derived from the actual
operating frequency of the receiver or transmitter. If they are not equal, a feedback circuit adjusts the transmitter or receiver frequency up or
VOLTS
do match. More details about
counters are better left to your course on digital
down
VBASE2
= ZERO
until they
electronics.
VOUT
THE SMALLER Rl 2 IS,
CHARGING THE MORE VERTICALLY
THROUGH R|_ 2 THIS VOLTAGE WILL RISE.
Ci
-
k
Bistable Flip-Flops as Memories
D.
Vcc-
The flip-flop can serve as a memory by remembering what state the output Q is in. By setting Q high or low and leaving it that way, the
o
Q can be read back later. It is something
tying a string around your finger. The string
itself doesn't remember anything, but hopefully
the presence of the string on your finger records
the fact that there was something that you
O
io
LU
<
I_J
o
>
state of
a.
like
o
a
wanted to remember. A single flip-flop doesn't
remember very much, but if you have thousands
IF IT
BASE CONDUCTING,
WOULD CONTINUE
01 TURNS 02 TURNS TO CHARGE UP TO
+Vcc VOLTS.
0N
0N
VBASE2
C2
be stored in the form of binary numbers begins to
be significant.
T
Referring to our counter example in Fig. 9-4,
suppose that after eleven input pulses, there were
no more pulses after that. Assuming that the
power supply doesn't fail during the following
months, the number eleven will remain "stored"
in those counters in the form of the binary
VOLTAGE
C2 DISCHARGING
CURRENT ENTERS
C2 THROUGH R2-
the flip-flop useful as a
able to set
Q
R2
high or low
trigger pulses in Fig. 9-3
Fig.
Q each time,
they don't set it to a particular state. Controlling the state of Q can be
done by separating the two diodes into separate
When
These trigger inputs are called set and reset.
a positive pulse comes into the set trigger,
if it is
An
RL-
astable multivibrator
and voltage
comes in on the reset line, Q goes
high, which means that Q goes low. If Q was
already low, then it stays there. If you can
remember that set makes Q a one, you will have
When
goes high or stays high
9-6
IS
TRANSISTOR
TURNS ON.
waveforms
trigger lines.
Q
CURRENT FLOWS
INTO BASE AND
VOLTAGE, V C c-
make
IS
EXCEEDED, SO
TOWARD SUPPLY
memory, we need to be
whenever we want. The
just change
I
BASE P-N
JUNCTION
1101.
In order to
WERE
NOT FOR THE
of flip-flops, the quantity of information that can
number
Q2
FULL OFF
Q2
FULL ON
UJ
_l
—I
a pulse
the terminology conquered.
already high.
147
E.
A stable
We
haven't
still
vibrator
can
explained
oscillate.
how
a
Self-oscillating
the
moment
The
resistor
the base of Q2 is out of the circuit.
R2 is also connected to the negative
end of C2 and positive current is free to flow from
V cc down through R2 to charge the right end of
C2 "up" to +V CC Looking at it another way, C2
Multivibrators
multimulti-
vibrators are called free-running multivibrators
.
discharging toward having zero volts
across it. Then later, if it could, it would continue
to charge so that the right end of C2 would eventually reach +V CC However, before the right side
of C2 ever reaches +V CC it will exceed the silicon
P-N junction voltage, +0.6 volts, and will begin
conducting current into the base of Q2. This turns
on the transistor Q 2 and completes the discharging of C2. At this point, the right end of C2 is at
about 0.6 volts and the left end is also at roughly
+0.6 volts. The voltage across C2 is now about
The word "astable"
means not stable and can be remembered as ain 't
stable. They can be built several ways but the
most common circuit as shown in Fig. 9-6.
or astable multivibrators.
is
first
.
,
Each
biased partly on by the
resistors Ri and R2. However, these resistors are
usually large so that the current through them is
small and the majority of the current that turns
the transistors full on or full off comes from the
capacitors. In fact, the purpose of these resistors
is to discharge the capacitors and determine the
rate at which the multivibrator flips back and
transistor
is
zero volts.
The period of time that Q2 remained turned
was determined by how quickly C2 charged (or
discharged if you prefer) to 0.6 volts on the base
of Q2. "Quick" is just a relative word, of course,
and if C2 and R2 have large values, this length of
time can be several seconds. As soon as Q2 turns
on, Ci is now jammed "below" zero volts and Ql
forth. Understanding the details of this circuit is
not easy, so don't be surprised if you have to go
through this more than once.
off
by assuming that the transistor
turned full on. Then we will
follow the events on the voltage waveforms as
time passes from left to right. Individual transistors are often labeled with "Q" followed by a
number. This is easily confused with outputs of
multivibrators which are called "Q" with a
subscript or no number at all. To avoid this proLet's start
on the
right, Q2, is
blem, we will
multivibrator
on the
call
turned off. Ci then begins to discharge until its
voltage rises high enough to turn on Q lf just as
is
we described
Now, let's look at how C2 recharges while Q2 is
turned on. Because the base of Q2 is conducting,
the right side of C2 is clamped to 0.6 volts and
can't be anything different until Qo turns off
again. As soon as Qi turns off, the left end of Co
will began to charge up toward +V CC The left
end of C2 is charging through the resistor Rlj
the output of this free-running
"V ou t." We begin with
the tran-
turned on, so the voltage
from collector to emitter is very- low, roughly 0.6
volts. The opposite transistor, Qi, is turned off
sistor
right, Q2,
.
which means that the collector of Q will have a
high voltage, V cc So at this point, the left side of
C2 is high and the right side is low. Therefore at
this time C2 will be charged up to a large voltage
.
t
The load
we
rate of charging. In fact, the faster the rate of re-
charging of the two capacitors, the more the collector waveforms of both Qi and Qj will resemble
square waves.
will get to shortly,
Qi abruptly switches full on. When this happens,
C2 can't discharge instantly, so the positive end
of C2 on the collector of Qi is now suddenly at the
collector saturation voltage: that is, about 0.6
volts or nearly zero. The negative end of C2 is now
pushed down below ground a voltage "distance'"
nearly equal to
V cc
resistors are usually very
.
with the left side positive. To be exact, this
voltage will be V cc — 0.6 volts.
for reasons that
much
smaller
than Ri and R2, so the capacitor charges very
quickly to V cc And of course, the collector voltage of Q\ will rise at a rate determined by the
.
Now,
for C2.
Because each transistor in the multivibrator
its ON time determined by a separate capacitor, the sizes of the capacitors can be different so
that the on times of the two transistors will differ. Depending on which transistor collector is
used as an output, this can give a series of very
has
.
The negative end of Cj is connected to the P
semiconductor base of the N-P-N transistor ()
Negative to P does not conduct, so current cannot
flow from the capacitor into the base of Q 2 For
short positive pulses with long off times or a
series of very long positive pulses with very short
off times.
.
US
F.
Synchronized Astable Multivibrators
There
designs.
We have already seen how a bistable flip-flop
can be made to change states by an external trigger pulse.
When
widely used (Fig.
set or reset trigger pulses are ap-
plied to an astable flip-flop,
it
can make the
9-7).
cuit
frequency speed up to match the trigger
is slower
than the frequency of the pulses, the trigger
pulses will make the multivibrator change state
early and will synchronize the flip-flop to the trigger pulses. One feature of the synchronized
astable flip-flop is that if the astable frequency is
very close to the trigger pulse frequency, you
don't need to have a synchronizing pulse on every
cycle. An occasional set or reset trigger pulse can
adjust the oscillator to keep it closely in step with
the master pulse. We shall describe a typical application for synchronization shortly.
flop
The astable multivibrator is a simple way to
generate a square voltage wave. However, FET
and bipolar integrated circuits are available in
and "timers." All
one type of astable multidiscrete parts which is still
This multivibrator is a power conversion cirwhich converts DC to AC. The circuit uses
the two halves of the primary winding of a
transformer in place of load resistors. The
collector-to-base coupling is done with a parallel
RC pair which perform exactly the same function
as those shown in the bistable amplifier flip-flop
in Fig. 9-3. But here the timing element is not the
capacitors, it is the inductance of the two halves
of the primary windings. In the capacitor controlled multivibrator (Fig. 9-6) the discharging
capacitor determined when the off transistor
would turn on. In the inductance timed circuit the
current stored in the inductor bleeds away until
there is insufficient base current to keep the on
transistor turned on. When the on transistor
comes out of saturation, the inductance between
its collector and the power supply is fully charged
and ready to bleed current into the opposite transistor base to turn it on and keep it on for a while.
flip-
pulses. If the astable natural frequency
flip-flops, logic circuits
is
made from
vibrator
of these
can easily be wired to generate square waves with
less cost, fewer parts, or less current drain. Consequently, you rarely see an astable multivibrator
built with individual parts in the most recent
Whenever current is changing in either half of the
primary winding, an AC voltage, the output, will
appear on the secondary.
Multivibrators like this can be used to
generate an AC voltage from a low DC voltage.
For example, this could be an AC converter which
is plugged into the 12 volt DC cigarette lighter of
your car. An electric shaver designed for 120
volts AC can be connected across the secondary
winding and you can shave while driving to work.
TRANSFORMER PRIMARY WINDINGS
ESTABLISH THE OSCILLATION
FREQUENCY BY "STORING" CURRENT
TO KEEP THE OPPOSITE
TRANSISTOR TURNED ON.120 VOLTS
AC
An electric shaver is usually not very fussy
about whether the AC frequency is 50 cycles or
100 cycles per second. However a record player
usually depends on the frequency of the AC to
determine how fast the records will turn. If the
frequency is 100 cycles, baritone singers will
sound like Mickey Mouse. If the frequency is
much below 60 cycles, the recording will sound
like prehistoric monsters groaning in the swamp.
The problem is even more severe because the resistance of the load, that
is,
the record player, will
be reflected back through the transformer to
become part of the LR circuit that is determining
the frequency. So the frequency will depend on
exactly
o
Fig.
used
9-7
to
how much
current the load draws.
-
One
A
transformer multivibrator can be
convert low voltage DC to high voltage
solution to this
is
to design the multi-
vibrator to run at a relatively low frequency, say
40 cycles. Then a separate timer circuit
AC.
149
is
added
VOUT
TRIGGER
VOUT
PULSE TURNS
Q1
Q2 OFF
ON AND
Q2 RETURNS
TO ON
Q2 0FF
THIS INTERVAL
IS
DETERMINED BY
C1
AND
R1
HELPS INSURE THAT
OFF
Q1 RESTS
Fig. 9-8
which
will
A
monostable multivibrator
Monostable multivibrators, also known as
one shot multivibrators, are usually designed so
that the output is low when the circuit is inactive.
Then when the trigger pulse comes in, the output
voltage jumps up for a length of time determined
by the charging capacitor, Ci, and its resistor, R lt
as seen in Fig. 9-8. At the end of this period the
output falls again and waits for the next trigger
produce frequency stable timing pulses
no
to synchronize the multivibrator to 60 cycles,
matter what the load
make
may
be.
The pulses
will
at
60
cycles even though the multivibrator astable
fre-
the multivibrator trigger "early'*
quency
is
inclined to
wander between 30 and 50
cycles.
pulse.
In another application, the
AC
voltage on the
secondary can be rectified and filtered to produce
DC at a much higher or lower voltage with little
loss of power. These are called DC to DC inverters. Simple inverters like this one are found in
the capacitive ignition discharge systems which
are discussed in Section five. The voltage desired
on the secondary must be designed into the circuit by the choice of winding ratios on the
There are thousands of applications
one
you are building an automatic machine which
packs one hundred oranges in each shipping box.
The oranges
roll
down
a chute
The oranges
much
too fast for
over a switch
which makes a voltage pulse every time an orange
goes by. A series of flip-flop counters counts the
oranges and a logic circuit recognizes when 100
oranges have entered the box. When the count of
100 is reached, the logic circuit delivers a short
pulse signifying "100.*" So far, this is just like the
counter illustrated in Fig. 9-4. This pulse is supthe eye to count.
transformer.
G.
for
shot multivibrators, but their usefulness is not
obvious until you need one. For example, suppose
Monostable Multivibrators
Half of an astable multivibrator and half of a
combined to form a
monostable multivibrator. The purpose of this
circuit is to make voltage pulses longer. If you
feed in a very short trigger pulse, say one millisecond long, the monostable multivibrator will
deliver one long pulse, say 10 or 1000 milliseconds duration.
bistable multivibrator can be
roll
posed to activate a large solenoid which releases a
trap door and redirects the stream of oranges to
an empty box. Unfortunately, the solenoid has
quite a lot of inductance and physical inertia due
to its mass. Before it can respond to the pulse, the
101st orange goes by and changes the count to
101. The logic circuit which recognizes the 100
150
.^^SROU.
TRAP
DOORS
SWITCH MAKES A VOLTAGE
PULSE FOR EACH ORANGE
THAT ROLLS OVER IT
V|N
1's
Fig. 9-9
Orange packing machine
illustrates use of
flip-flops
and one-shot
multivibrator.
Schmitt triggers, zero crossing detectors, and
comparators are not oscillators, but they are frequently used with oscillators to make square
waves out of sine waves. Digital circuits including computers are usually regulated with a
frequency-stable square wave called a clock pulse.
count returns to zero before the trap door is able
to open. As seen in Fig. 9-9, a one shot is used to
make the 100 count pulse last long enough for the
solenoid to respond.
H.
counting
Schmitt Triggers, Zero Crossing Detectors,
and Comparators
The clock
151
is
a sort of electronic
drill
sergeant
HIGH GAIN
AMPLIFIER
CLIPS
SINE WAVE
OFF AT
VOUT
V|N
Vcc
i
SINE WAVE
INPUT
VlN
1/2
>
VOUT
OF
CLASS B
AMPLIFIEF
SQUARE" WAVE OUTPUT
/7T7
Fig. 9-10
A
/777
high gain amplifier serving as a zero crossing detector.
which counts cadence and keeps the various parts
circuit takes drastic action
of the circuit synchronized so that they don't in-
on or
and turns either full
compares one voltage
to another and switches full on or full off is called
a comparator. In general, comparators can compare a voltage to any voltage within the range of
the power supply. So the zero crossing detector is
with each other. The usual way to genpulses is to start with a crystal
oscillator which produces sine waves. It might
seem more efficient to begin with a relaxation
oscillator or a multivibrator which produces
square waves directly. However, these circuits
usually do not produce a stable enough frequency
terfere
erate
clock
full off.
A
circuit that
just one special kind of comparator.
comparator so that
By
wiring a
compares an input voltage
the comparator becomes a zero
it
to zero volts,
crossing detector.
standard.
Crystal controlled digital watches, for exam-
can improve the perfortwo ways.
First, it increases the apparent gain of the
amplifier and makes the square wave voltage
changes more rapid or vertical. In other words, it
makes the square waves more square. The second
advantage of positive feedback is that it makes
the conversion of sine waves to a square wave immune to high frequency noise that may be riding
on the sine wave. A circuit built like the zero
crossing detector but with positive feedback is
ca'led a Schmitt trigger, named after the guy who
first built one with vacuum tubes.
Positive feedback
mance
use a crystal oscillator for a time standard.
After the sine wave has been made square, a long
series of flip-flop counters divides the square
wave down to second, minute, and hour intervals.
These numbers are recognized and displayed by
ple,
clock pulse controlled logic circuits.
A sine wave can be converted into a good
square wave by passing it through a very high
gain amplifier. Let's assume that the amplifier is
biased like class B so that only the positive half of
wave is amplified. The amplifier has so
gain that, whenever the sine wave signal
goes the slightest bit positive, above the zero
point, the amplifier will produce a high voltage.
As soon as the sine wave signal goes slightly
the sine
of a zero crossing detector in
much
It is fairly easy to understand how positive
feedback can increase the gain of an amplifier.
Suppose the amplifier is a class A non-inverting
and the quiescent point places the
below the zero point, the transistor will turn full
on causing the output to fall to the lowest voltage
on the load line. An amplifier like this is called a
amplifier
zero crossing detector.
put signal rises a large distance above the output
zero point. This is fed back directly to the input
In effect, this circuit
amplifier half turned on.
As soon
as the input
signal rises slightly above the zero point, the out-
is
comparing the sine wave to zero. Every time the
sine wave departs from zero the slightest hit. the
via the resistor
L52
Rf shown
in Fig. 9-11.
The
feed-
SQUARE WAVES
MORE SQUARE
BECAUSE OF INCREASED
GAIN.
SQUARE WAVE
DELAYED IN
PHASE
NOTE SWITCHING POINTS
ABOVE AND BELOW ZERO
Fig. 9-11
back
The Schmitt
trigger, a
comparator with hysteresis.
the sine wave returns to the zero point from
below, the positive feedback will hold the output
promptly amplified until the output goes
power supply will allow. The reverse is true for the below zero point, the output
will be considerably below zero and feedback will
make the output go lower still.
is
as high as the
low until the input sine wave
above the zero point.
WHEN
INPUT
SCHMITT TRIGGER
SIGNAL GOES
DOWN,
Vqut OUTPUT
FOLLOWS
-THIS PATH
Positive feedback introduces hysteresis into
The word "hysteresis"
means that something is lagging or falling short
of some expected level. In electronics it means
the circuit characteristic.
that the behavior of a circuit
is late in
responding
HIGH STATE
o
>
sometimes magnetic flux in a transformer core.
In the Schmitt trigger, "hysteresis" describes
irj
the partial latching effect that the positive feed-
o
-WHEN INPUT
SIGNAL GOES
UP, OUTPUT
FOLLOWS THIS
o.
tz>
back causes. In the multivibrator we have already seen the result of what a great deal of
positive feedback can do. Once the output leaves
dead center, it quickly moves to either of its two
stable states and stays there permanently.
PATH.
LOW STATE
V|N
J
In the Schmitt trigger much less positive
feedback is used so that when the input sine wave
goes back below the zero point, the circuit is
capable of responding by switching back to the
other extreme. The important point here is that
the switching point is no longer zero volts. When
wave returns
TRANSFER CHARACTERISTIC
<
to rising or falling values of voltage, current, or
the input sine
rises considerably
INPUT VOLTAGE
1
SWITCHING
-SWITCHING
THRESHOLD
THRESHOLD
GOING
DOWN
Fig. 9-12
•ZERO"
POINT
GOING
UP
Schmitt trigger hysteresis
Because of the hysteresis, the Schmitt trigger isn't really a zero crossing detector because it
doesn't switch at exactly zero volts. The average
of the two switching thresholds is still zero so
that if a sine wave is fed into it, the output will
look like that of a zero crossing detector, but will
be delayed in phase by several degrees.
to zero, the positive
feedback will still hold the Schmitt trigger switched as if the input were still above the zero point.
The Schmitt trigger will not switch to a low
voltage output again until the input sine wave
goes considerably below the zero point. Before
the trigger can switch, the input must exceed the
effect of the positive feedback which is holding
the output at the opposite extreme. This is the lag
or hysteresis. After the sine wave has gone below
zero and switched, it will again have hysteresis or
a lag when the sine wave comes back up. When
To explain how the Schmitt trigger ignores
high frequency noise riding on a sine wave, let's
feed a noisy sine wave into a zero crossing detector and a Schmitt trigger and compare the difference.
153
NOISE CAUSES
EXTRA PULSES
NOISE RIDING
ON SINE WAVE
THRESHOLD
rrn
zero
CROSSING
DETECTOR
VOUT
PHASE SHIFT
DELAYS PULSES
THRESHOLD
I
L SCHMITT TRIGGER
ONLY RESPONDS
TO FIRST TIME
SIGNAL GOES
PAST THRESHOLD
•PHASE
SHIFT
V
t
VlN
EXCEEDS
VlN
SCHMITT TRIGGER
EXCEEDS
+
THRESHOLD
THRESHOLD
FIRST
TIME
Fig. 9-13
Because
Comparison of zero crossing detector and Schmitt
of the noise spikes
on the sine wave,
/.
FIRST
TIME
trigger.
Unijunction Transistor Oscillators
the zero crossing detector responds several times
wave
per sine
cycle. Instead of the big sine
Yes, there is another kind of transistor we
haven't mentioned. The unijunction transistor is
a device that resembles the junction FET in its
construction and circuit symbol. Its volt-ampere
characteristic has roughly the same shape as that
of a tunnel diode, including the negative resistance region. This makes it useful as an oscillator.
They do not work very well at high frequencies
and are not as reliable as other transistors. For
these reasons there is little to recommend them
over other ways of building oscillators. Still, you
may occasionally find one in the depths of
avionics instruments, so we did not want you to
be surprised.
wave
crossing the zero point just twice each complete
sine wave cycle, the signal may cross four or more
times because of the noise. The higher the noise
spikes, the more often they are liable to cross.
These extra crossings result in narrow, extra output pulses which preceed and follow the desired
large pulses.
When
the Schmitt trigger is given the same
does not switch until the signal rises
above a relatively high threshold. Not only that,
signal,
it
it
will ignore
any further crossing changes
until
the signal goes below a relatively low, below zero
threshold. Any noise on these signals will be ig-
A unijunction transistor (UJT) oscillator is
seen in Fig. 9-15. This circuit happens to be a
metronome for piano teachers, but practically all
UJT oscillators use the same circuit. In pulse circuits frequently the loudspeaker is replaced with
nored unless the noise amplitude is so high that
noise peaks extend from the upper threshold all
the way down to the lower threshold. Notice that
the Schmitt trigger is a kind of filter that ignores
low amplitude noise, but responds to the biggest
signal as if the small signals weren't present.
an inductor or load resistor. The capacitor, C,
154
is
+ V CC
VOUT
V|N^-
*
BASIC AMPLIFIER SYMOBL
(NON-INVERTING)
lb
INPUT
B1
Ve-
SIGNAL
C N-TYPEUJT
VOLTAGE
>
>
VOUT
LL
—
-?
^
COMPARISON
VOLTAGE
COMPARATOR SYMBOL
CAN ALSO BE OPERATIONAL
INPUT
rrn
s\ss&'
|
L-vj
TOCK!
r
°c Kf
LOUDSPEAKER
SERVES AS
LOAD
AMPLIFIER
SIGNAL
CAPACITOR CHARGES
CAPACITOR DISCHARGES
, THROUGH TRANSISTOR
VOLTAGE
CO.
GROUND
OQ
o
yj
tu
io
Q. z 5 o S
S:<Otl2
3 -> $ a.
to I
IS
ZERO
VOLTAGE
/777
COMPARATOR WIRED AS ZERO
t
CROSSING DETECTOR
CURRENT PULSES WHICH
OCCUR EVERYTIME
INPUT
CAPACITOR DISCHARGES.
PULSES THROUGH
LOUDSPEAKER
lb
MAKE
NOISE.
/777
ZERO CROSSING DETECTOR
WIRED AS A SCHMITT TRIGGER
NOTE HYSTERESIS DRAWING
ON AMPLIFIER SYMBOL
Fig. 9-15
neon or P-N-P-N relaxation
">
J.
SYMBOL FOR SCHMITT TRIGGER
Fig.
9-14
Symbols
and Schmitt
for amplifiers,
comparators
triggers.
resistor, R. When the
discharges the capacitor suddenly
and causes a large current pulse to flow from the
+ supply down through base 1 to base 2 and
fires,
oscillator.
Integrated Timing Circuits
We now enter the wonderful world of integrated circuits. The NE555 timer integrated circuit (IC) contains circuits already discussed in
this section and it is used as a universal oscillator
which can generate square waves and sawtooth
waves (oscilloscope sweep waveforms) and can be
used as a monostable multivibrator. It can also
detect when a pulse is missing from a series of
equally spaced pulses. Some push button tele-
charged by the variable
UJT
unijunction transistor oscillator
pulses give the metronome that "tock-tock-tock"
sound. To summarize, it works very much like a
VOUT
VlN^
A
it
through the loudspeaker to ground. These current
155
phones use this "timer" IC to generate the
musical tones you hear when the buttons are
pressed to "dial" a number. This IC has so many
uses we could spend a section on it.
One
of the
advantages that makes ICs won-
that most of the nitty-gritty details like
diodes, base resistors, and biasing are all hidden
away in the silicon chip. To repair a circuit which
uses "ICs," all you need to do is make sure that
derful
is
each IC
is
doing the overall job assigned to
it.
example, suppose your timing circuit is supposed
to make square waves and it doesn't. You check
to be sure that the IC has all the correct voltages
and inputs that are supposed to make it perform.
If it has everything it needs, but refuses to work,
you remove the old one and replace it. There is no
way to repair defective ICs, which is probably a
blessing.
If
For
you
you study the
will notice
circuit
diagram
some unfamiliar
for the timer
transistor sym-
At the upper left a transistor is drawn with
base connected backwards. A transistor at the
upper right has two emitters and two collectors.
When ICs are printed on a silicon chip, transistors are often built in unusual configurations
or have parts doubled. Fortunately, you don't
need to understand IC technology in detail unless
you are actually building ICs. It is enough to
understand that the block of circuitry which contains the double-emitter transistor is a bistable
bols.
its
Fig. 9-16
package
NE555
IC
flip-flop
two separate timer
studied.
integrated timer circuit. The
on the right contains
and
behaves
like
the
flip-flop
you
circuits.
o
+ v cc
PIN 8
il
\
> —
THRESHOLD}—•
CONTROL
VOLTAGE
\
/
'^
4
—/\
RESET
OR
IKIUI
INHIBIT
5
*
RESET
BISTABLE
OUTPUT
}
FLIP-
SET
FLOP
PIN 2
TRIGGER^)
—•—
<CAPACITOR
U
5k
PIN
1
rrn
rrn
Fig. 9-18
Block diagram of the
156
NE555
timer
DISCHARGE
157
the timer
RESET IS
HELD HIGH TO
KEEP IT FROM
INTERFERING
more
is
much more
reliable
precise, more versatile,
and costs about 39c retail. This is
less than the price of a unijunction transistor,
you can find one
Like the UJT, the timer
2
TRIGGER INPUT
^HELPSTO
X
STABILIZE
/TT7
UPPER VOLTAGE.
2/3
CAPACITOR
VOLTAGE
V cc
In Fig. 9-19 the timer
A
is
wired as a square
R a + Rb.
charges the capacitor, C. When the high voltage
threshold (2 3 of V cc is reached, the flip-flop
switches and the capacitor discharge transistor
inside the IC turns on, it is like a switch that
shorts pin 7 on the IC to ground. If R D is very
large (high resistance) then the voltage pulse on
the output stays low a long time because the
capacitor is discharged slowly. But if Rb is very
small, perhaps even zero ohms, then the output
pulse will be very short because the capacitor will
discharge quickly. In this case the timer will be
operating just like the UJT oscillator above.
wave
R a + Rb
CAPACITOR DISCHARGES
or pulse generator.
resistance,
)
THROUGH Rb
A VOUT (PIN
sensitive to high
application.
CAPACITOR CHARGES
^THROUGH
is
and low voltage stages that cause it to switch.
These two voltage levels are detected by the two
comparators. These comparators go full on or full
off when the input voltage rises above or below
the two switching thresholds. In the circuit
diagram you will see three 5K resistors in series
going from V cc to ground. The high and low
voltage thresholds are established by the high
and low end of the center 5K resistor. The outputs of the comparators go to the set and reset
triggers and cause the flip-flop to change state
when these two voltage thresholds are crossed.
Because of the complexity, it has a number of inputs and outputs which are not needed for every
TT7
"-pX.
if
for sale.
3)
By
not connecting the "trigger'* input, pin
2,
to the capacitor, C, the timer will not retrigger
and the oscillator will not be stable. Instead
becomes a monostable multivibrator. A short
itself
it
Astable square wave oscillator
Fig. 9-19
The block diagram
veals that
voltage pulse into the trigger input causes the
timer to make one long pulse on the output. The
length of the pulse is determined by the size of
the resistance charging the capacitor. C.
circuit.
of the timing circuit re-
QUESTIONS:
consists of a bistable flip-flop, two
comparator amplifiers and a few more transistors
it
for discharging the
the timer
together
timing capacitor, inhibiting
1.
and amplifying the output \i
has 23 transistors, two diodes, and 15
it
For
complexity, the function of
is really not much different
than a single unijunction transistor. However,
resistors.
all
What
are
bistable
(non-oscillating)
multivibrators used for?
(reset),
its
2.
the circuit as a whole
In
what form
flip-flop
is
memory
the circuit power
1
58
information stored
cell?
is
in
a
What happens when
shut off?
3.
The
trigger pulse input circuit in Fig. 9-3
12.
always makes the flip-flop change state.
How can this be since the trigger pulse is
applied to both transistors equally?
13.
What is a clock pulse? Why are they often
derived from sine wave oscillators?
What
4.
In the flip-flop counter circuit shown in
what would be the states of the
flip-flops during the 13th pulse? What
14.
would be the states
15.
Fig. 9-4,
of the flip-flops just
In a typical astable multivibrator (Fig. 9-6)
16.
the bases are biased partly on by the resistors R\ and R2. Why aren't these
resistors connected to
ground so that the
transistors can be sure of turning
If
the purpose of the astable oscillator
17.
is
a zero crossing detector improved
is
accurate to say that a Schmitt trigger
Is
it
is
a kind of zero crossing detector?
What
hysteresis?
is
Is it possible for a
too
to
generate "square" waves, what must be
true about the relative sizes of the load
resistances and the capacitor charging re-
19.
R\ and R2.
sistances,
How
a comparator?
is
off.
18.
6.
What
is
by positive feedback?
after the 13th pulse?
5.
What
is a zero crossing detector?
response to a zero crossing?
its
much
Schmitt trigger to have
positive feedback?
Why does a Schmitt trigger circuit symbol
have an "S"-like symbol drawn on it?
What is this symbol and what is its
significance?
7.
If
an astable multivibrator
by a
set trigger pulse,
is
synchronized
what happens
if
20.
the
8.
21.
What are some uses for the
integrated circuit?
Suppose an astable multivibrator (like Fig.
used as a power inverter to convert
12 volts DC to 6 volts DC. The transformer
22.
Why
is
wound
6
so that the secondary
volts
RMS.
This
is
AC
voltage
23.
What
What two
to
all
one
monostable
multi-
vibrator?
11.
like
the
timer preferred over a nice simple
repairing circuits
containing ICs,
it
necessary and important to
Suppose the orange packing machine in
Fig. 9-9 were expanded so that it could load
10,000 oranges in boxes of 100 in a truck.
How many more flip-flops must be added
to the present string of flip-flops to enable
the circuit to count up to 10,000?
159
know
the details of the circuitry inside the
tegrated circuit?
similar circuits are usually com-
build
When
why is
isn't it
are the
What is the basic purpose of a monostable
multivibrator?
bined
circuit
timer
usually enough to know the block
diagram of the integrated circuit? Why
advantages of the zener diode circuit?
10.
complicated
NE555
rectified
filtered to
of the multivibrator circuit?
a
is
NE555
UJT?
and
produce 6 volts DC. Another
way to convert 12 volts DC to 6 volts DC
would be to use a 6 volt zener diode and a
dropping resistor. What is the advantage
is
what
a unijunction transistor and
is
flop?
9-7) is
9.
What
are they used for?
slower rate than
the natural, astable frequency of the fliptrigger pulses occur at a
in-
SECTION X
Operational Amplifiers
A.
Introduction
3.
Perfect linearity
Operational Amplifiers, or op-amps for short,
4.
Complete input-output
isolation
are a high gain amplifier circuit that attempts to
They
achieve the perfect amplifier.
"operational" because they
are called
5.
may
Infinitely fast switching
be wired to perform mathematical operations on voltages and
currents. For example, the addition of voltages
means literally adding 3 volts to 2 volts to get 5
volts. Circuits based on op-amps may be used for
subtraction, division, multiplication, taking
Operational
logarithms, and solving differential equations.
mance
6.
are used.
any application
in
They can be wired
Perfect current source or voltage source output.
amplifiers
can exceed the perfor-
of single transistor amplifiers in every
aspect except the infinitely fast switching and
unlimited slew rate.
Operational amplifiers can also be used for
practically
which transistors
Operational amplifier voltage gain is not inbut is extremely high, 100,000 or more.
Negative feedback through resistors is used to
adjust the gain to any level desired. Different
kinds of components in the feedback path such as
diodes and capacitors can produce the unusual
as high gain non-
finite,
inverting amplifiers or inverting amplifiers of
almost any voltage or current gain desired. They
can be used in sine wave oscillators, multivibrators, Schmitt triggers, frequency filters, and
much more. Their primary limitation is that they
generally lack high frequency response. Today,
1980, they are rarely used above about 2 MHz.
Nearly all op-amps today are in the form of integrated circuits and the cost per amplifier can be
as little as ten cents each. Entire books are written on particular uses for operational amplifiers,
so it is hard to do them justice in two sections.
mathematical operations.
C.
Differential Amplifiers
Operational amplifiers are very high gain,
sophisticated differential amplifiers, so
we
explain operational amplifiers without
first ex-
plaining differential amplifiers.
the differential amplifier
B.
The Op-Amp— The Ideal Amplifier
In Section 4
ideal amplifier.
we introduced
We
and unlimited slew
rate.
is
can't
The purpose
of
to amplify the dif-
ference between two signals. For example,
if one
input signal is 10 volts and the second input
signal is 6 volts, the signal we wish to amplify is
the 4 volt difference between the two.
the idea of an
listed the following attributes
as being ideal for an amplifier.
1.
A
2.
Unlimited gain
Isn't amplifying
the difference between
voltages like amplifying the hole in a donut? Who
perfect switch
cares,
nals
161
you ask! The difference between two
is
important
because
not
all
sig-
important
You might imagine
voltages are referenced to ground. For instance,
suppose you want to amplify the voltage across
one of the capacitors in an astable multivibrator
that a differential am-
that could handle 10,000 volt inputs and
only look at the small difference voltage would be
plifier
hard to build. You're right,
as in Fig. 9-6 in the last Section. Neither of the
voltages at the ends of the capacitor are the
A
If you have the
chance to build such a multivibrator in the lab,
you should try to look at the capacitor voltage
with an oscilloscope. You will find this is impossible unless you have a two channel oscilloscope
that can be set up as a differential amplifier.
voltage across the capacitor.
it is
difficult!
is common to both input
common mode voltage. If this
large voltage that
signals
called a
is
common mode
voltage
is large, real differential
amplifiers have a hard time ignoring
it. This large
voltage will be partly amplified along with the
voltage difference. Therefore differential am-
plifiers
Adiff-
VlNi>-
that
have two voltage gains, the one we want,
and the voltage gain that we don't want
amplifying the
is
^com- The usual way
->
common mode
voltage,
to rate the quality of a dif-
by its common mode rejecnumber you get when you
divide the differential voltage gain by the comferential amplifier is
V1N2V
= AV
V
(V|N-|
tion ratio. This is the
- V|N2>
DIFFERENCE VOLTAGE = Vi^ - V1N2
mon mode
Adiff
COMMON MODE VOLTAGE IS THE ARITHMETIC
AVERAGE OF THE TWO INPUT SIGNALS
THE
VCOM
Fig. 10-1
=
1/2 (V1N1
Differential amplifier
Common Mode
The emitter coupled
symbols and pro-
the most
=
A c om
.
(Vin)
amplify the
ference between two input voltages, Vjnl
Vin2- Therefore,
difference amplifier
dif-
and
emitter-to-ground voltages. If either collector
V = Av
<V in i
- V in2
taken as the output, the collector voltage
)
ly
A
is
common
kind of differential amplifier.
Referring to Fig. 10-2, the emitter coupled differential amplifier is made from two common
emitter amplifiers that share the same emitter
resistor. Currents from both emitters flow down
through the emitter resistor. Therefore, the
voltage across the emitter resistor goes up when
either transistor begins to turn on. If two large input voltages are applied to the bases of both transistors, the emitter resistor voltage will try to
turn both transistors off again by raising the
As you know, voltage amplifiers have voltage
gain which means that the output is the input
voltage times the amplifier voltage gain, A v
Differential voltage amplifiers
Rejection Ratio
+ V|N2>
perties
V = Av
voltage gain.
change substantially when there
is
is
will on-
a big dif-
ference between the two input voltages.
would ignore the
two input voltages and just look at
the difference between the voltages. Suppose the
amplifier had a voltage gain of 2. Therefore, if
perfect differential amplifier
The bigger the emitter
sizes of the
V in
= 10andV in2 =
i
V
And
\
',,
And
2 (10
-
=
10 ° and
2 (100
-
96)
if
Vin]
=
=
6.
=
6)
Vini
if
=
=
2
(4)
v in2 =
=
8 volts.
8 volts.
=
it is
is,
the more
at cancelling the
.
96, then
10.000 volts and Vj n 9
resistor
response to large
common mode voltages. The higher the emitter
resistance, the more dramatic the change of the
voltage will be between the emitters and the
below ground power supply, Y ee However, if the
emitter resistance is too big, even the smallest
emitter current will cause a voltage drop across
the resistor that is as big as the power supply
voltage. This extreme case would make any
change in the output voltage impossible.
effective
9,996
To
volts.
get around this problem, practical
dif-
ferential amplifiers replace the emitter resistor
V =
2 (10.000
-
9,9961
=
with a network of interconnected transistors and
8 volts
162
diodes that
is
a constant current source.
An
amplifier on the positive input, the output will
not be inverted. A voltage gain of one is shown,
but the gain could be any number. In Fig. 10-3 the
ex-
ample of a complex current source can be found
in
the operational amplifier in Fig. 10-4. This circuit
can be thought of as an automatic variable emitter resistor that varies its resistance in response
to the size of the
common mode
signal on the non-inverting input
not inverted
is
or phase-shifted in the output.
voltage. Large
common mode
voltages cause the resistance to
become very high. Small common mode voltages
If
the inputs in Fig. 10-3 were reversed so
wave were fed into the inverting,
that the sine
cause the resistance to stay very low. Using a current source instead of an emitter resistor, the
common mode rejection ratio is usually 1000 or
minus
input, the signal
lector of the
same
would be taken
transistor. This
off the col-
means that the
sine wave on the output will be 180° out of phase
with the input. Whether the signal is inverted or
not at the output depends on which of the two
higher.
ABOVE GROUND
transistor collectors the output
POWER SUPPLY
REFERENCED TO
GROUND
Sometimes
differential
taken from.
is
amplifiers
are
built so
that the output signal can be taken off either or
both collectors. This configuration is called differential outputs.
RLi
«L 2
E.
VOUT
->
*
- INPUT
/
+ INPUT
<-
VlNi
V|N2
/
A POSITIVE INPUT
A POSITIVE
INPUT ON THIS
ON THIS SIDE MAKES
VOUT GO DOWN.
MAKES
VOUT GO UP
SIDE
ei
"e2
-V ee
Symmetrical
emitter
usually a
output stage receive inputs of the proper polarity
and correct DC average voltage levels.
All the internal amplifier stages of an
op-amp
are direct coupled. There are no energy storage
coupled
components like capacitors or inductors in between the stages which would decrease the gain
dif-
ference amplifier
at high or low frequencies. It
D.
is
complementary class B stage. The class B stage
is driven by a DC level translator network which
insures that the two transistors in the class B
BELOW GROUND
POWER SUPPLY
REFERENCED TO
GROUND
10-2
As we have said, operational amplifiers have
very high gain— the more gain the better. To
achieve this high gain, the transistors in Fig. 10-2
can be replaced with Darlington transistors.
Usually two or more differential amplifiers are
put in series to increase the gain still further. This
is done using the two differential outputs to drive
the differential inputs of the next stage. The last
amplifier of the operational amplifier
BOTH EMITTER CURRENTS
ADD TOGETHER ACROSS
R e RAISING THE EMITTER
VOLTAGE OF BOTH
TRANSISTORS.
Fig.
Operational Amplifier Design
Inverting and Non-inverting Inputs
is difficult
to build
capacitors into an integrated circuit and extremely difficult to make inductors. Therefore, in-
Notice in Fig. 10-2 that the two inputs are
and minus. This convention means
that when a signal on the positive input goes
positive with respect to the signal on the negative
tegrated circuits are designed to avoid using
these components, even if it means using a dozen
input, the output will also
From an applications point of view, the advantage of direct coupling is that the op-amp is a
DC amplifier and will amplify DC voltage levels.
For example, let's say that we shunt an operational amplifier with negative feedback so that it
has a voltage gain of two. We use it to "amplify''
a 1.5 volt flashlight battery so that the output is
3 volts DC. Because the battery voltage does not
labeled plus
transistors to avoid using one capacitor.
go positive. In other
words, the polarity of the signal on the positive
input is not inverted. Suppose that the negative
input of a differential amplifier is connected to a
constant DC voltage, say +5 volts DC, and suppose the positive input is connected to a sine
wave signal which has its zero point established
at +5 volts. Since the sine wave is applied to the
163
A SINE WAVE ON THE
NON-INVERTING INPUT YIELDS
A NON-INVERTED OUTPUT.
NOTICE THAT A +5 VOLT
DC SIGNAL IS COMMON
TO BOTH INPUTS SO IT
IS CANCELLED IN THE
OUTPUT.
Fig. 10-3
A
signal on the non-inverting input
is
not inverted
in the output.
square wave signal at the output was proportional to the DC level of the input DC signal. The
chopping consisted of turning the input to the
first stage on and off rapidly. After the square
wave had been greatly amplified, the output
square wave was then filtered to remove the AC
component much the way that low pass filters are
used to filter out the ripple in power supplies.
change, the output remains at 3 volts indefinitely. Not all signals that are worth amplifying
change rapidly like radio or audio signals. Some
signals, like temperature readings, barometric
pressure, or humidity, change slowly over hours
or days.
Before integrated circuits, building high gain
DC
amplifiers was difficult because temperature
changes made the amplifier very unstable. As the
gains of individual transistors changed, the transistor outputs would drift up and down changing
the quiescent points. These changes in DC
voltage would be directly coupled to the next
stage where they would be amplified and fed to
the next stage. The result was that small changes
in voltage would be amplified until the last
amplifier stage would turn full on or full off
without any regard for the input signal.
to
Integrated circuits
tion
can have extremely closely matched
and temperature characteristics. These
matched transistors are used in the circuit so that
changes with temperature will be cancelled by the
same changes occurring in identical transistors.
Another trick used to temperature stabilize transistor amplifiers is to put a diode between the
base and emitter. As the transistor heats and
tries to turn more on, the diode also heats and its
forward offset voltage decreases. This change
shunts more base current to the emitter and turns
the transistor back off again. Three examples of
silicon chip
In contrast, a high gain AC amplifier is easy
stable because the biasing of each of the
is
independent. The
made chopper stabilizamade on the same
Transistors
gains
make
several transistor stages
obsolete.
DC
from one stage is not biasing the next stage
because the stages are separated from each other
base diode technique can be found in the integrated operational amplifier circuit in Fig. 10-4.
this
by capacitors or transformers. As the quiescent
point of one transistor changes,
this
change along
it
Probably the most well known operational
is the type 741 integrated circuit. The
cannot pass
to the next stage.
amplifier
"MCI"
In the old days, slowly changing
DC
were usually amplified by chopper stabilized DC
The DC signal was "chopped" into a
fied
by
AC
AC
signal that could be ampli-
amplifiers.
The amplitude
of
means
that
this
circuit
is
the
.
amplifiers.
square-wave-like
prefix
Motorola Company version of the IC. The 741 opamp needs two power supplies, +V CC and — V ee
The output is designed to rest at a quiescent
point of exactly zero volts. So if the operational
amplifier were used as a high fidelity amplifier,
signals
the
164
Many of the operational amplifiers are not as
fancy as the 741 and do not have an external null
offset adjustment. For example, the MC1558 con-
(TYPICAL +12)
sists of two 741's in the same 8 pin IC package.
The MC4741 contains four 74 Is in a 14 pin
NON-INVERTING
INPUT
>
>
package. In these IC's there are not enough pins
OUTPUT
to allow offset null leads for
*
INVERTING
INPUT
of the individual
Some operational amplifiers, such as the
LM324, are designed for use with a single power
supply. This means that "zero" must be defined
at some voltage half way between zero and V cc
just the way we did for the class A amplifier.
FFSET NULL ADJUSTS
OUTPUT TO EXACTLY
ZERO VOLTS WHEN
THERE IS NO SIGNAL.
-V ee
all
operational amplifiers.
,
(TYPICAL -12 VOLTS)
pull
NOTE THE
COMPLEMENTARY
Notice that the output of the 741 is a pushcomplementary amplifier. This allows the
output to change from a very high voltage, nearly
+V CC down to a very low voltage, nearly V ee
without ever having very large currents flow
directly from +V CC down to — V ee Large currents are prohibited because only one of the two
output transistors needs to be turned full on at
one time. This is the same energy saving principle
we discussed in the CMOS inverter circuit. By
having the two transistors on at different times,
the current through the two transistors is never
,
CLASS B OUTPUT.
,
.
INVERTING
INPUT
OFFSET
NULL
great and
(25
50 k
VEE
is
by the two resistors
programmable operational
further limited
and 50 ohms).
A
amplifier has an input lead that allows the circuit
V_
minimum power
whatever power supply voltages
quiescent current to be tuned for
consumption
are being used.
THE CURRENT SOURCE IS THE CIRCUITRY
AROUND THE OFFSET NULL INPUTS.
10-4
Fig.
MC1741
diagram and
circuit
(741)
operational
amplifier
is
symbol
how fast the voltage on the output can change.
For example, the slew rate of a 741 is 0.5 volts
change in one microsecond, or 0.5 V7 \i s. A newer,
high speed version of the 741, the MCI 74 IS has a
slew rate of 10V7 u s.
wave output signal would be symmetabout the zero voltage axis. For some applications, such as an amplifier for a thermocouple thermometer, the output must be adjusted to
the sine
do
when
this the 741 is
lead.
the input
Frequency response in operational amplifiers
measured in terms of slew rate. This just refers
to
rical
exactly zero volts
for
is
zero volts.
equipped with an
To
F.
The Comparator
offset null
The best way
"Offset" means a voltage set off to one side
to understand the operational
thousandths of a volt is not important because
the signal is AC and any offset will disappear
how it is used. The simplest
use for an operational amplifier is to use it as a
comparator. By connecting the two inputs to different voltages, these two voltages can be compared. In other words, whenever one voltage is
bigger than the other, the op-amp will turn full on
or full off. This off-on switching action is often
across the coupling capacitors.
used to control machinerv or
and "null" means to set something to
zero. This is done by means of a potentiometer
wired to the op-amp as shown in Fig. 10-4. For
many applications, such as a high fidelity
of zero
amplifier
preamplifier, adjusting the zero to within a few
165
is
to study
circuits.
A
REFERENCE VOLTAGE
REPRESENTING THERMOSTAT
SETTING
70°
? +v cc
(5
VOLTS)
65°
FURNACE
ON
60°
TA
VOLTAGE REPRESENTING
ROOM TEMPERATURE
FURNACE
OFF
(5
FURNACE
OFF
VOLTS)
rrn
Fig. 10-5
A
comparator used as a thermostat.
For example, a comparator could be the heart
of a thermostatic control for a
home
Fig. 10-6
shows a
single IC
which contains
ten comparators. These comparators can be used
to drive a bar graph voltmeter. The bar graph
furnace.
When the voltage representing room temperature
drops below a voltage representing the desired
room temperature, the comparator would switch
and turn on the furnace. The thermostat temperature control is just a potentiometer which is
calibrated in degrees of temperature, but it produces reference voltages that are compared with
the voltage from an electronic thermometer.
voltmeter is simply ten lights lined up in a row or
column. The lights could be labeled in volts from
one volt up to ten volts, miles per hour, or any
other calibration. The lights are light emitting
diodes (LEDs) and each light is driven by its own
comparator. Each comparator is wired to look for
voltage: one volt, two volts, three
and so on up to ten volts. These reference
voltages are obtained from a string of eleven
resistors in series. The voltage to be measured is
a
specific
volts,
When the positive (non-inverting) input is
connected to a more positive voltage than the
negative input (inverting input), the output
voltage will go full up to nearly V cc a high
positive voltage. If the inverting input is more
positive than the non-inverting input, the output
will drop to a low voltage, nearly the negative
supply voltage, — V ee Actually, comparators are
more likely to be used with just one power supply
since there is no need for the output to be balanced half way on at exactly zero volts.
introduced to the positive inputs of all of these
comparators. All comparators whose reference
voltages are exceeded will turn on. As the column
of lights is activated, the visual effect resembles
the red column rising in a thermometer.
,
.
Comparators do not need
way on
ference
to be balanced half
when
be put
the input signal difprecisely zero. Also, they do not need
at zero volts
is
This voltmeter is rather crude, since it has
cnly ten steps. However, it is cheap and it is
beginning to be used for fuel gauges or other uses
where the quantity cannot be precisely measured.
If more precision is needed, two or more ICs can
in series to drive
columns of 20 or 30
lights.
high linearity. As a result, special comparator integrated circuits are usually used for this purpose
instead of op-amps. These circuits are usually
simpler and cheaper than op-amps, and because
they have fewer parts, they often switch on and
off faster than the complex op-amps.
G.
The Voltage Follower
Another simple use for the op-amp is a
voltage follower. The voltage follower is used for
matching high impedance sources to low imped166
+ 12 VOLTS DC
10
VOLTS
9
VOLTS
THE IC (DASHED LINE)
CONTAINS 10 COMPARATORS
AND THE 10 RESISTORS
WHICH ARE THE REFERENCE
VOLTAGE STRING.
£>
8
VOLTS
£>
/\_
7
W/
VOLTS
6
"\
•M-
=D>
\l /
VOLTS
s>
7
5
VOLTS
\l/
VOLTS
t>
_/\
4
/
VOLTS
t>
3
/
VOLTS
t>
2
LIGHTS GO ON IN
> 7RESPONSE
TO 7 VOLTS INPUT.
W-
>r-
\l /
VOLTS
=D>
1VOLT
3> /
/
_j
/777
Fig. 10-6
A
I
\
rrn
bar graph voltmeter which uses a single
167
IC containing
ten comparators.
ance loads. In Section 7 the emitter follower and
source follower were used the same way. Suppose
you had a signal with a large voltage but a high
source impedance. If you try to use this voltage
directly to drive a meter or an amplifier, the load
will draw too much current and voltage will sag
and become inaccurate or distorted. The voltage
follower configuration will amplify the signal current but the output voltage will be identical to the
input. This circuit will provide whatever current
is needed, but the output voltage will always
feedback will stop the output from
changing as soon as the difference voltage is
negative
again zero.
The voltage gain
of the voltage follower
is
one. However, the current gain can be quite high
depending on the design of the op-amp and will
vary with the size of the load impedance. The load
on an operational amplifier can be from the output to ground, the output to +V CC or even from
the output to — V ee This versatility is a major
reason why op-amps are so widely used.
,
.
"follow the input."
NEGATIVE FEEDBACK
H.
Precision Diode
Now
here
is
a cute
little circuit
variation of the voltage follower.
that
is
a slight
You may have
been bothered by the lack of a rectifying diode
with a break point at exactly zero volts. On the
other hand, maybe not! All the diodes we have
looked at so far either haver a voltage offset of 0.2
or 0.6 volts or do other strange things. In contrast, this diode is almost "perfect."
V|N = V
So A v = 1
10-7 A voltage follower used to match a
high impedance source to a low impedance load.
Fig.
shows an op-amp wired as a voltage
The output is wired directly to the
Fig. 10-7
follower.
- 0.6 +
VOLTS
COMPENSATION
an example of negative
feedback. Whenever the output tries to rise, this
rise is coupled back to the input and tries to turn
the output back down again. Of course, if the output falls below the positive input voltage, the
feedback will cause the output to rise again.
negative input. This
is
VOLT-
Ok
AMPERE
now, listen up! If you miss the next sentence, you have missed 13 of the whole section.
In an operational amplifier circuit with negative
feedback, the extremely high gain will change the
output until the negative op-amp input voltage
equals the positive op-amp input voltage.
By
SILICON DIODE
CHARACTERISTIC
FORWARD
CHARACTERISTIC
+V
+ 0.6
VOLTS
the
definition of a differential amplifier, the output
voltage will be the difference between the positive
and negative input voltages multiplied times the
gain. Because of the way it is wired, the voltage
follower output voltage is the negative input
means that whenever the positive
input tries to differ from the negative input, the
voltage. This
Fig.
L68
10S
F
}
recision diode
made
with an op-amp.
made from an
tional amplifier to cancel out the offset voltage.
back at all, the voltage difference between the
two inputs is zero. It is as though the two inputs
were connected together and they are shorted.
We know
This strange connection
This precision diode
germanium
silicon or
is
ordinary
diode, but uses an opera-
that this circuit has negative feedback
because circuitry connects the output with the in-
verting
the
Therefore,
input.
amplifier
tween the two inputs is always zero and that the
inputs draw no current is all you need to figure
out what the amplifier is doing.
will
change the output until the voltage on the negative input equals the voltage on the positive input. Because of the feedback, the op-amp will
assume the same voltage drop across it that is
across the diode and cancel out the voltage offset.
Now tiny AC current signals can be rectified precisely at zero volts. Notice that even the forward
resistance of the real diode is largely compensated and the resulting volt-ampere characteristic is an almost perfect rectifier. Of course this
diode
and
is
still
limitations
and the
of
the
Non-inverting amplifier with controlled gain
1.
The
easiest
way
to understand
how
a ne-
gative feedback network determines the gain is to
start by looking at the output voltage. Then we
will
work our way back
to the input.
Fig. 10-9, the output of the
op-amp
The current flows from the
subject to the frequency, current,
voltage
amplifier
is called a virtual short
that the voltage difference be-
Knowing
circuit.
ground through the 2K Q
operational
resistor.
real diode.
6
in
6 volts DC.
volts
resistor
The voltage divides
As drawn
is
down
and the
to
1KQ
across these two
resistors. So,
/.
Operational
Amplifiers
With
Controlled
R
R + Rf
Voltage Gain
VinThe input resistance to an op-amp, as
measured from either input to ground, is very
high. This is because the current source circuit
that acts like an "emitter resistor"
makes
voltage drop between the emitter
IK
a large
and — V ee
Vin
:
.
P-N junctions of the transistors can not
conduct current from one input to the other, so
the impedance between one input and the other is
quite high, ideally infinite. In op-amps made from
FETs these input resistances really do approach
IK + 2K
6
=
—
6
=
2 volts
Also, the
This is consistent with the high gain and negative
feedback forcing the two inputs to have the same
That is, zero volts difference
tween the two op-amp inputs.
voltage, 2 volts.
be-
infinity.
Because of
Just to make sure you have the right idea,
look at a negative input signal to the same
amplifier: the output voltage is —4.5 volts so
this large input resistance, the
current that flows into an op-amp input
microamperes
is
These
the purpose of
let's
very
cur-
positive current
calbe ignored for
culating voltage gain and predicting the overall
performance. You should consider the input
resistance to an op-amp as infinite. Therefore, no
current flows into an op-amp input. As a result,
the voltage on an op-amp input should not be
determined by this current. Later we will see that
the tiny input currents are important for balancing an op-amp output to zero volts. However,
from the point of view of the op-amp's assigned
task in the circuit, the input current is negligible.
the —4.5 volts.
small. 0.1
rents can
is
typical.
must flow from ground down
As
across the resistors and the two input voltages
should come out equal, —1.5 volts.
Vin- =
R
R + Rf
IK
We
saw
in the voltage follower
how
the huge
in"
voltage gain of the op-amp made the negative input the same voltage as the positive input. The
gain of the op-amp is so high, that for all practical
purposes, whenever there
is
any negative
to
before, the voltage divides
feed-
169
4.5
IK + 2K
=
1
4.5
3
=
-1.5 volts
POSITIVE
CURRENT FLOWS FROM
6
VOLTS, PAST V|N-.
AND DOWN TO
GROUND. ZERO.
VOLTS
rm
+ 6 VOLTS DC
6 -v ee
>
R + Rf
Vo
Av =
V|N +
rrn
p±
tr
Fig. 10-9
POSITIVE
]
Non-inverting amplifier
CURRENT FLOWS FROM
VOLTS. PAST V|N-.
AND DOWN TO
-4.5 VOLTS.
4.5
6 ~
>
Vo = A v
VOLTS
v ee
V| N
Vo = (3X-15) = -4.5 VOLTS
rrn
Non-inverting amplifier with negative
input voltage.
Fig. 10-10
We
can summarize the gain of non-inverting
2.
amplifiers with the formula:
Av = +
This formula
is
R + Rf
R
memorize
it
operational
amplifiers
with
con-
Wiring the op-amp so that the output signal
is not very different, but the voltage
gain changes because the two inputs are always
is
so basic to figuring out opera-
tional amplifier circuits, that
Inverting
trolled gain
inverted
locked together at zero volts instead of at the input voltage. In the inverting amplifier the positive input is soldered to zero volts, so the
negative input must follow it.
you should either
or be able to figure out the gain in a
few seconds using the reasoning process we just
went through.
170
POSITIVE
CURRENT FLOWS FROM +2 VOLTS,
PAST ZERO, AND
DOWN TO -4
'
R
VOLTS, PAST ZERO,
VOLTS.
AND DOWN TO THE
1K^
+ 2 VOLTS
)
VlN
>—
1.5
n 77
INPUT, -1.5
i
ZERO
WW*
V|N^>
CURRENT FLOWS FROM +3
POSITIVE
1KQ
>n
rrn
V
ZERO
VOLTS
vAAAA-
VOLTS
ZERO
VOLTS
VOLTS
= -4 VOLTS
ZERO
VOLTS
V
= +
3
VOLTS
Vo
= -2
VOLTAGE GAIN =
-1.5
VlN
I
=
-Rf
-<2K)
R
1K
= -2
= -
V
=
3
Fig. 10-12
Fig. 10-11
Inverting amplifier
put
In Fig. 10-11 the inverting amplifier is using
same resistors as in the non-inverting amplifier, but the gain is less. Knowing that the two
inputs will be equal and zero, makes it easy to
figure out what the output will be. If we assume
that the input voltage is +2 volts DC, then this
voltage will force a current to flow down to zero
volts through the 1KQ resistor. No current goes
into the negative op-amp input because of the
nearly infinite input impedance. Therefore the
current must keep right on flowing past zero
volts and into the 2k Q resistor. Since there is a 2
volts drop across alKQ resistor, there must be a
4 volts drop across a 2K Q resistor. We already
know that this is an inverting amplifier, so the
output voltage must be negative when the input
is positive. We also know that since the current is
going "past" zero volts, it must be headed
toward a negative voltage. Therefore the output
voltage in this example must be —4 volts.
J.
Rf
=
(1.5
ma
ma)(2KQ)
Inverting amplifier with negative
in-
Rf_
R
Balancing Operational Amplifiers
If operational amplifiers are used for amplifying small DC voltages, then balancing the amplifier is very important. Balancing is adjusting
the output voltage to zero volts DC when the inis at zero volts DC. Most quality oscillo-
put
scopes have a DC balance control on the vertical
input amplifier. It is usually adjustable with a
screw driver from the front panel. If the scope is
not balanced, then the zero volt line will move up
or down dramatically when you change the sensitivity range. If you put one volt into your
oscilloscope and you think it is 5 volts because
your amplifier was not balanced, then this error
could lead you to
make
a mistake.
The currents flowing into the plus and minus
inputs of the op-amp are insignificant for calculating amplifier gain, but they are not insignificant for balancing the amplifier. Even though
these currents are tiny, the high gain of the
amplifier will greatly exaggerate any difference
between them. The difference between the input
currents is called the inpu+ current offset. The
it
— 1.5 mA, came
from the 2K Q resistor.
Since there is a 1.5 volts drop across a IK resistor, there must be 3 volts across a 2KQ
resistor. We can summarize inverting amplifier
gains with the following formula:
rent,
i
Av = -
again using a negative input
voltage, —1.5 volts. We know that the negative
input terminal on the op-amp is zero, so a positive
current will flow from zero down to —1.5 volts
through the 1KQ resistor. Because of the huge
input resistance, no current comes out of the negative input terminal. Therefore, all of this curtry
VOLTS =
1.5
voltage.
the
Let's
VOLTS
1KQ
adjustment used with the 741 and
is an internal compensation for this imbalance of currents which
may or may not be enough to compensate for unnull offset
other operational amplifiers
equal
171
DC
input currents.
To keep
200 ° C below zero, and extremely high
temperatures up to the melting points of the
the amplifier quiescent point balanc-
tures,
ed at zero, the DC currents flowing into both inputs must be equal. To keep these currents equal,
the resistance between the inputs and ground
should be equal. This explains why there is often
a resistor in series with the positive input when
there would be no need for one if the op-amp really had an infinite input impedance. In most applications the input signal source is in series with
metals.
Sometimes the thermocouple voltage is read
by a sensitive voltmeter or galvanometer. This thermometer assembly is called a
pyrometer. The exhaust gas temperature (EGT)
meter in an aircraft is an example of this. An addirectly
vantage of the thermocouple over the thermistor
is that it is self-powered. No other power source,
like the battery, is needed to run the thermocouple. The ambient heat is enough to make the
metals exchange charge and establish the thermocouple voltage. Sometimes a temperature
meter reading is needed a great distance from the
thermocouple or perhaps the temperature reading
is needed at several locations at once. To do this,
the thermocouple voltage must be amplified.
one of the input leads, so the resistance between
the input and ground should include the resistance of the voltage source itself. These principles
are illustrated in the thermocouple amplifier
discussed next.
K.
A
Thermocouple Amplifier
The thermocouple thermometer
is
a
basic
electronic device that every technician should be
familiar with. When two different metals like cop-
per and iron
are
joined
together,
a
tiny
DC
Fig. 10-13 shows an operational amplifier
used to amplify the DC level of a thermocouple
temperature sensor so that this DC level will be
able to drive an insensitive voltmeter. Depending
on how the voltage gain is adjusted, the voltage
output of the op-amp can either be the same as
the thermocouple itself, or many times larger.
The important thing is that the power output
from the operational amplifier can be very large.
Notice in the diagram how the amplifier has two
appears
50
across the junction. This voltage increases linearly with temperature. A small thermocouple cannot provide any significant power and it is difficult to measure this small voltage. Usually particular metal alloys are chosen which produce
large, linear voltage changes over the range of
temperatures needed. Unlike thermistors, thermocouples can be used at extremely low temperavoltage,
typically
1
to
millivolts,
GAIN
CONTROL
,+
v)
"-
IRON WIRE
NULL
OFFSET
CONSTANT AN
METAL ALLOY
WIRE
/7T7
TWO WIRES FUSED
TOGETHER TO MAKE
A THERMOCOUPLE.
TYPICAL THERMOCOUPLE
RESISTANCE IS 10
6-12 VOLTS
WHENEVER POSSIBLE. THE TWO INPUTS
SHOULD HAVE THE SAME RESISTANCE
TO GROUND SO THAT V WILL
EASILY BALANCE AT ZERO
Fig. 70-/.?
Thermocouple amplifier
172
DC VOLTMETER
CALIBRATED AS
A
THERMOMETER
calibration controls; one to set the zero point and
Op-Amp Output Impedance
M.
a second one to adjust the gain. Ideally the input
should be shorted out to make a true zero voltage
when adjusting the null offset control.
L.
Single
Now that we have discussed how voltage
gains are determined and how the output is
balanced, we need to cover how these amplifiers
Power Supply Amplifiers
power
transfer
In the op-amps
assumed
we have discussed
so
far,
the
to
amplifier
in
Fig.
10-14
resistance
is
it
had
is
a fixed quantity.
drawn from
Whenever current
this source, there will be a voltage
drop across this resistance and the voltage seen
.
Another drawback
re-
low. In fact,
In an ordinary voltage source, the internal
de-
extra resistors and capacitors are
needed to do this, but the circuit is basically the
as in Fig. 10-11.
is
section.
see, a lot of
same one
The output
behavior was one of the goals of the "perfect
amplifier" we discussed at the beginning of the
monstrates how a single power supply can be
used if a new "zero reference point'" is established
halfway between ground and +V CC As you can
this circuit is that the
loads.
zero output resistance. This ideal voltage source
.
The inverting
their
the op-amp attempts to behave as though
have two power supplies,
+V CC and — V ee In addition, the ground is used
as a fixed reference point for zero volts. Because
two separate power supplies can be expensive and
cumbersome, op-amp circuits are sometimes used
which require only one power supply.
circuit is
to
sistance of an operational amplifier
across the terminals of this source will drop.
more and more current
of
drop
still
is
drawn, the voltage
As
will
more.
use of coupling capacitors
input and output restricts the circuit to AC
The coupling capacitors are needed to
translate between the different zero points used
in the amplifier and the input and output. If the
zero reference at the input and output were the
On
in the
the other hand, an operational amplifier
much current as necessary so
output voltage will never fall below the
voltage determined by the input voltage and the
feedback resistor. In other words, the op-amp
automatically compensates for decreasing load
resistance by supplying more and more current at
will try to deliver as
signals.
that
same, 1/2 of V cc then these capacitors could be
omitted and the amplifier would amplify very low
frequency or DC signals.
,
its
virtually the
same
voltage.
COUPLING CAPACITORS
ADJUST THE AMPLIFIER
ZERO REFERENCE, 1/2 V cc TO
THE REFERENCE POINT, ZERO, OF
THE INPUT AND OUTPUT.
NEW ZERO
-^vout
POINT
REFERENCE
1/2
V CC
rrn
Fig. 10-14
Inverting
AC amplifier
173
with a single power supply.
NEGATIVE OUTPUT VOLTAGE SWING
LOAD RESISTANCE
POSITIVE OUTPUT VOLTAGE SWING
VERSUS LOAD RESISTANCE
versus
-15
-14
-13
o.
> -12
a.
tu
-11
i-
-9.0
a -10
<
—
+15 V SUPPLIES
'
<
+ 12V
o -an
>
K -/.o
=3
0.
1Z3
o
6
>
500 700
200
RL.
1.0 k
2.0 k
10k
5.0 k
±9 V
-fiO
-b.U
-4.0
-3.0
-2.0
-1.0
+
100
200
LOAD RESISTANCE (OHMS)
Fig. 10-15
500 700
RL,
Maximum
2.0 k
1.0 k
V|N
'OUT
ma
io9
+
r
.01
.02
.03
EQUALLY SPACED LEVELS
OF VOLTAGE
CONTROL.
_^V_
.04
.05
.06
.07
.08
.09
.1
8
VOLT-AMPERE
CHARACTERISTICS OF
A 741 OP-AMP
WIRED AS A VOLTAGE
AMPLIFIER WITH A
VOLTAGE GAIN OF 100
7
6
5
4
3
2
1
"V"
Vin
NEGATIVE
CONTROL VOLTAGE
Fig. 10-16
-I
Volt-ampere characteristic for an op-amp with a gain of 100.
174
V
5.0 k 7.0 k
LOAD RESISTANCE (OHMS)
output voltage versus load resistance
6
10 k
When we
calculated voltage gains for am-
we assumed
operational amplifier
output voltage
would be independent of the output resistance.
We assumed that, no matter what the load resistance was, 10,000 ohms or 0.1 ohm, the op-amp
would generate enough load current so that the
output voltage would depend only on the input
voltage. It is not realistic to expect a tiny IC
smaller than a corn flake to produce hundreds of
amperes. There must be some load resistance at
which the op-amp can no longer supply enough
current. In practice the operational amplifier does
have an internal resistance and its ability to compensate for big loads is ultimately limited by this
resistance. For example, according to the specification sheet, the 741 op-amp has a 75 ohm output resistance.
plifiers
that
is
a sophisticated transistor
amplifier and through the use of negative feed-
the
back, the operational amplifier has an excellent
voltage source output with a programmable
voltage gain. Compare Fig. 10-16 with Fig. 4-5
and you will see that the op-amp does with
voltage what the "ideal transistor" is supposed
to
do with current.
QUESTIONS:
1.
2.
Operational amplifiers are supposed to be
"ideal amplifiers." In what ways do opamps fall short of this goal?
What
they
are differential amplifiers? What do
have to do with operational am-
plifiers?
Rather than asking what the output resistance is, perhaps we should be asking how big a
load or how low a resistance can be put on an
operational amplifier without the output voltage
falling below what it ought to be? Fig. 10-15 is
two graphs showing the output voltage swing of
a 741 op-amp versus different load resistances.
Voltage swing refers to how far the voltage can
change away from zero in response to an input.
The limits of this swing are partly determined by
the power supply voltages and for this reason
there needs to be a separate curve for each power
supply voltage.
3.
Using words instead of an equation, exwhat is meant by the common mode
plain
rejection ratio?
4.
5.
If the input voltage on a non-inverting input of a differential amplifier goes up,
while the input voltage on the inverting input stays the same, would the amplifier
output go up or down? If the differential
amplifier has differential outputs, what
would you expect these outputs to do?
Integrated circuit op-amps generally have
or more direct coupled amplifier
stages inside them. What is the advantage
four
Curves for four different pairs of voltage supshown. As long as the load resistor is
greater than 1KQ, the maximum voltage swing
is largely independent of the load resistance. This
is reasonable because 75 ohms is pretty tiny compared to 1000 ohms. But when the load gets down
to 100 ohms, 75 ohms is almost as large. The opamp is only able to compensate for this heavy
load over a small range, about 2 volts positive
and negative. Notice that the voltage swing is
always less than the sum of +V CC and — V ee For
instance, with 12 volt supplies the output signal
can be only 11 volts positive and 10 volts neplies are
of direct coupled amplifiers like this?
6.
makes them
stable?
What
chopper stabilized
a
is
DC
What
am-
plifier?
7.
Some op-amp ICs are equipped with offset
What are they used for and why
null leads.
don't
all
op-amps have them?
What
is
a bar graph voltmeter?
What
is
a voltage follower used for?
.
8.
gative.
9.
In Sections 4 and 6
we looked
Why
do the positive and negative op-amp inputs
have the same voltage?
at a lot of volt-
ampere diagrams
for transistors. In all those condevices the output appeared to be a current
source. Then later in Section 7 we saw that when
these constant currents were forced to flow
trol
10.
is shown in Fig.
work as well with a
precision diode circuit
10-8. Will this circuit
germanium diode or does the offset voltage
compensation work only with silicon
through load resistors, the current source was
converted to a fairly good voltage source in the
various transistor amplifier configurations.
A
diodes?
The
175
11.
When
is
there a "virtual short circuit" be-
20.
What does
13.
When
the op-amp
amp? What symptom
is
non -in verting amplifier similar
tell
"R"
1KQ,
is
voltage
what
to Fig.
feedback
the
is
amplifier
input
—2 volts, what will the
be? What kind of power
output
supply
Rf?
the
If
is
voltage
voltages would be needed to make it possible to amplify — 2 volts without distortion?
14.
inputs
the
If
an
to
inverting
op-amp
amplifier always remain at zero volts and
they don't accept any input current, how
can the amplifier amplify voltage?
15.
The feedback
resistor
in
amplifier like Fig. 10-11
resistor, R, is
is
—3
volts,
2.5K Q.
what
If
an
5K
is
Q.
inverting
The input
the input voltage
will the
output voltage
be?
16.
What
the importance of the input offset
is
current?
17.
Suppose you were
to build a non-inverting
amplifier like the one in Fig.
notice that
voltage
is
to fix this
when Vj n
not quite zero.
10-9.
zero, the
You
output
What can be done
problem?
18.
What
19.
The input and output
is
is
a pyrometer?
of the amplifier in
coupled with capacitors.
What are these capacitors for? Other than
extra cost, what is the disadvantage of
using coupling capacitors? What effect do
they have on very high frequency signals?
Fig.
10-14
wired with negative
or symptoms would
you that the output load resistance is
too low?
10-9 has a voltage gain of 7. If the resistor
resistance.
is
feedback resistor. What does this tell you
about the output impedance? What eventually limits the current output of the op-
wired as an amplifier
with a finite amount of gain, what does the
current flowing into the op-amp input terminals have to do with this gain?
A
an op-amp
feedback, the output voltage depends almost entirely on the input voltage and the
tween the two op-amp inputs?
this virtual short circuit have to do with
the high gain of the op-amp?
12.
When
are
176
SECTION XI
Applications for
Operational Amplifiers
A.
Introduction
In this section
we
are going to cover
operational amplifier circuits.
Whenever
more
possible,
examples of practical uses for these circuits will
be given. You shouldn't get the idea that these
op-amp circuits are the only way that these
various tasks can be accomplished. Cheap, IC opamps are relatively recent, 1970, and most of
these same circuits can be built using discrete
parts or with design philosophies that are quite
Vo = (V|N2 -
For example, TV set sweep circuits are
used to illustrate a use for an op-amp integrator.
Don't be surprised if your particular TV set
doesn't use op-amps in its sweep circuits. Most
TV sets use multivibrators or tube or transistor
circuits that resemble relaxation oscillators to accomplish this same job.
different.
Adiff
Operational Amplifiers as Differential
Am-
The operational
The
= -r—
Rl
SO,
= 10
Operational amplifier used as a
dif-
Summing
Amplifier
amp
the inverting amplifier and remains at zero volts
11-1 has a voltage gain of 10.
Adiff
1K
with controlled gain by com-
differential amplifier
ferential amplifier
rT
amplifier can be wired as a
bining the inverting and non-inverting circuits
to one.
10K
The inverting amplifier can be used to add
two or more different signals together. Several input resistors can be fed into the same negative op-
plifiers
differential amplifier
Rf
=
ferential amplifier.
C.
B.
11-1
Fig.
V|N-|)
shown
The gain
(Vin2
The negative input
is at
zero volts in
because of the feedback. The current that flows
through the feedback resistor is the sum of the
several input currents through the input resistors. This is because virtually no current flows
into the negative op-amp input, so the currents all
join together in flowing through the feedback
resistor. The negative op-amp input is at zero
volts and remains at zero, independent of the input currents that are being added. Therefore the
several currents being added do not interfere with
each other.
in-
in Fig.
of this dif-
is:
Vo =
input.
1
" V in i) J !
Ri
177
VOLTS
'TOTAL = Oa +
'b
INVERTER
+
«c)
/
V a + Vb + V c
rm
rrn
Fig. 11-2
OUTPUT
t
Operational adding circuit
D.
1
Active Frequency Filters
db
HIGH PASS FILTER
A common use for operational amplifiers is
low frequency filtering. By "low" we mean
below 1 MHz. A typical application for this might
be a telephone number tone decoder for use with a
push button telephone. With push button phones,
the telephone number is coded into a sequence of
musical tones. Therefore, at the telephone exchange, there must be sharp filters that separate
one musical tone from another so that the
numbers may be recognized.
for
attenuate/i
/
PASS
'
FREQUEN
J
'V CUT OFF
FREQUENCY
OUTPUT
db
I
LOW PASS FREQUENCY
You
PASS
ATTENUATE
FREQUENCY
V
Fig. 11-3
even three
RC
RC
filters are
put
filters
two or
in series, the relative
attenuation of one frequency over another can be
dramatic. The frequency filtering characteristics
CUT OFF
and low pass filters are shown in Fig.
graph like this that shows the output of a
of high pass
FREQUENCY
Frequency characteristics of high pass
and low pass
are already familiar with using
to attenuate high or low frequencies. If
11-3.
A
circuit in decibels versus the frequency of the
filters.
processing is called a Bode Plot and is
Mr. Bode. This graph shows how
some frequencies are passed or amplified while
others are suppressed or blocked.
signal
it is
named
Notice that the operational adder must be an
inverting amplifier. If a similar circuit were attempted with the non-inverting amplifier, a large
input voltage on one of the input resistors would
change the voltage on the op-amp inputs. As one
adding input voltage changes, the voltage shift
would change the currents received from the
other adding input resistors. These changes
would produce an error in the addition. If the
The
after
closer the
the more difficult
two frequencies are together,
is to attenuate one without
attenuating the other. Suppose several RC filters
are put in series to try to seperate two close frequencies, for instance, 500 Hz and 450 Hz. By the
time the two signals have sufficiently different
amplitudes, the amplitudes of both signals will be
attenuated to the point where the desired signal
reversed polarity of an inverting amplifier output
is not acceptable, the best way to correct it is to
invert the signal again with a second inverting
amplifier that has a gain of one.
will
ITS
it
be too weak to use.
High pass and low pass active filters are
shown in Fig. 11-4. An active frequency filter consists of one or two RC filters incorporated into an
operational amplifier so that the desired signal
amplified at the
same time
very large, 2 or 3 at the most. If more gain is attempted, the amplifier becomes a poorly designed
phase shift sine wave oscillator, like the ones we
studied in Section 8. The exact frequency at
which the amplitude is cut off is determined by
the R and C time constant. Several of these filters
can be put in series to make the discrimination
between two frequencies very sharp.
is
Usually
R2 + R1/R1, is not
it is filtered.
the gain of these amplifiers
When
active filters are used in a circuit
important
to
have
the
output
it is
V
voltage,
,
centered at zero volts so that the AC output
signal can swing positive and negative as widely
as possible. This can be done by having both inputs of the operational amplifier tied to ground
V|N
(zero volts)
This
is
with equal amounts of resistance.
easiest to see with the high pass filter
where Ri should equal R. In the low pass
the input resistance to the positive input
filter
is
two
both with resistance R. If the
source impedance of Vj n is low, as it would be if
Vj n were the output of another op-amp, then
when the input voltage, Vi n is zero, the positive
input will be grounded by a resistance of approximately 2R. This will satisfy the condition that
series resistors,
V
J\.
v
RC LOW
Y
RC LOW
PASS
PASS
NOTE:
R-i, V
WILL BE NICELY
BALANCED AT ZERO
IF
R + R =
,
WHEN
V|N IS ZERO
AND THE SOURCE
RESISTANCE OF
V|N
LOW PASS ACTIVE
IS
ZERO.
FILTER
NOTE:
R = R-i, V
WILL BE
NICELY D.C.
IF
BALANCED
AT ZERO
WHEN
V|N
IS
A
ZERO.
HIGH PASS
/!
freq
1
,L
V,N>—|f
A
i
l
LOW PASS
i\
freq
IfH
fv„
CR
HIGH
CR
PASS
1
HIGH
PASS
/!
HIGH PASS ACTIVE FILTER
Fig. 11-4
A — Low pass
pass active
BAND PASS
active filter
fL
B—
1
v
fH
THE "PASS BAND"
High
Fig. 11-5
filter
179
Band pass
filter
freq
the two inputs be grounded equally and the output V will be nicely centered on zero. If the output is not centered at zero, an AC signal at the
output may have the peaks of its sine waves clip-
filters have done their work, the two signals are
recombined in an operational adder.
ped
E.
off at either
the positive or negative ex-
The Logarithmic Amplifier
tremes.
The low pass and high pass
filters
By using silicon or germanium diodes as the
negative feedback element in an inverting amplifier, it is possible to build an amplifier that has
an output voltage equal to the natural logarithm
of the input voltage. A complex but similar circuit made with two op-amps can be used to take
the anti-natural-logarithm of the input voltage.
can be put
in series so that their cut-off frequencies overlap
and make a band pass filter. This means they will
pass a narrow band of frequencies, but will exclude all other frequencies. The telephone number
recognizing circuit that responds to a specific
musical tone but ignores all other tones would be
an example of this.
So what? Think back to those dull, sleepy spring
days in the 10th grade when you learned about
You
logarithms.
The low pass and high pass
can be put
in parallel to form a notch filter. This filter
system excludes or suppresses a narrow band of
frequencies, but allows all other frequencies to
pass through without attenuation. After the two
logarithms can be
will recall that
used to multiply, divide, take square
filters
raise
numbers
roots,
and
to exponential powers.
Back in Section 2 we discussed three defects
P-N junction that make it differ from an
in the
"ideal diode."
HIGH
PASS
resistance.
>-
Vo
ADDER
description of real diodes
The forward
resistance
is
was
not a
simple linear equation because this resistance is
not constant and does not plot on a graph as a
OP-AMP
—»
Our
oversimplified.
—*-
»H
VlN
These defects were the forward offand the forward
set voltage, the zener breakdown,
?
1
LOW
PASS
SILICON
«L
DIODE
1
1
HIGH PASS
1
'
1
I
'/'
\J
1
'
1
1
1
1
OFFSET
VOLTAGE
fH
THE FORWARD
RESISTANCE REGION
THE VOLTAGE
VARIES AS THE
NATURAL LOGARITHM
OF THE CURRENT
IN
ZENER
BREAKDOWN
i
V
V = K
ADDER
+V
A .06
,rec^
1
,
iyfc
OUTPUT
WHERE
(0.6
VOLTS)
In
K VARIES
WITH DIODE TEMPERATURE
i
Fig. 11-6
Notch
V
Fig. 11-7
i
filter
180
Diode volt-ampere characteristic
I
straight
We
line.
forward charexpressed by the
said that the
acteristic of a diode could be
LOG
following linear equation:
V =
A
I
(Rforwardl
more accurate equation
diode
SYMBOL
+
offset voltage
for the voltage across a
is:
n
V = K
^W\A
Vin)>
In
I
where K is a factor that depends on the
diode temperature and the offset voltage.
By using
ment
a diode as the negative feedback
INVERTER
rm
VOUT =
K ln(V|N)
ele-
WHERE
K IS A FACTOR WHICH
DEPENDS ON DIODE TEMPERATURE AND
DIODE OFFSET VOLTAGE.
an inverting amplifier, the output voltage
will equal the natural logarithm of the current
passing through the diode. The diode current is
proportional to the input voltage which drives a
current through the resistor R on its way toward
in
Fig. 11-8
A
logarithmic amplifier
zero volts. Unfortunately, practical logarithmic
amplifiers need
more stages
fed into the antilogarithmic amplifier, the output
to keep the logarithm
of the
function independent of temperature.
antilogarithmic amplifier would be the
original signal.
As you can see from the illustramuch more complicated than
tion, this circuit is
the log function. Not only
In order to perform mathematics with logit is necessary to find anti-logarithms as
is a second stage needbut a current source circuit is needed to bias
the first op-amp. We shall not attempt the math
and frantic arm waving needed to explain this cir-
arithms,
well as logarithms.
The
ed,
circuit in Fig. 11-9 will
reverse the logarithm process. In other words,
if
the output from the logarithmic amplifier were
cuit.
CURRENT
SOURCE
ANTILOG
CIRCUIT
SYMBOL
^
VOUT
INVERTING
VOUT - -K' ln-1
WHERE
(V|N)
K' IS ANOTHER FACTOR
WHICH DEPENDS ON DIODE TEMPERATURE
Fig. 11-9
An
anti-logarithmic amplifier
181
-(InX + lnY) =
ANTILOG AMPLIFIER
INVERTS SIGN OF
/777
SIGNAL, SO AN
^
J
INVERTER IS NOT
NEEDED AFTER
THE INVERTING
ADDER
ADDER
In x
+
In y
=
xy =
Fig. 11-10
A
multiplier
In
-
1
(Inxy)
made from
logarithmic and anti-logarithmic amplifiers.
In order to multiply two numbers together
using logarithms, first the numbers are converted
to their logarithms. Then the logarithms are added together. The antilogarithm is then taken of
the
sum
of the logarithms.
The
result
is
To
Division can be done in almost the same way.
number into another, the logarithm
divide one
That is, the sign of the
changed from plus to minus
of the divisor is inverted.
divisor logarithm
before
the pro-
it is
is
fed into the adder.
We
will illustrate
division with a circuit that will calculate
duct.
Given
log e
CIRCUIT.
In xy
X
and Y, find XY:
X +
loge
Y =
how
of flying time an aircraft has for the
present rate of fuel flow.
log e (XY)
XY =
antiloge (loge XY)
loge and antilog e
-1
equivalent to In and In
respectively.
so,
many hours
NOTE: The symbols
are
,
FUEL TO
ENGINE
-(Ln FUEL - Ln FLOW)
FUEL
= -In
VSAAS
>
FL0W
*
ANTILOG
FUEL
FLOW
m FLYING
TIME
ADDER CIRCUIT
Fig. 11-11
A
flying time calculator circuit which divides fuel remaining in the tank by the rate of fuel
flow to the engine
1
o2
The circuit in Fig. 11-11 is designed to produce a reading on how much flying time remains
at the present rate of fuel consumption. The
faster the engine is burning fuel, the shorter
period of time the fuel will last.
remaining
fuel
and voltage Y are fed into the multiplier and the
output is a voltage equal to X times Y. Usually
the product is made smaller by a scale factor, say
1/10, so that the product will be less than the
power supply voltage. Without the scale factor, 5
volts times 10 volts would be 50 volts which
would need at least a 50 volt power supply. Since
the multiplier can tolerate only 15 volts, scaling
the output down to less than 15 volts is essential.
To use the multiplier as a divider, the multiplier
IC is wired in series with the negative feedback
resistor of an inverting amplifier.
gallons/minute
minutes of flying time remaining.
The output
of the circuit is a voltage
which can be
read on a voltmeter calibrated in minutes of
to multiplier integrated circuits, the
time circuit can be built using one
multiplier IC and one op-amp. Multiplier ICs
multiply two analog signals together. Voltage X
gallons
fuel flow/minute
=
Thanks
flying
fly-
ing time.
Referring to the circuit, the fuel tank gauge
fuel flow meter both produce DC voltages
proportional to these quantities. The logarithm of
and
voltages
these
then
taken,
is
the
fuel
F.
flow
logarithm has its sign changed by an inverting
amplifier. In logarithms, this is equivalent to
taking the reciprocal of the number. That is,
dividing the number into one. Then the two
logarithms are added together and the antilogarithm is taken just as it was in the
The
tank
-s-
fuel flow
=
—
so,
In (fuel flow)
flying time
flying time
=
In
-1
=
[In
in-
ed with op-amp circuits. These circuits are used in
analog computers which are used to solve differential equations. Integrator circuits, built with
or without op-amps, are also widely used for
many non-mathematical chores in circuits. Probably the most common use for integrators is in
sweep
In (fuel in tank)
calculus mathematical operations of
tegration and differentiation can also be perform-
multiplier circuit.
fuel in
Integrators and Differentiators
TV's and
circuits in
oscilloscopes.
In (flying time)
It is
(flying time)]
how
strange
a circuit as "simple'' as one
and one capacitor can have so many different properties and uses. It seems as though 1/4
of the science of electronics is devoted to explaining RC circuits! Electronic integrators and differentiators made with op-amps are other uses for
resistor
XR2208
XY
MULTIPLIER
INTEGRATED
<>
CIRCUIT
the existing
RC
circuit.
XR2208
multiplier (r
Fig. 11-13
integrated!/
shows an
RC
integrator circuit.
CIRCUIT
ways
applications are just different
vz
>
vVNAA
the
R1
~7
VY =
Vx
rfn
vx^11-12
A
multiplier integrated circuit can
also be used for division.
same
effect.
the capacitor
-Vz
Fig.
We
have already used the same circuit in Fig. 11-4 as
a low pass filter and in Section 5 it was a sine
wave phase shift delay circuit. All three of these
is
related
to
of looking at
the voltage across
the current which
charges the capacitor. To use the mathematical
terminology, the voltage across the capacitor is
the integral of the current which charges the
capacitor. This same relationship can be expressed by saying that the current charging the capacitor is the derivative of the voltage across the
capacitor.
183
As you know,
VlN
VOUT =
—mv—
THE INTEGRAL OF A
l-SQUARE WAVE IS A
TRIANGLE WAVE
^
=
c
|
time
•
VOUT
dt
I
4
t
y
1/c
1
time
ufd
i
N/
1
MILLISECONDS
/
i
ZSS
2
4
i
'
5
MILLISECONDS
VOUT
R C = T
(10KK1 Jd) = 10
\
~jf(r
dt
•VOUT MUST BE VERY
MUCH SMALLER THAN
FOR ACCURACY
MILLISECONDS
Vin
ORDER FOR THE INTEGRATION
V|N TO BE ACCURATE. THE
RC TIME CONSTANT SHOULD BE AT
IN
OF
LEAST
TIMES
10
1
MILLISECOND.
Fig. 11-13
The voltage across the capacitor
RC integrator circuit
.dt
-*«.
ca P
-o
The current charging the capacitor
more mathematical way
to look at in-
graph of the quantity being integrated versus the variable that is changing. Examples would be inches of rainfall versus time in
days. Another would be the current charging the
tegrals
cap
A
season.
is:
is:
is
to plot a
capacitor plotted versus the time in seconds.
The
In other words, differentiation and integration
water or current delivered over a
period of time will be proportional to the area
under the curve on the graph. Or to say it another
way, the integral of the rainfall over the rainy
season is proportional to the depth of the water in
are opposite processes.
the rainbarrel.
J
cap
total quantity of
dV cap
—
dt
Unless you have spent a
calculus
squiggles.
lot of
The problem with simple RC integrators is
waveform must be very
time looking at
symbols are more
Perhaps a better way to
these
frightening than useful.
look at the integration circuit
that
is
it
that the output voltage
much
produces a
smaller than the input voltage waveform.
RC time constant must be
longer than the cycle time or period of the
In other words, the
much
voltage that is proportional to the quantity, of
current that has flowed in (or out) through the
waveform being
something like a rainbarrel which
collect^ water from the roof of a house in the
desert. The depth of water in the barrel is propor-
pacitor.
integrated. This is because the
voltage across the capacitor must not significantly change the current that is charging the ca-
resistor. It is
tional
to
how much
rain
fell
But in a simple RC integrator, the
voltage across the capacitor decreases the volt-
during the rainy
CAPACITOR WOULD CHARGE TO
VOUT
AV|N
V| N
-
time
MILLISECONDS
RC
11K
Fig. 11-14
^>V
^/W^
^>
-if
Vin IN
IF
V|N
ABOUT 4 TIME CONSTANTS
WERE DC VOLTAGE.
UT
5
c
= 0.5
time
..td
MILLISECONDS
= T
xO.5 ,fd) = 0.5
An RC
MILLISECONDS
TRIANGLE WAVE IS
BADLY DISTORTED
integrator with too small a time constant.
1S4
TRIANGLE WAVE
INVERTED
IS
LARGE BUT
AV|N
1K
AW
Vin^>
ZERO VOLTS
6 - v ee
Fig. 11-15
Operational amplifier integrator
in Fig. 11-15 is the
age across the resistor. Therefore this voltage
decreases the current charging the capacitor.
This is comparable to the filling of the rain barrel
causing the rain to stop!
An
operational amplifier integrator
in Fig. 11-15.
Because the positive input
ground and because the
is
shown
is
tied to
in
across
voltages, zero.
The current flowing
as the
RC
time constant
R
,
not distorted. Now the only limitation on how high the output voltage can go without distorting the integral waveform are the
power supply voltages, +V CC and — V ee The output waveform is inverted but this could be corrected with a second amplifier with a gain of
is
.
back, the output will try to change the negative
op-amp input so that the positive op-amp input
will
same
However, because the voltage
depends only on V m the output
11-14.
waveform
circuit has negative feed-
and the negative op-amp input
Fig.
minus
one.
have equal
into the in-
tegrator input will always be flowing toward zero
Integrator Sweep Circuits
op-amp input. Because the
voltage across the input resistor R is only caused
by the input voltage, V
the current that is
stored in the rain barrel capacitor C will depend
G.
on Vj n and
integrators
volts on the negative
m
will
,
Probably the most
the capacitor.
This
so
to
if
Vin
a
is
is
an inverting amplifier configuration,
positive, then the capacitor will charge
negative
capacitor
starts
voltage
out
"below
zero."
discharged,
the
If
amounts
in order to force the negative
op-amp
in-
Using the operational amplifier to keep one
end of R at zero volts, the voltage across the
capacitor no longer effects the current charging
RC
use for electronic
in
wave in Fig. 11-13. As the capacitor
slowly charges with a small current, its voltage
gradually rises in a straight, linear voltage ramp.
If the capacitor or the resistor are too small, the
capacitor will charge too quickly and the charging current will fall off. This will cause the
voltage to lose its straight, ramp-like characteristic. We have already seen how using an inverting operational amplifier can keep the charging current constant and the output voltage a
volts.
the capacitor. Notice that the
is
of a square
the
voltage
across it will be zero. As the voltage across the
capacitor grows higher and higher. The voltage
across the op-amp will have to increase equal
put to remain at zero
common
sweep circuits for TV sets and
oscilloscopes. These circuits generate sawtoothshaped voltage or current waves which steer the
electron beam across the screen. The basis for
most sweep circuits is similar to the integration
not depend on the voltage across
straight ramp.
time constant
185
A TRANSISTOR
SWITCH SHORTS
OUT CAPACITOR
TO RESTART
RAMP WAVEFORM.
/CO4016 CMOSA
IC SWITCH
J
A
VlN
^
SYNCHRONIZATION PULSES
TO CLOSE SWITCH AND BEGIN A NEW
SCAN-RAMP.
+ V CC
-RAMP WAVEFORM
-
-Vee-
"RETRACE"
WHEN SWITCH
CLOSES. NEW
RAMP STARTS WHEN
SWITCH OPENS
6-Vee
Fig. 11-16
A
sweep
circuit built with
In the sweep circuit we are only interested in
ramp portion of the output waveform.
The return or retrace back to the bottom of the
ramp should be as quick as possible. For this
reason, the ramp sweep circuit just integrates a
constant voltage until the ramp reaches the
desired high voltage. At this point the capacitor
is shorted out and the ramp abruptly returns to
zero. This retrace function is accomplished by a
switch across the capacitor which shorts it out
after the completion of each cycle. There is no
need to integrate a square wave because the
retrace portion is done with the shorting switch.
This switch could be a transistor or it could be the
output side of a monostable one-shot multivibrator. A CMOS integrated circuit analog
switch would work as shown. In the case of a TV
set, the scanning sweep is triggered by pulses
that are derived from the transmitted TV picture
signal. These pulses tell the switch across the
capacitor when to turn on and start a new scanning line.
an op-amp integrator.
Just to make
the rising
it
complicated, every other pic-
up one
half line so that the next
scan of 262 V2 lines will produce horizontal lines in
between the lines from the last picture. This interlaced scanning fills in more detail and gives
the resolution of a 525 line picture. By vertically
scanning each picture twice instead of doing all
525 lines in one vertical scan, the picture flickers
ture
less
is
shifted
and seems more
realistic.
The light and dark areas of the picture are
represented by high amplitude and low amplitude
signal strength in the amplitude modulated (AM)
TV
signal. This radio signal is detected and converted to a proportionately varying current. This
current is amplified and applied to the cathode of
the TV picture tube where the current becomes
the electron beam. The electron beam is accelerated toward the TV screen and, depending on
the strength of the radio signal at the moment,
beam makes light or dark areas as the beam
moves along each scanning line. If you have
the
forgotten
how TV
picture tubes work, review Fig.
1-7.
A TV uses two sweep generators like this to
generate the horizontal and vertical sweep ramp
H.
waveforms. The vertical sweep moves relatively
slowly, 60 times per second and produces one
sweep ramp per picture. The horizontal sweep
The operational differentiator is the same as
the integrator except that the resistor and capacitor are reversed. The voltage output is the
moves much more rapidly, 15,750 times per second and produces 262' t horizontal scanning
lines for
The Operational Differentiator
voltage across the resistor while the capacitor is
charging. Therefore, this voltage is proportional
every vertical scan of the picture.
186
ANTENNA
AUDIO
AMPLIFIER
SUPERHETRODYNE
FM
SOUND
RECEIVER
LOUDSPEAKER
DETECTOR
AM
VPICTURE
PICTURE
DETECTOR
HORIZONTAL SYNCHRONIZATION
PULSES (262 VERTICAL SCAN)
/
LIGHT AND DARK INFORMATION
FOR BOTTOM 2 SCANNED LINES
VPICTURE
VERTICAL
/LAI
SYNCHRONIZTION
/M w\A /vVlA PULSE
kfl/w W^
SIMPLIFIED PICTURE SIGNAL
/\
WAVEFORM
HORIZONTAL
SYNC. PULSES
SYNCHRONIZATION
PULSE
SEPARATOR
HORIZONTAL
PULSES
VERTICAL
SYNC. PULSES
VIDEO
AMPLIFIER
VERTICAL
-
PULSES
H
VIDEO
AMPS ARE
USUALLY
CLASS A.
C*
VERTICAL
1%
SWEEP
AMPLIFIER
VERTICAL
SWEEP
GENERATOR
CIRCUIT
HORIZONTAL
VERTICAL
DEFLECTION
SWEEP RAMPS
COIL
\ SCANNED
LINES
HORIZONTAL
SWEEP
GENERATOR
CIRCUIT
HORIZONTAL
SWEEP
AMPLIFIER
HORIZONTAL
DEFLECTION
COIL
L|QHT
FR0M
PREVIOUS
FRAME
FADING AWAY
Fig. 11-17
A
block diagram of a
187
TV set.
TV PICTURE
TUBE
to the current which is the first derivative of the
voltage across the capacitor. Did you get that? If
not, don't worry about it. Operational differentiators are not very practical
and
are seldom used.
cient at accelerating the rocket. Therefore the
more and more too.
Problems like this would be extremely tedious to
solve by hand because you would have to figure
rate of acceleration increases
all the new variables every second. Or to be
more accurate, the calculations should be re-
out
The
differentiator
supposed to make a
is
voltage at the output that
is
proportional to
how
Since it is sensitive to the rate at which the input voltage
changes and not the amplitude of those signals,
even very small fast signals can make a huge out-
peated every tiny fraction of a second.
fast the input voltage changes.
put signal appear at the output. The result is that
the circuit goes crazy with the slightest bit of
high frequency noise riding on the input signal.
The analog computer output simulating the
would be voltages rising and
These voltages would represent increasing speed, decreasing fuel supply, and so on. The
voltages would be graphed on oscilloscopes or
chart recorders and this performance data would
flight of the rocket
falling.
represent the "solution'* to the differential equa-
V,N>
tion.
nMAA
|f
Out
in the real world, the digital
computer
is
rapidly taking over the solution of differential
equations.
}
dV| N
rrn
v
dt
Fig. 11-18
When
Operational differentiator
V|N =(i|NPUTXR|N)
needed, designers usualconcept around so that
they can use an integrator instead.
a differentiator
FEEDBACK RESISTOR
ALSO LOAD RESISTOR-
is
ly try to turn the circuit
An
The computer solves the problem
using the tedious, slow method that was too
cumbersome by hand. A digital computer is perfect for doing boring arithmetic at high speed, so
this approach to the problem is quite practical. A
major advantage of the digital approach is that
analog computer
is
IS
a collection of opera-
ZERO
VOLTS
modules which can be wired
together with test leads to make up circuits composed of op-amp integrators and amplifiers. This
computer is used to solve differential equations
by wiring up a circuit that simulates the equational
amplifier
equations contain differentials
In order to simulate them, the
tion. Differential
or derivatives.
whole equation
the equation is
integrated several times until
is
all
integrals
Then the equivalent
circuit is wired
tegrators and the voltage
to find out
how
iLOAD = 'INPUT
and no derivatives.
using
in-
Fig. 11-19
waveforms are graphed
the equation behaves.
A
voltage-to-current converter
numbers and the
on a sheet of paper that
must be calibrated and converted into numbers.
the calculations produce hard
results are not
Differential equations are problems that involve lots of variables changing simultaneously.
For example, an analog computer simulating the
flight of a rocket would have to take into account
that the rocket is going faster and faster every second the engine is accelerating the rocket. The
engine burns fuel so quickly that the rocket is
becoming
is
lighter
lighter, the
and
lighter.
Because the rocket
engine becomes more and more
effi-
All of the
in this section
great
wavy
lines
mathematical operations described
can be done by digital circuits to
precision.
As
digital
circuits
become
cheaper and cheaper, it is quite possible that
analog mathematical circuits will become obsolete. But while the digital circuits have become
better and cheaper, so have the analog integrated
circuits.
At present it appears that analog mathematical circuits will continue to be used in simple
applications where the rest of the circuitry is
analog and only one or two calculations must be
beam, this field is generated by a ramp-shaped
current waveform passing through the coil. The
voltage across the coil is incidental to the process.
It is the current that is doing the steering. One
made.
way
/.
The Voltage-To-Current Converter
to produce a ramp-shaped current is to start
with a ramp-shaped voltage and convert it to a
ramp-shaped current waveform.
will
Occasionally there is a need for a circuit that
convert a voltage to a proportional amount of
Deflection coils are inductive and generally
have capacitance and resistance as well. The
might be converted
the op-amp equi-
is ideal for jamming
ramp waveforms through the
yoke. No matter how the yoke impedance will try
to distort the nice clean ramp shape, the op-amp
current. For example, 3 volts
to 3 amperes.
This circuit
voltage-to-current converter
straight current
is
valent of the grounded-base or grounded-gate
Remember how the input voltage
amplifier.
source was below ground and supplied the output
The same principle is used here and if
not take "no" for an answer and will see that
ramp waveforms pass through the
deflection yoke.
will
current?
you have forgotten, look back
perfect current
at Fig. 7-6.
11-20 shows a hypothetical sweep am-
Fig.
In this circuit the feedback resistor and the
input resistor, Rf and Rj n are also the load on the
operational amplifier! Instead of the current go-
plifier built
from a voltage follower and a voltage-
to-current
converter.
,
ing to an external, separate load, the load current
the current that passes through the feedback
is
and the input resistor. The intended load
and the feedback element are one and the same.
Notice that this circuit has no current gain
whatever. However, it can have a very large
resistor
voltage gain
if
the load resistor (Rf)
larger than the input resistor, Rj n
.
is
much
The point
through the load
be whatever comes through input resistor on
way
what the load
The load can even be inductive or capacitive. The load will receive that same current
whether it wants it or not! This circuit will jam
current through the load no matter what wierd
its
might
to zero volts, regardless of
be.
across the deflection coil to force the
tion coil
Rm
An application for this might be the vertical
and horizontal sweep amplifiers in the TV set in
Fig. 11-17.
Whenever
steer the electron
it
is
common
practice to
a coil that
is
the
same
cur-
,
and keep one end
of the input resistor,
at zero volts.
were driven through a current step-up
transformer. In that way, the op-amp would not
have to supply so much current directly to the
deflection
coil
These coils are mounted in a donut-shaped
assembly called a deflection yoke. The yoke slips
over the thin neck of the picture tube and is
mounted just where the "bell" of the glass picture tube begins. The deflection coil has two pairs
of coils; one for vertical deflection and another for
horizontal deflection. Since it is the magnetic
coils.
field inside
that
As a practical note, the op-amps used for the
voltage follower and converter would have to be
high current ICs to drive a large deflection coil.
Cheaper op-amps could be used if the deflection
a picture tube has a screen
beam with magnetic
assume
rent to flow through the inductance of the deflec-
impedance characteristics the load may have.
This circuit is a current source controlled by an input voltage.
larger than 5 inches,
Let's
is
.
of
this circuit is that the current
will
ramp
generated by the circuit in Fig.
11-16. This voltage ramp generator is not able to
supply enough current to drive a deflection yoke
for a big picture tube. Therefore, the current is
first amplified by a voltage follower to the level
needed without changing the perfect voltage
ramp waveform. Then the voltage ramp is applied
to the voltage-to-current converter input, Rj n
The current through the resistor is alway proportional to the voltage output of the voltage
follower because the current is always flowing to
zero volts. By means of negative feedback, the second op-amp will put whatever voltage is needed
voltage
deflection
coil.
There are also techniques
in
which
the current and power capability of an op-amp
can be boosted by wiring an additional transistor
amplifier stage onto the op-amp output. The feedback loop is still maintained as if this additional
stage were part of the integrated circuit.
steering the electron
189
A vload
V|N
VOLTAGE RAMP
WAVEFORM
t
V|N
iLOAD
LOW IMPEDANCE
VOLTAGE RAMP
ACROSS R|N
CURRENT RAMP WAVEFORM
/777
t
L
R
DEFLECTION COIL
vVW^'
LOAD
RlN
ZERO
VOLTS
HIGH CURRENT
GAIN OCCURS
'LOAD
HERE
VOLTAGE-TO-CURRENT
CONVERTER HAS NO
Fig. 11-20
CURRENT GAIN
sweep amplifier which converts voltage ramps to current ramps
A
for driving a deflection
coil.
you have a chance, you should look at a TV
book and study some of the sweep circuits
used in older TVs. You will find that the whole
sweep generating process for each ramp waveform is done with just three or four tubes or a
the individual circuit modules are so easy and
If
cheap to build, modern equipment remains comby using more and more simple modules
to accomplish more and more complicated tasks!
repair
plicated
QUESTIONS:
handful of transistors. In order to live with the inductance of the deflection coil and the imperfections of the tubes or transistors, the sweep circuits usually have a confusing tangle of inductors, transformers, resistors, and capacitors surrounding each tube or transistor. Each component seems to be busy compensating for the
shortcomings of the other parts. The result is an
orchestra of distorted waveforms that somehow
all come together in the deflection yoke to pro-
1.
Modern
Each
do
for
circuit design like the
just discussed
circuit
is
much more
module does what
in a precise
for the operational
2.
What
is
an active frequency
3.
What
is
a
4.
op-amp design
it is
the inverting amplifier configura-
adding
cir-
cuit?
straightforward.
way with very few
is
used
tion
duce the perfect ramp-shaped current needed to
produce an undistorted TV picture.
we
Why
supposed to
The need
Bode
filter?
plot?
A high pass filter like the one Fig. 11-4 is
used to separate two close audio frequencies. The filter does not attenuate the unwanted frequency enough even though the
cut-off frequency is properly located for a
maximum attenuation. How can the filter
be made sharper so that the unwanted frequency is eliminated?
parts.
compensating components and careful alignis largely gone. Modern circuits based on in-
5.
If
active filters like the ones in Fig. 11-4 are
ment
built with voltage gain greater than 2 or 3
tegrated circuits are not only easier to design,
they are easier to fix. Modern circuits have not
in these circuits
they tend to oscillate.
yet liberated every application from these problems, but the trend is well established. Now that
ble?
What
makes
190
is
make
What
three factors
self-oscillation possi-
there about the design that
oscillation unlikely?
6.
7.
How can low pass and high pass
combined to make a notch filter?
How
filters
be
17.
can low pass and high pass filters be
to make a band pass filter?
What makes
a logarithmic amplifier log-
18.
arithmic?
9.
voltage
Suppose you were going to build a circuit
that would calculate instantaneous milesper-gallon for your car. A small DC generator driven by the speedometer cable
20.
A fuel flow meter produces
a DC voltage proportional to the gallons
per hour flowing into the engine. Draw a
gallons/hour
gallons
is
similar to the com-
Back
in
Section 4, Fig. 4-5 showed an
volt-ampere characteristic for a
many op-amps
as needed, sketch a circuit that will simulate this ideal bipolar transistor.
cuit should
using a multiplier IC:
fuel flow
configuration
collector amplifier?
bipolar transistor. Using as
perform the following calculation
miles
configurations are similar to
the grounded emitter amplifier? Which op-
"ideal"
miles-per-hour.
miles/hour
What op-amp
amp
mon
proportional to
speed
current
form through the complex impedance of
19.
is
and
for a deflection coil are
the deflection coil?
input.
produces a voltage that
waveform
shown in
Fig.11-20. How did the op-amp "know"
what this bizarre voltage waveform had to
be in order to force a perfect ramp wave-
Using op-amps, logarithmic and antilogarithmic amplifiers, draw a circuit that
circuit to
The
waveform
can raise a voltage to the third power.
What simple change in this circuit could
change the output to the cube root of the
10.
the voltage-to-current converter
and the inverting amplifier configuration
have in common? What characteristics do
the grounded base amplifier and the voltage-to-current converter have in common?
combined
8.
What do
have a current gain
Your
of 100,
cir-
The
output should act like a current source controlled by the input current. The input
voltage can be any voltages that are convenient for you and the load can be located
anywhere that
is
convenient. (Hint: The
current delivered to the load resistor in
many op-amp configurations is determined
11.
How
is
by the
size of the load resistor because the
output of the op-amp acts like a voltage
source and will supply all the current "asked for."
integration related to differentia-
tion?
12.
13.
Why
How
a simple RC integrator inaccurate?
does an op-amp integrator correct
this inaccuracy?
is
Why
are operational differentiators usual-
ly impractical?
14.
15.
Referring to Fig. 11-15, sketch a graph of
the current flowing into the integrator.
What two
applications for integrators were
described in the text? How could an integrator be used to count the number of
short, equal voltage pulses occuring over a
period of several seconds or minutes?
16.
What
is
meant by the "solution"
to a dif-
ferential equation?
191
SECTION XII
Power Supplies and
Voltage Regulators
Introduction
A.
The power
directly
to
run electronic circuits
available in the
is
rarely
m
DC
voltages needed.
Even in battery powered equipment, the batteries
may start out with the correct voltages, but as
the battery is used, the voltage gradually falls
and the circuit performance falls with it. In this
section
we
gulating
methods
are going to look at
DC
of re-
voltage so that the circuits will
always receive a constant voltage supply,
re-
what the actual source of the power is
will also look at methods of converting
gardless of
doing.
We
voltages from high levels to low
DC
W
voltages.
Power supplies are not the most glamorous
part of electronics, but every circuit has to have a
power supply, so we may as well grit our teeth
and learn about them. I have met engineers who
think power supplies are so fascinating that they
have made entire careers out of designing them.
An
Fig. 12-1
assortment of solid state voltage
regulators.
From their point of view, a TV or a radar set is
just an excuse to use an exotic power supply. Per-
B.
Power Supply Design Goals
Power supplies have many design goals. The
power supply should provide a constant DC
I can't get that excited about power supand you probably can't either. However,
they aren't dull, and thanks to the new integrated
circuit voltage regulator devices, fancy power
supplies are no longer hard to understand and
sonally,
plies
voltage with no noise or ripple.The voltage should
remain constant even though the load current
may vary widely.
repair.
The
Much
of
what we
are going to study,
have already been introduced
A
regulators.
Then
and
power
supply
should
limiting capability so that,
if
have
current
a short circuit oc-
few of the
are going
curs in the load or even in the power supply itself,
the electronics in the load and the power supply
parallel voltage
be damaged as little as possible. Generally
done in two ways. First, a fuse or a small
circuit breaker usually protects the instrument as
a whole. Second, the voltage regulator often con-
to.
principles will be entirely new. First
to talk about simple series
you
we
we'll discuss switching
will
power
this is
supplies and regulator methods based on exotic
kinds of power transformers.
193
tains a current limiter circuit in addition to the
voltage regulation function. The current limiter
restricts the output current to the level the sup-
C.
ply can deliver safely.
example
Parallel Voltage Regulators
Zener diode regulation of power supplies
of parallel regulation.
The idea
is
is
an
that a
relatively high, unregulated DC voltage is divided down to a lower, regulated DC voltage by
important that the power supply
has no appreciable inductance or resistance as
seen by the circuits it is powering. This is very
important for high frequency, high current electronic loads such as radio transmitters. The output voltage signal from a transmitter final amplifier will be wasted across the power supply inductance and the output power will be dissipated
uselessly in the supply internal resistance.
It is also
means
of a resistive voltage divider. As you
know, a voltage divider consists of two resistances which span the power source. The load is
usually placed across the divider resistance that
has one end grounded. In the case of a parallel
grounded resistance element varies
resistance in order to hold the voltage across
the load constant. The zener diode is an example
regulator, the
its
of the variable resistive element that is in parallel
It is
often desirable to electrically isolate the
with the load.
power source. No, I
power cord. I mean
isolating the load from the power source so that
there is no voltage reference between the two.
This is necessary whenever the output voltage
has no reference to ground. In other words,
electronics load from the
don't
mean snipping
off the
This circuit will do a good job of holding the
load voltage constant at the zener voltage, provided that two circumstances remain true. First,
the input
neither the positive or negative voltage terminals
are grounded.
main
voltage
is also important as a
leakage voltage from the
power source (usually 120 volts AC) is not present
on the cabinet or in the electronics circuitry
where it might be a risk to the operator.
it is
AC
usually important to
make
voltage drop across the dropping resistor, R^, exceeds the difference between the input voltage,
V and the zener voltage, V z
m
.
As we
shall see in this chapter, heroic
UNREGULATED
DC VOLTAGE
- V
2
both the dropping resistor and the zener diode. In
a typical application, only one third of the energy
used to accomplish these goals.
vin
Vin
The zener regulator is a practical, simple
voltage regulator system as long as the load requires only small currents or wasting power is not
important. The zener regulator wastes power in
very efficient so that very little heat needs to be
dissipated by fans or heavy, finned aluminum
heat sinks.
(Rd)
the
power supply as small and lightweight as possible. In order to do this, the power supply must be
efforts are
V
/
Rd
Vi
VOUT
(LOW
RESISTANCE)
IS
A
CONSTANT VOLTAGE
AS LONG AS V|N > V Z ENER
THE TWO
ELEMENTS
OF A
RESISTIVE
VOLTAGE
DIVIDER
^
^"v ZENER
NETWORK^
RL
DIODE CONDUCTS
AT VZENER
LOAD
RESISTANCE
^>
>
IF
LOAD DRAWS TOO MUCH
CURRENT, THE VOLTAGE DROP
Fig. 12-2
A
re-
in its zener
(i)
Finally,
must remain higher than
breakdown region. Second, the
load must not draw so much current that the
Isolation
safety feature so the
DC
the zener voltage so that the zener diode will
ACCROSS Rd WILL EXCEED V|N - Vz.
AND REGULATION WILL WILL BE LOST
zener diode regulator as an example of parallel voltage regulation.
194
UNRECULATED
DC
VOLTAGE
ZENER
DIODE
SERVES
AS A
REFERENCE
VOLTAGE
FOR
THE
VOLTAGE
FOLLOWER
VOUT = V Z
J
RL
LOAD
RESISTANCE
/777
NOTE HOW THE
COMPLEMENTARY CLASS
rm
Fig. 12-3
An
WITH
ACTIVE
op-amp voltage follower used as a voltage
is
"amplifying" a constant
Rd,
parallel regulator is to
power
regulator.
DC
reference voltage.
The reference voltage comes from a zener diode,
just like Fig. 12-2. But now the dropping resistor,
sistances.
replace the zener diode with a
2
RESISTIVE ELEMENTS
drawn from the unregulated supply may go to the
load while two thirds is burned up in these re-
Another way to build a
B
OUTPUT IS SERVING
AS A PARALLEL
VOLTAGE REGULATOR
is
a very high resistance and the "load" on
the zener diode
transistor.
is
the op-amp input which draws
essentially no current. Therefore, the zener diode
This transistor would be turned part way on or
response to the load voltage so that its
behavior would be identical to the zener diode.
Such a regulator can be made more efficient by
replacing both the zener diode and the dropping
resistor, Rd, with power transistors. By making
both of these resistive elements variable, the
resistance divider can be "tuned" for maximum
reference circuit
consumes
little
power.
off in
power
in the resistance divider.
Fig.
amplifier
voltage
12-3 demonstrates
voltage
regulator.
follower
As you
a complementary class B transistor
you follow the course of the majority
of the input current which eventually goes to the
load, it is going to get there by means of a
voltage divider consisting of two
resistive
resistances— the "dropping resistor" is the resistance of Qi and R lt and the "parallel resistor"
consists of Q2 and R2. In this example both the
dropping resistance and the parallel resistance
are active and vary to hold the load voltage cons-
put stage
how an operational
can be used as a
recall,
the
voltage
both resistors are active, the voltage
divider can be "tuned" for maximum efficiency.
For example, if Vj n dropped toward the desired
load voltage, Qi can turn nearly full on while Q2
can turn nearly full off. This makes the divider as
efficient as possible. Whereas, when one of the
tant. Since
follower will follow an input reference signal and
the output will behave like a voltage source. That
the output will attempt to provide all the cur-
is,
rent "asked for"
of amplifying
is
amplifier. If
and minimum power
transfer of
burned up
to the load
This op-amp circuit could be classified as a
because the typical op-amp out-
parallel regulator
by the load resistance. Instead
some interesting voltage signal,
such as a Beetle record, here the voltage regulator
195
elements is fixed, the regulator is
always stuck with that energy and heat dissipa-
resistance
cuit.
Large op-amps capable of delivering amperes
not as precise.
In the parallel regulator, power is dissipated
two resistive elements. The regulation can be
more efficient by using the active transistor as
the voltage dropping resistor and eliminating the
For larger power supplies,
parallel resistor.
whenever the zener diode would have to dissipate
more than a watt or two, the system generally
used is the series regulator in which the active elein
ment
is in series
Most of
shown in Fig.
cuit.
with the load. In other words, the
in
which
a neat
all
its
own
voltage divider
cir-
12-1 are actually series regulators
plastic or metal
is
contained in
package that
12-5
in a
the
same
series
re-
sembles an ordinary transistor.
In Fig. 12-3 we showed a voltage follower
amplifying a zener diode reference voltage. Fig.
12-4 shows a simple series regulator which is
nothing more than an emitter follower transistor
amplifier amplifying a zener diode reference cir-
Vin)-
UNREGULATED
DC VOLTAGE
POWER TRANSISTOR
•PASS ELEMENT"
S
VARIABLE RESISTANCE
REGULATED
DC REFERENCE
VOLTAGE
»-CAPACITOR
HELPS HOLD Vz
VOUT - Vz -
CONSTANT
0.6
VOLTS
>
v
ZENER REFERENCE
VOLTAGE
Fig. 12-1
^_
~V
EMITTER
FOLLOWER
Simple
regulator
is
voltage. A bridge rectifier converts the AC to
unregulated DC voltage. The difference between
the regulated DC voltage and the unregulated DC
voltage is selected by the designer to allow for the
lowest AC line voltage that might be used with
the power supply. Frequently supplies like this
are designed to operate at levels as low as 90 or
100 volts AC. It is also important that increases
in line voltage can also be tolerated without parts
overheating. Supplies are often designed to tolerate 130 to 140 volts AC without overheating.
the solid state regulator devices
the complex circuitry
little
In Fig.
shown used
complete power supply. Notice
how cleverly the emitter follower has been
redrawn so that you would never recognize it as
an emitter follower. Starting from the left, this
power supply is intended to be powered directly
from the household AC line, 120 volts AC. It contains a switch to turn it on. A fuse is used to prevent spectacular damage to all the currentcarrying parts in the event that some part of the
power supply or the load were shorted. If the current drawn through the fuse is too high, the fuse
will blow and the current to the rest of the circuit
will be automatically turned off. This supply also
uses a transformer to isolate the AC line from the
load and to convert the AC voltage an AC voltage
slightly higher than the desired regulated DC
The Series Voltage Regulator
load becomes part of
learned that the emitter
common.
These op-amps are generally hybrid circuits that
combine a small integrated circuit chip with
power transistors in the same unit. The whole circuit is potted in plastic so that it looks and acts as
if it were a single integrated circuit.
D.
we
1, but can
have a large current gain. Functionally, this circuit behaves just like the voltage follower but is
tion.
of current are available, but are not yet
In Section 7
follower has a voltage gain of about
I
/Vbe WILL BE GREATER
THAN 0.6 VOLTS FOR
A LARGE SILICON
^TRANSISTOR
scries voltage regulator
196
FUSE BLOWS
IF
HIGH CURRENT
DRAW EXCEEDS
SAFE
LIMIT-
SERIES
CAPACITORS HELP
FILTER OUT HIGH
REGULATOR
FREQUENCY NOISE
Rload,
RADIO OR
WHATEVER
ONOFF
ISOLATION
AND
SWITCH
STEP-DOWN
TRANSFORMER
~N^"
BRIDGE
RECTIFIER
AND
CAPACITOR
FILTER
Fig. 12-5
A
complete
AC power supply
Current Limiter Circuits
E.
The
fuse current limiter is crude since fuses
usually respond slowly. In the event that the
regulator output were shorted, it is quite likely
that the pass transistor or some other part would
fail before the fuse had time to respond. A fancier
series regulator
imum
that
is
added to the pass
when
the power supply
current than
1
SENSING
RESISTOR
is
A
^^
is
voltage
lLOAD
used to
delivering
more
can safely handle.
it
The supply
of
CURENT
an additional circuit
transistor.
reference diode, often another zener,
detect
LIMITING CIRCUIT
/
|
would be equipped with a max-
current limiter. This
is
CURRENT
in Fig. 12-6
current limiter circuit
regulator in Fig. 12-4.
Rload
shows a simple form
added to the series
The output current
is
made
to pass through a current sensing resistor. This
resistor
preciably
is
a
small
value
which
will
not ap-
>^
the voltage regulation perforzener diode is wired between the base
affect
mance. A
and the positive output voltage. Except for the
small drop in voltage across the resistor R s these
two components have no effect on the voltage
regulation until too much current is drawn from
VZ 2 -
,
Fig. 12-6
A
rent limiter
19'
0.6
+
lL
MAX (Rs)
series regulator with a simple cur-
When this occurs, the sum of the
voltage drop across the resistor, (iL x R s plus
the base-to-emitter voltage becomes greater than
the zener voltage. When the zener voltage (V Z2 is
can be controlled with smaller currents and the
zener diode references and resistors do not have
to carry so much current. Second, the turn-on and
turn-off currents for the pass transistor(s) are
generated by two separate circuits. The pass
transistor is turned on by a current source circuit
made from a zener diode and a transistor, Q3.
This circuit is a P-N-P emitter follower amplifier
which provides a constant current to turn on the
base of Q2. The "load" of the emitter follower is
the fixed resistor, R c Because emitter followers
have a voltage gain of 1, the voltage across R c
will be whatever voltage is across the reference
voltage, V zl Therefore, the current through R c
will be a constant current. From the collector of
Q3 this same constant current passes on to the
base of Q2 to turn it on. If you get confused about
which way positive current is flowing, just follow
the arrowheads in the transistor symbols.
the supply.
),
)
exceeded, it begins to conduct current away from
the base of the pass transistor. This steals base
current and the transistor can not turn on more
heavily. This limits the supply current to the level
at which the zener voltage was exceeded. It is desirable to have R s as small as possible. Instead of
a zener diode, a stabistor diode is often used.
Fig.
12-7
.
looks more like series regulators
found out in the real world— complicated! The circuit uses the same basic principles used in Fig.
12-6, but the complexity improves the temperature stability and the degree of voltage regulation. First, the pass transistor has been replaced
with two transistors, Qi and Q2 wired as a Darlington transistor. A Darlington pass transistor
.
Q5 STEALS BASE CURRENT
FROM Q2 WHEN VOLTAGE
ACROSS
DARLINGTON PASS
R s RISES
ABOVE
VBE-
TRANSISTOR(S)
"PREREGULATOR
Rload
rrn
•DIFFERENCE AMPLIFIER"
TURNS OFF Q1 and Q2
AS VOUT RISES ABOVE
Vz2
Fig. 12-7
A complex
scries regulator with a transistor current limit circuit.
198
A separate circuit is used to control the output voltage and generate base current to turn off
Qi and Q2. This turn-off circuit does not effect Q2
until the output voltage is roughly the same or
greater than the zener reference voltage, V z2
This zener voltage is usually about 1/2 of the
desired output voltage. When the output voltage
exceeds the zener reference voltage, it becomes
Motorola LM109, LM209, and
.
Q4
ground through
,
it
basically the
LM309
regulators
5 volt regulator.
The wider the temperature operating range and the heavier the current rating,
the higher the price for each regulator. Operating
at —55° C (67° F below zero) may not be impor-
to turn on.
Q4
all
ating ranges.
As current flows to
begins to turn off Q 2 The
exact voltage at which the output voltage is
regulated can be adjusted by changing the current into the base of Q 4 This is done with the
potentiometer R a dj- By dividing the control of
the pass transistor into separate turn-on and
turn-off circuits, the voltage regulation can be improved by a factor of 100 over the relatively crude
possible for
same
But as
you can see from the specifications, this regulator
is available in two current ratings, 0.2 ampere, 1
ampere, and several different temperature operare
tant in your
.
TV
set,
but
tant in your space ship.
it
could be very impor-
The metal case
is
design-
ed to dissipate heat, so it is convenient to ground
the metal case. Therefore, there are separate regulators designed for negative power supplies
which have the positive side gounded to the case.
Positive voltage regulators have the negative
side connected to the case.
.
circuit in Fig. 12-4.
When
voltage regulators are used in any cirimportant that the wires between the
regulator and the load be as short as possible. The
inductance in the wires going from the voltage
regulator to the load can be very significant.
cuit, it is
This circuit also has an improved current
The current limiter consists of
transistor Q5 and the current sensing resistor,
R s The voltage drop sensing diode is the base-toemitter P-N junction of the transistor Q5.
Whenever the voltage across the sensing resistor
rises above about 0.6 volts, the transistor Q5 will
turn on and steal base current away from the
transistor Q 2 The sensing resistor can be smaller
than that used in Fig. 12-6 because only the baseto-emitter junction voltage needs to be exceeded.
A smaller current sensing resistor means that the
voltage regulation will be more accurate and will
consume less energy. This regulator circuit is
complex, but if you think about the separate functions of each of the four parts, it will not seem so
complicated.
limiter system.
.
When
is a serious problem in digital cirwhere binary numbers are being processed
at high speed. For instance, suppose a large
number of binary circuits turn on simultaneously
to make "zeros." Altogether they will draw a
large current pulse. If enough zeros come on at
once, the combined pulse may be large enough to
make the supply voltage momentarily drop to
such a low voltage that a digital logic circuit may
confuse a "one" with a "zero." This could cause
the computer to make an error. If you have a
charge card, you know what a hassle a computer
error can cause. The small size of 3-terminal
the wires. This
cuits
.
F.
Three
Terminal Integrated
Voltage Regu-
you thought the
circuit in Fig. 12-7
was
complicated, just look at the innards of a typical
integrated circuit voltage regulator in Fig. 12-8.
As
the
name
They
power supply.
Three terminal regulators can be used in difThey can regulate voltages larger
than the nominal voltage by using a resistive
voltage divider to produce a reference voltage
equal to the actual regulator voltage. For example, a 5 volt 3-terminal regulator can be used to
regulate 10 volts. The ten volt output is divided
which have three exter-
them
right
ferent ways.
tegrated voltage regulators are integrated circuit
nal leads connecting
makes
regulators
large, centrally located, regulated
implies, the three terminal in-
series voltage regulators
regulators
to put voltage
it practical
on each circuit board where
regulated voltage is needed. This gets rid of the
long wires and is usually better than using one
lators
If
the load draws large current pulses, a large
voltage drop will occur across the inductance in
to the outside world.
packaged in transistor cases and
resemble ordinary power transistors. They contain complex temperature compensation and current limiting circuitry. Three terminal regulators
come in a wide variety of voltages and rated curare usually
in half
by a
resistive divider so that the regulator
has a 5 volt feedback signal.
By making
the
resistive divider variable, the regulated output
voltage can be varied over a large range.
The designer picks out the type that will
supply the need of his particular circuit. The
rents.
199
LM109
LM209
LM309
MONOLITHIC POSITIVE THREE TERMINAL
FIXED VOLTAGE REGULATOR
•
A
for
positive
versatile
easy
regulator
fixed
POSITIVE
as
VOLTAGE REGULATOR
application
for
logic
+ 5.0-volt regulator designed
on on-card, local voltage
systems. Current limiting and
digital
thermal shutdown are provided to make the units extremely rugged.
In
most applications only one external component, a
capacitor, is required in conjunction with the LM109
Series devices. Even this component may be omitted if
the power-supply filter is not located an appreciaable
distance from the regulator.
• High Maximum Output Current
TO-3 type Package Over 200mA in
TO-39 type Package.
— Over
1.0
Ampere
INPUT OUTPUT
K SUFFIX
METAL PACKAGE
CASE 11-01
in
(TO-3 TYPE)
—
(BOTTOM
GROUND
VIEW)
• Minimum External Components Required
OUTPUT
2
•
Internal Short-Circuit Protection
•
Internal
/T\
'NPUT
1
(
Thermal Overload Protection
°
C
°) 3
S
GROUND
(BOTTOM VIEW)
• Excellent Line and Load Transient Rejection
H SUFFIX
• Designed
for
METAL PACKAGE
CASE 79
Use with Popular MDTL and MTTL Logic
(TO-39)
ORDERING INFORMATION
Device
LM109H
CIRCUIT SCHEMATIC
y
t—« INPUT
t
Temperature Range
Package
= -55 = C lo +150'C
Metal Can
Tj
-55 C
LM109K
Tj
=
LM209H
Tj
= -55°C lo + 150=C
Metal Can
Tj
= -55°C lo +150 = C
Metal Power
LM209K
LM309H
LM309K
=
lo +
15(FC
Tj
= 0°C lo + 125°C
Tj
= 0=C lo
+125°C
Metal Power
Metal Can
Metal Power
TYPICAL APPLICATION
FIXED
OUTPUT
5.0
V REGULATOR
'NPUT
2
LM109
C1*
C2
39GROUND
0.22 uF
5V
—t—'OUTPUT
-o
-REEQUIRED IF REGULATOR IS LOCATED AN APPRECIABLE
DISTANCE FROM POWER SUPPLY FILTER.
ALTHOUGH NO OUTPUT CAPACITOR IS NEEDED FOR
STABILITY. IT DOES IMPROVE TRANSIENT RESPONSE.
Fig. 12-8
Motorola 3-terminal integrated voltage regulator
Another common use
Because the 3-terminal regulator can be used
to regulate the voltage across a load resistance,
can
also
regulate
the
voltage
across
a
gulators
it
will
be fixed
at a
them
for three terminal re-
as control circuits for high
In
Fig.
12-11
a 0.2
ampere 5 volt regulator is shown controlling a 10
ampere pass transistor. Notice that this circuit
has no current limiting capability for the large
power transistor. A current limiting circuit like
constant
value. This current can then pass on to a load
which needs to be driven by
to use
current series regulators.
fixed
resistor. Since the resistor is fixed, the current
through the resistor
is
a current source.
200
LM109
5
5
VOLT
REGULATOR
VlN
- 15
VlN
~ 10
1
>VOLTS
^VOUT
10
TRANSISTOR
= 10
DC
UNREGULATED
3(CASE)
10
VOLTS
REGULATED
300 Q
REFERENCE
.22 M fd
300 Q
A
*
A 3-terminal voltage regulator used to
regulate voltage larger than the rated voltage.
Fig. 12-9
G.
Energy Gap Voltage Standards
ference
3(CASE)
RL
iREG
is
an integrated circuit based on the tran-
sistor circuit
shown
in Fig. 12-12.
In this circuit, the transistor at the
^>
X
5
•REG =
Fig. 12-10
A
cur-
Sometimes the performance of a zener diode
is not accurate enough for use in a
precision power supply or as a voltage reference
for a precision voltmeter. The energy gap re-
^>
VOLT
REGULATOR
>
if
rent limiting were needed.
or stabistor
LM109
5
rrn
VOLT VOLTAGE
DROP ACROSS CURRENT
SENSE RESISTOR
V|N^>-
x
3(CASE)
the one in Fig. 12-6 would have to be added
rrn
5
/TT7
3-terminal regulator used to control
a heavy current series voltage regulator.
Fig. 12-11
>
10 ufd
(200 Ma)
VOLTS
5
*
Q
LM109K
5 VOLT
<>-*-
AMPERES
n
y
v/
UNREGULATED
VOLTS
AT UP TO
POWER
VOLTS
3-terminal regulator used as a cur-
rent source.
left,
UNREGULATED
VOLTAGE
->
>
SMALL BUT
VERY CONSTANT
AS A MATCHED
CONSTANT
f VOLTAGE
(USUALLY
MICROAMPERES)
RESISTOR
Vbe(i)SERVES
v
CURRENT.
Rload
VOLTAGE
DROPPING
*
/
0.6
y
^
02
VBE =
VOLTS
VOLTAGE REFERENCE
FOR Q2
J
AND Q2 ARE
MADE SIMULTANEOUSLY
THE SAME INTEGRATED
Qi
SO THEY ARE
CLOSELY MATCHED.
CIRCUIT,
rm
Fig. 12-12
Qi,
is
used as simple forward diode. The collector is
shorted to the base so that all that remains is the
base-emitter junction. The "diode" at the left is
biased by current from the unregulated supply,
through Rd- This develops a voltage across the
base-to-emitter junction of about 0.6 volts. This
voltage just barely turns on the second transistor, Q2. The current that flows through Q2 is
very tiny, but very constant. When this constant
Basic "energy gap' circuit
201
IN
NOTE ENERGY
GAP CIRCUIT
AT TOP CENTER
METAL CAN PACKAGE
«^-ro9
NOTE: PIN
CONNECTED TO CASE.
2
TOP VIEW
LM113H
204
Fig.
12-13
circuit
current
is
LM113
voltage reference integrated
Corp.
made by National Semiconductor
passed through a fixed load resistor,
a constant voltage. The reason this
show up on the
DC
regulated output voltage. A
designed to attenuate 120
low pass
filter
circuit is so special is that these transistors are
Hz
may
both part of an integrated circuit and are extremely closely matched. The temperature and
of these spikes because these filters
the result
is
gain characteristics of the two transistors are virtually identical because they are manufactured
simultaneously. This circuit causes the temperature dependent characteristics of the two transistors to cancel each other.
The energy gap
used to build the naenergy gap voltage
reference diode. The basic energy gap circuit is
located in the upper center of the diagram. The
reference voltage provided by the basic circuit is
amplified by separate turn-on and turn-off amplifiers which drive an output transistor, Q9. The
result is a circuit that acts like an almost ideal
tional
circuit is
semiconductor
LM113
zener diode or stabistor with a breakdown of 1.22
volts over a wide range of temperature and
voltage. It can handle from about 0.5
to 20
mA
mA
of current. It generates far less radio noise
than zener diodes so
it
is
also
preferred
for
regulators in radio receivers. In short, this device
is
the world's most accurate and complicated
zener diode.
H.
ripple
that
is
not be very effective in getting rid
have a great
deal of inductive reactance and resistance at high
frequencies.
capacitors
capacitance of filter
decreases dramatically at
the
Also,
actually
high frequencies. The solution to this problem is
to attenuate the high frequency spikes before the
AC
is rectified.
The metal oxide varistor (MOV), is a semiconductor resistor made of zinc oxide semiconductor
crystals. When the voltage across this specialized
resistor becomes too high, the resistor breaks
down and becomes quite a good conductor. The
action of a varistor can be compared to a pair of
zener diodes wired back-to-back in series. Whenever the AC voltage exceeds the breakdown voltage, in either the positive or negative direction,
the varistor conducts and clips off the noise
spike.
The key
is
extremely fast and can
noise spikes.
across an
AC
The
between the zener
difference
diodes and the varistor
the varistor switches on
clip
varistor
very short duration
connected directly
is
voltage, usually across the second-
ary of a transformer.
Varistors
The
varistor
is
/.
Switching Power Supplies
1.
Introduction
a semiconductor device used
for clipping noise spikes off
AC
voltage.
Heavy
duty motors and relays can generate very large
voltage spikes which are impressed on the AC
power line voltage. These noise spikes have such
a short duration and large amplitude that they
often pass right through a power supply and
Switching power supplies can achieve all of
power supply design goals and still be
lightweight and compact. The efficiency with
which they transfer power to the load can be very
the
+
TYPICAL
1
ductors do not dissipate any energy, the voltage
can be changed without power dissipation.
POSITIVE
MOV
BREAKDOWN
VOLT-AMPERE
CHARACTERISTIC
Switching power supplies accomplish two
basic tasks. First, they convert the voltage level
from whatever the source may be to whatever is
DC
needed. Second, they regulate the output
voltage. Both of these functions are accomplished
in the same operation. In switching power sup-
-200
+ 200
VOLTS
plies
designed to operate from an
source, the switching function
the
AC and isolate
AC lines.
may
AC
power
also rectify
the voltage output from the
main
NEGATIVE
BREAKDOWN
To appreciate why switching power supplies
can be such a good deal, you need concrete examples of what it takes to replace one. Suppose
you are a high-flying financial wizard and you
wish to install a stock exchange computer terminal in your jet plane. The plane has a 28 volt
DC power system and the computer terminal
needs 5 volts DC at many amperes. There is a 23
volt difference between the average source volt
age and the voltage you need. If you use a series
regulator it will work fine, but over 80% of the
power consumed will be burned up in the series
pass transistors. This means that you will have to
use huge heat sinks and fins to get rid of hundreds of watts of waste heat.
-I
MOV
SYMBOL
EQUIVALENT CIRCUIT
SIMULATED WITH ZENER
DIODES. ACTUALLY,
ZENER DIODES ARE TOO
SLOW TO DO THE JOB.
VARISTORS ARE MADE
BY GENERAL ELECTRIC
CORPORATION.
12-14
Fig.
and volt-ampere
symbol,
Varistor
diagram.
90%
high, over
in real
power supplies are
power
supplies.
These
Another design approach would be to convert
DC to AC by means of chopper transistors, then pass the AC current through a
the 28 volts
efficient because, unlike the
series voltage regulators, these supplies do not
use a resistance to lower the unregulated voltage
transformer to reduce the voltage
regulated
voltage level. Instead, the
unregulated DC voltage is chopped into AC. The
AC current is passed through an inductor or
vicinity of 5 volts.
transformer to change the voltage
but
to
the
level.
Since
tified
and
Of
filtered to
make DC.
in-
it
VOUT
BREAKDOWN
VOLTAGE
VlN^
1
170 V-P
A
1
\
J
/
,
h
i
\
•
i
^
120
VOLTS AC
RMS
>V0UT
FILTER
AND
REGULATOR
NOISE
y
ov
TO RECTIFER,
+
>
c
BREAKDOWN
VOLTAGE
Fig. 12-15
Varistor noise spike clipping action
203
to the
do this
would probably have to be more complex to
cuit similar to Fig. 9-7 could be used to
V|N
+
down
AC must be recA DC inverter cir-
course, the
work
At
reliably in this application.
The pulse width modulator generates
this point the
the
DC
DC
control pulses that turn the transistor switch on
supply.
and off. The width of the pulses generated by the
modulator is determined by the negative feedback from the output voltage. As the output voltage begins to fall, the pulse width modulator
output voltage is still not regulated, so you
might use a series voltage regulator with a small
voltage drop to produce the regulated 5 volts. The
result is a fairly efficient, but cumbersome power
makes the
2.
A
voltage-reducing switching power supply
control pulses wider so that the tran-
sistor switch will let wider current pulses into the
inductor and capacitor. This will charge the stor-
A
age capacitor at a faster rate and the voltage
across the capacitor will rise. If the output voltage rises too high, the pulse width modulator
will make narrower pulses so that the storage
capacitor is not charged at such a rapid rate.
block diagram for a voltage-reducing
switching power supply is shown in Fig. 12-16.
The voltage is reduced and regulated with five
basic circuit components: a switch, an inductor,
a diode, a capacitor, and a pulse width modulator.
First, the 28 volts DC is chopped into DC voltage
The pulse width modulator principle resembles the SCR and TRIAC light dimmers in
Section 5. Rather than dissipate the unwanted
pulses by a transistor switch. These pulses drive
current ramps through the inductor to the stor-
age capacitor. The capacitor is large enough so
that it serves as an energy reservoir and the
voltage across the capacitor
is
voltage in a resistance, the current is let into the
load in short pulses so that the time average of
the transistor current delivers the desired energy.
relatively cons-
tant, even though it is being charged by a rapidly
varying current. The regulation and switching is
controlled by the pulse width modulator. This
"component'* is itself a complicated circuit and is
the brains of a switching power supply.
In fact,
SCR's serve as both the "switch'* and the
some switching power supplies that
rectifiers in
are designed to operate directly from the
power
AC
line.
WIDTH OF PULSES
IS PROPORTIONAL TO
5
VOLTS.
VOUT
PULSE
WIDTH
SWITCH
CLOSED
MODULATOR
5
DURING EACH
PULSE
+
t
VL
OUTPUT VOLTAGE
HELD CONSTANT
BY CAPACITOR
28
VOUT
I
L
28
VOLTS
I
5
INDUCTOR
TRANSISTOR
SWITCH
+
S
"FREEWHEELING
DIODE
DIODE CONDUCTS
WHENEVER
SWITCH OPENS.
VOLTS
DC
4>
yisms&s
I
VOLTS DC
LARGE
STORAGE
CAPACITOR
>>
Rload
CURRENT THROUGH INDUCTOR
CHARGES CAPACITOR
CURRENT
RISES WHILE
SWITCH IS
CLOSED
Fig
12-16
Diagram
for
VOLTAGE AT TOP
OF DIODE
MA/
DIODE CONDUCTS
WHILE INDUCTOR
DISCHARGES.
a voltage-reducing switching power supply
204
Although the
filter
capacitor
must be
Think
of the charged inductor as a battery
ready to deliver current to the load.
However, this "battery" must be properly wired
to the load during the time when the switch is
open. Since the current in an inductor cannot
large,
the inductor can be quite small because the
that
fre-
quency of the pulses is usually very high, 20 kHz
or higher. Very little inductance is needed to
make the necessary reactance at 20 kHz. If the
same circuit were operated at 60 Hz, the inductor
would have to have a formidable chunk of trans-
change instantly, we know that the inductor
rent will continue to flow in the
free
its
Let's think about
what would happen
if
the
wheeling diode were left out. When the
switch opened, a huge voltage would appear
across the inductor. This voltage will become as
large as necessary to keep current flowing in the
same direction as before. This voltage easily
could be thousands of volts and could damage the
switching transistor. If the voltage did not succeed in breaking down the transistor, it would
find some other way to discharge itself. Inductors
do not remain charged indefinitely in the way
that charged capacitors are content to remain
charged. In any case the energy stored in the inductor would have no way to pass into the load.
The supply efficiency would be very poor and
there would be no advantage over using a voltage
dropping resistor instead of an inductor. Whenever a transistor has an inductive load, it is often
necessary to use a diode to protect the transistor
from the voltage that appears across the inductive load when the transistor shuts off. The class
E amplifier circuit, Fig. 7-20, uses a diode in this
manner.
The diode between the top of the inductor and
ground needs a careful explanation. This diode is
often called a free wheeling diode. This refers to
the rachet-like action of this diode which keeps
the inductor current flowing in one direction into
the load and capacitor. If you are normal, you
we
path.
free
wheeling diode
didn't understand that explanation, so
cur-
direction as
before.
cuitry.
The
same
But like any battery, both ends of this
"battery" must be connected to the load to
deliver current to it. The diode therefore connects
the transistor switch end of the inductor to
ground so that the charging current can complete
former iron to maintain the reactance for such
long half cycles. This principle of using high frequency to make inductors and transformers lighter is very wide spread. For example, electric railways can use 25 Hz AC current because there is
no shortage of steel in an electric locomotive. In
contrast 60 Hz is used for household AC systems
since no one would want to pay for a 5 pound, 25
Hz transformer in their table radio. Weight is
even more important on aircraft. Aircraft AC
power systems use 400 Hz so that transformers
and motors do not need so much iron. In some
missiles 1400 Hz AC power is used to cut weight
still more. The bad news about the use of the high
switching frequency is that it can generate noise
which can interfere with radio and computer cir-
3.
is
will try
you know, the current through an inductor can not change instantly. This means that
again: as
when
a constant voltage is applied to an inductor,
the current through the inductor will rise slowly
to produce a current ramp. In this case, when the
transistor switch closes, the current through the
4.
The pulse width modulator converts a
voltage level into a series of pulses which have a
width which is related to the original voltage
inductor will rise slowly as the magnetic field of
the inductor is charged with energy and the inductive reactance falls.
If
a resistor had been used instead of an
Pulse width modulators
This can be done with a voltage ramp
generator and a comparator. The comparator has
two inputs; the voltage ramp and the relatively
constant "error signal" voltage level derived
from the regulated output voltage. The "error
signal" is a voltage that represents the difference
between what the output voltage is and what we
would like it to be. The comparator makes a pulse
level.
in-
ductor, the energy lost across this impedance
would be burned up as heat. The efficiency of the
switching power supply comes from being able to
use the energy stored in the magnetic field by
delivering it to the capacitor and the load. The op-
whenever the triangle shaped voltage
portunity to use this stored energy arises when
the switch opens and the inductor is left alone to
discharge energy into the capacitor and load.
than the error signal voltage
level.
is
Since the
larger
ramp
waveforms come to a point, the comparator output pulses become narrower and narrower as the
205
error signal voltage
signal is the difference between the reference
voltage and the actual supply voltage multiplied
by the gain of the differential amplifier. The
reference voltage and the differential amplifier
gain are carefully chosen to locate the error signal
becomes higher and higher.
In other words, the pulse width
is
inversely pro-
portional to the error voltage level.
many ways
to build a pulse width
one example. This circuit
is made from circuits you have studied in previous sections. The voltage ramp generator is the
one we studied in the last section. The integrator
capacitor shorting switch is controlled by a
square wave so that the ramp signal is turned off
50% of the cycle. The square wave is generated
by starting with a phase shift oscillator and making a square wave by feeding the sine wave into a
comparator. As wired, the comparator makes a
positive pulse whenever the sine wave goes below
its zero point. We could have generated the
square wave with an astable multivibrator, but
the frequency would not have been as stable.
There are
modulator. Fig. 12-17
on the voltage ramp triangle. As the load draws
current ranging from zero to the full rated
amount, the error signal will travel smoothly
down the ramp triangle and make wider pulses.
is
This circuit is a
modulator because the
is
only
50%
shall see shortly,
switching system.
pulse
The output from the voltage ramp generator
is
the load
is
drawing
little
makes the
or
become
drawing the
as thin as hairs.
maximum
When
allowable
the load
current,
of the
ramp
circuit
is
available
as
an
in-
Silicon General Corporation
used
in Fig.
12-18. This par-
two
complementary half wave pulse width modulated
pulse trains. So it may be used in both full wave
and half-wave switching circuits. This power sup-
is
the
ply has three features not
error voltage signal will be very close to the bot-
tom
The
ticular integrated circuit is able to generate
no current, the error voltage signal will rise so
high that the pulses the comparator generates
will
guess, a circuit as complex as a
width modulator
tegrated circuit.
fed to a comparator to produce the final switch
When
pulse width
width of each
of the total cycle time. As we
some kinds of switching power
supplies use complementary pairs of half wave
waveforms for driving a push-pull amplifier
switching system. The result is a full wave
pulse
As you might
control pulses.
half-wave
maximum
shown
earlier in Fig.
12-16. First, a Darlington transistor
is used for
high gain in the switch function. Second, the
pulse width modulator also contains a current
triangles to generate pulses as
wide as possible. It is important that the error
voltage never reach zero volts. Because if it did,
the comparator would stop generating pulses and
comparable
limit circuit that is
ones
would just make one long, continuous turn-on
signal. This would quickly burn up the switching
we discussed
series regulators.
A
series with the line
transistorls).
earlier
in
in function to the
conjunction with
sense resistor, 0.1
ohms
is in
from the 28 volt power supply.
A
The
error signal is rather abstract.
At
pair of current sense leads look at the voltage
across this resistor. If the current becomes too
first
high, the switching pulses are kept skinny to
glance you would think that if this modulator
were going to regulate a 5 volt power supply, then
the voltage reference signal should be 5 volts DC.
amount of current the switching transupplying to the inductor.
restrict the
sistor is
seems reasonable that the regulation error
what the
supply voltage actually is and the 5 volts reference. But if this were so, the error voltage
would be "zero" whenever the supply voltage
equaled the reference voltage. We have already
seen that zero will not work because the
switching transistors would always turn full-on.
It
signal should be the difference between
5.
Full
wave switching power supplies
We
will illustrate full
might be used to power a 500 watt radio
mitter. This circuit
When
the system
error signal can be
waveform except
ing
is
that there
is
regulating properly, the
anywhere on the ramp
will
tasks:
is
say-
1.
signal.
he exaggerated by
the gain of the differentia] amplifier.
trans-
accomplishes five different
triangle
at zero volts. What this
must always be an error
Moreover, the error signal
wave switching power
supplies by describing a supply designed to work
directly off the AC power line. A supply like this
The
error
206
increases the average voltage level from
120 volts AC RMS to 300 volts DC. By using
a step-up transformer, the same design could
also lower the voltage.
It
v
20 kHz SINE
WAVE
REFERENCE
VOLTAGE
O +V CC
PHASE
SHIFT
OSCILLATOR
20 kHz
SQUARE WAVE
VOLTAGE
RAMP
GENERATOR
THE REFERENCE
VOLTAGE AND
DIFF.
AMP. GAIN
ARE CHOSEN TO
PLACE THE ERROR
VOLTAGE ON THE
RAMP CORRECTLY.
SUPPLY OUTPUT
VOLTAGE
/
I
ERROR
VOLTAGE
PULSE WIDTH
Fig. 12-17
IS
INVERSELY
PROPORTIONAL TO THE ERROR
VOLTAGE LEVEL AND DIRECTLY
PROPORTIONAL TO THE OUTPUT
CURRENT LEVEL.
Half-wave pulse width modulator
207
It
2.
When
regulates the output voltage.
all
the current pulses pass through in one
do in a half-wave system, then
direction as they
It rectifies
3.
and
It limits the
4.
the
filters
AC
the permanently magnetized iron in the core is
unusable and is extra weight added to the power
supply. With a full wave system, the current
pulses generate magnetic flux in both directions
and all the iron is remagnetized on every cycle.
Since all the iron is generating useful magnetic
fields, less weight of iron is needed.
power.
current from the supply to a level
that can be delivered safely.
It electrically isolates
5.
the
DC
the
power delivered
AC
power
from
line
to the load.
Thanks to the dual outputs of most pulse
width modulator integrated circuits, full wave
designs are easy to build. Each of these control
pulse outputs produces pulses that are 180° out
of phase with each other. In other words, only one
output is producing a pulse at one time. In fact,
there are logic circuits inside the IC that make
certain that both pulses are never on at the same
time. If they were to come on simultaneously, the
switching transistors would energize both primary winding halves simultaneously. This would
cancel out the inductance of the primary winding
and the transistors would be shorted to ground.
They would conduct huge currents and be de-
+ 28V
z i 500 ^
i^AA0.1 Q
1
RETURN
Fig.
12-18
A
GND
stroyed.
power supply
General SG2524 IC pulse width
practical switching
using the Silicon
modulator.
As was done
series with the load senses the load current and
converts this into a voltage. The modulator contains circuits that interpret the load current and
"decide" when the power supply is delivering too
This full wave power supply must seem like a
Rube Goldberg machine. In spite of its complexity, this
power supply can be
built far smaller
much
and
means
on 60 Hz transformers
transformer that can
handle 20 watts at 60 Hz can often handle over
1000 watts when the AC frequency is raised to 20
kHz. A transformer is used in this circuit to raise
the average voltage level.
lighter than designs based
and
series
regulators.
in the half-wave circuit, the out-
put voltage and current are sampled by means of
a resistor network. A low resistance resistor in
current.
The output voltage
is
sampled by
would not
of a resistive voltage divider. It
be practical to feed a 300 volt signal into a tiny integrated circuit, so the output voltage is scaled
down by the resistance divider so that, say 2-1/2
volts represents 300 volts.
A
J.
Isolated
Power Supplies and
the
Photo-
The full wave design also makes the transformer more efficient. By having the DC current
pulses pass through the primary windings in two
Isolator
directions relative to the transformer, the trans-
reference to ground or to other power sources.
former core is demagnetized on each half cycle.
Again, this allows less iron to be used in the
example
Isolated
power sources have no
electrical
An
power source would be a
flashlight suspended from the ceiling by a dry
cotton string. There is no way that electrical
energy from the flashlight battery can reach people or circuits in the room unless a deliberate effort is made to connect two wires to the two bat-
Any iron core becomes permanently
magnetized to some degree whenever a DC pulse
is passed through a winding around the core.
Although some iron cores do this less than
others, this problem is unavoidable. This characteristic of a residual, permanent magnetism in
transformers is another example of hysteresis.
transformer.
of an isolated
tery terminals. Notice that one wire will not be
enough. Both wires must be connected in order to
get power out of an isolated power source.
'JOS
DC
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209
With an isolated power system, the bathroom
power sockets are isolated from the rest of the
CONVENTIONAL
GROUNDED
HOUSEHOLD
house wiring with a transformer. The transformer
does not change the voltage, it just eliminates the
connection with ground. Now when the shaver
falls into the bathtub, the shaver may still be
shorted, but the current will have no interest in
traveling to the grounded drain and the bather
will not be seriously threatened.
WIRING
ISOLATION TRANSFORMER
BUILT INTO WALL OF
GROUND
BATHROOM.
REFERENCED
AC CURRENT FLOWS THROUGH
BATHER TO REACH GROUNDED
DRAIN.
12-20
Fig.
A
danger of a ground referenced
power supply.
So what? Suppose that you are bathing your
sweaty (salty) body in a bath tub. The tub has the
usual ceramic construction with a grounded
metal drain pipe at one end. Some careless person
has left an electric shaver on a shelf over the bath
tub. The shaver falls into the end of the tub opposite from the drain. The household AC power
line is ground referenced. This means that the
current will flow from the AC power line to any
grounded object whenever a pathway is available.
In the bathtub situation, the bather's body is the
lowest resistance path between the shaver and
the grounded metal drain. If enough of the AC
current passes through the vicinity of the heart
on its way to the drain, the bather will be electrocuted. A couple hundred microamperes of AC
through a healthy adult heart is all that is re-
NOW WHEN SHAVER
FALLS INTO TUB.
SHAVER MAY BE
SHORTED BY WATER.
BUT NO CURRENT
FLOWS TO GROUND.
Fig. 12-21
is
power system
in
which the
not referenced to ground.
rooms are equipped with
transformer to
make
AC
power system
for a
struments of all kinds, especially test equipment
and laboratory power supplies. In the laboratory,
isolated power supplies can be put in series to
make
higher voltage sources. Either polarity of
the supply output can be grounded without fear
of ruining the power supply or having unexpected
currents flow to ground.
called fibrillation.
This risk can be prevented by using an
lated
isolated
For safety reasons, power supplies sometimes have isolated outputs. This is common in
medical equipment and is found in quality in-
quired to disrupt the orderly beating of the heart
and kill the bather. This disruption of the rhyth-
mic beating of the heart
An
bathroom
AC
current
Some modern
an
isolation
As we have seen, the isolation transformer is
way to achieve this isolation. If the
a simple
iso-
power supply uses a transformer
is
bath-
voltage
to
change the
then the transformer can accomplish both purposes. Unfortunately, the full
wave power supply we just looked at not only has
power
this accident very unlikelv.
210
level,
conduction band creating holes and conduction
band electrons. The light falling on the base is
equivalent to base current turning the transistor
on. Sometimes the transistor base lead is brought
out separately. Grounding the base lead through
a base resistor helps the photo-transistor turn off
promptly for better high frequency response. Optical isolators usually look like standard 8 pin integrated circuits. However, when the isolator is
designed to tolerate thousands of volts across it,
it is usually built as a short, round or square rod
about as thick as a pencil and two or more centimeters long.
a transformer, it also has two feedback signals,
the output voltage and the output current. These
signals
must be communicated back
to
ground
re-
ferenced circuitry. If this feedback information
were returned to ground referenced circuits by or-
dinary wires, the isolation would be destroyed.
These wires would make a low impedance connection to one side of the output and both output terminals would now have a definite voltage with
respect to ground.
Since both signals are voltages and not curdo not need to pass much current to
rents, they
communicate this information. If high resistances were placed in these feedback lines, say 10
million ohms, these resistors could not pass
significant current and should not effect the isolation. The trouble with this idea is that if a large
In the
voltage were placed across these feedback resistors, the voltages seen by the modulator IC
would change. And besides, using one wire to
communicate a voltage level implies that there is
a ground or some other return path for a voltage
reference. To summarize, the feedback signal
would become confused with the common mode
voltage between the isolated output and ground.
Isolating these feedback
communicate voltage
ference
C
is
so that they
signals
levels with
no ground
re-
not a trivial problem.
PHOTOTRANSISTOR
LIGHT
Si
LIGHT EMITTING
DIODE
S
B
Fig. 12-22
K.
Optical isolator
A way to isolate electronic
is
to use
an optical
devices are usually
These electronic
made from
knocked out
of the valence
AC
Voltage Transformers
voltage regulating transformers: the ferroresonant transformer and the paraformer. Neither
of these two devices is easy to understand, but at
least try to get the general idea.
are built with
the base exposed so that light can alter the conductivity of the base semiconductor. Electrons
are
Constant
regulate the voltage, they limit the AC current.
They are not used verv often, so we will keep this
discussion brief. There are two major types of
the photo-transistor. Photo-transistors are usual-
They
application, the
are
AC
a light emitting
diode and a bipolar photo-transistor. These components are usually mounted at the ends of a
short plastic tube so that the photo-transistor can
"see" the LED. When a small current passes
through the LED, it lights and the light turns on
ly bipolar silicon transistors.
wave power supply
isolators
The only significant voltage regulation
techniques we have not covered are the constant
voltage transformers. These are highly
modified transformers which can vary the coupling between primary and secondary to hold the
secondary AC voltage constant. They not only
signals completely
isolator.
full
used to transfer analog
voltage and current information from the output
back to the ground referenced pulse width modulator. This implies that the photo-transistor
must be operating as a class A linear amplifier.
Even if the photo-transistor end of the light connection happens to be linear, we know that the
light emitting diode does not have a linear voltampere characteristic. So it is no surprise that
analog information will be distorted when it is
passed through the optical isolator. Additional
circuitry must be used to correct or calibrate the
distortion. If the analog information can be
transmitted by the light in the form of on-off
signals, such as pulse width modulation, then the
non-linearity will not be a problem. One way of
doing this is to use pulse width modulators which
turn the light on and off in pulses which have a
width proportional to the voltage level being
transmitted. An op-amp RC integrator can convert the pulse width modulation back to an
analog signal.
optical
band and into the
211
->• LOAD
CURRENT
REGULATED
AC VOLTAGE
»
>
UNREGULATED
AC
>
PRIMARY
VOLTS AC,
120
uzuj
u.
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O^<
u5o
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u.
SECONDARY
5
=
oc
a
WINDING
>
MAGNETIC
SHUNT ALLOWS
SOME FLUX NOT
THIS WINDING
GENERATES FLUX WHICH
APPOSES PRIMARY FLUX
TO LINK PRIMARY
WITH SECONDARY
CAPACITOR DRAWS
A LARGE CURRENT
Fig. 12-23
Ferrore sonant AC
voltage regulating transformer
Ferro-re sonant transformers are heavier than
conventional power transformers designed for the
same frequency, and it takes them two or more
sine
wave
voltage.
A
compensating winding uses output
current as negative feedback to turn down the
magnetic flux from the primary winding.
cycles to react to transients in line
When
any significant
works in the usual
way. Magnetic flux generated by the primary
winding travels around the iron core where it in-
voltage or load. Nevertheless, they provide excellent voltage regulation. Less than 2% varia-
operating
without
regulation, the transformer
from 100 to 130 volts input voltage is
Although much heavier, the AC output
from this transformer can be rectified and used as
a substitute for a high power, isolated, switching
power supply. The ferro-resonant power supply
design is far less complicated than an equivalent
switching design.
tion
typical.
duces a similar AC voltage in the secondary winding. The magnetic shunt in the middle of the
transformer core enables some of the flux from
either the primary or secondary to "take a short
cut" and avoid going through the opposite winding. This shunt has an air gap which produces
enough magnetic resistance (reluctance) to the
flux so that not all the flux will be shorted out by
the shunt. If the shunt were solid and had no air
gap. nearly all the flux from either winding would
take that path and it would be impossible to
transfer power from the primary to the second-
The general idea behind the ferro-resonant
transformer is that the magnetic coupling between the primary winding and the secondary
/finding is variable and is controlled by feedback
and by saturating the iron core. As more voltage
applied to the primary winding, the coupling
between the secondary is reduced so that the
secondary AC voltage will remain constant. The
ferro-resonant transformer in Fig. 1L'-'J3 uses two
different ways of varying the magnetic coupling.
A so-called resonant winding saturates the secondary side of the core and reduces the secondary
is
ary.
The
so-called resonant
winding on the second-
ary side has a capacitor across
it so that a large
current will flow in this winding. Usually this
winding
2
1
is
not resonant, but just draws a large
FLUX PATHS
INTERFERE AT
THE 4 CORNERS
WHERE THE CORES
JOIN
"C" CORES
MOUNTED 90°
2
NOTE THAT THERE
NO MUTUAL FLUX
LINKAGE BETWEEN
THE CORES
IS
EACH OTHER
SECONDARY
OSCILLATING
LC CIRCUIT
>
REGULATED
AC VOLTAGE
UNREGULATED
AC VOLTAGE
Fig.
12-24
former"
AC
current. This current generates a flux
>
Wanlass Electric Company
which
The paraformer voltage regulator invented
by the Wanlass Electric Company is really different. This is a compact "transformer "that
regulates its own voltage and limits its own current. Not only that, it acts like a "filter" and only
allows pure sine wave voltage to appear on the
paraformer secondary winding. The voltage
waveform on the primary winding can be a square
wave or a noisy sine wave but only pure sine
waves appear on the secondary winding. This
"transformer" makes RF filter capacitors or
varistors unnecessary because noise cant get
ap-
poses the primary flux. The high current in this
winding saturates or "uses up" the iron on the
secondary side so that the primary flux finds less
magnetic resistance by taking the path through
the magnetic shunt. The more current that is
drawn from the true secondary winding, the less
current that is available for the resonant winding
loop. This situation diverts more flux back into
the secondary winding.
The compensating winding is a second regulation component that reinforces the activity
of the resonant loop winding.
As
through
When
it
in either direction.
The paraformer is made from two thick, "C"
shaped iron cores. The cores are put together so
that the end of each arm of the "C" is spanning
the open end of the "C" of the other core. Primary
and secondary windings are put on each core, but
because of the weird core coupling, there is no
mutual inductance between the primary and secondary windingsl Instead, the energy is coupled
across from the primary to the secondary by a
phenomenon that is not found in other transformers. The secondary inductance changes in
response to the primary current. As current rises
in the primary, flux from the primary enters the
secondary side of the paraformer. This flux interferes with the flux from the secondary winding. Unlike a normal transformer, these flux flows
are always competing for the same iron. This
the primary
voltage rises, the secondary voltage tends to rise
too.
"Para-
voltage regulator
this happens, the current to the load
and this causes more current to flow
through the compensating winding. The compensating winding is oriented on the primary side of
the transformer so that it cancels out magnetic
flux from the primary. In other words, the compensating winding is a form of negative feedback
that turns off the primary if it starts to deliver
too much energy to the secondary. The compensating winding is also a current limiting circuit. If
the load tries to draw too much current, the compensating winding will partially cancel out the
flux from the primary and shut off some of the
coupling to the secondary winding until a balance
between flux and current is reached.
rises
213
QUESTIONS:
reduces the inductance of the secondary. Since
the energy stored in an inductor can not change
instantly, decreasing the inductance means that
any current flowing in the secondary must increase in order for the energy to remain constant.
This surge of current is used to sustain an LC
oscillation in the
secondary
1.
2.
The general idea is that the paraformer is a
wave oscillator with an output AC voltage
determined by the frequency of the oscillacapacitor is placed across the secondary
winding. The inductance of the secondary forms
an LC parallel resonant circuit with the capacitor. When the resonant frequency of the LC
circuit matches a dominant frequency in the
voltage on the primary, the secondary circuit
oscillates. As long as the secondary winding is
oscillating, the AC voltage across the secondary
remains essentially constant and this is the AC
that
tion.
many common
goals of power
A
zener diode regulator is an example of
of regulator design?
what kind
circuit.
3.
sine
List as
supply design as you can.
is
4.
A
Why
zener diode regulators in
power voltage
regulation? What role do they often play
in high power voltage regulators?
aren't
Fig.
12-2 used for high
Why
are series regulators
more
efficient
than parallel regulators?
5.
What
transistor amplifier configuration
most
like an op-amp voltage follower?
can these two circuits be used as
voltage regulators?
is
How
6.
voltage regulation.
When an LC circuit oscillates, the voltage
from the capacitor "charges*' the inductor with
current. Then on the next half cycle, the inductor
charges the capacitor with voltage. The amount
of energy stored in the L and C must be equal. In
any ordinary LC oscillation, the L is fixed. But in
this circuit, the frequency is fixed by the frequency of the voltage on the primary. The amount of
energy the capacitor can store is fixed for a cer-
7.
A transistorized series regulator can be
equipped with a current limiter circuit
by adding just two components. What
are these parts and what does each one
do?
is a 3-terminal regulator? Why are
separate 3-terminal regulators made for
What
positive
and negative voltage regula-
tion?
8.
tain maximum (peak) voltage across it. It turns
out that the peak voltage across the capacitor is
fixed by the frequency. The inductance adapts to
provide current to the load and to deliver the fixed amount of energy to the capacitor.
How
can a three-terminal voltage
regulator be used to build a current
source? If you needed to build a current
source to provide a large current,
how
would you decide what regulation voltage the 3-terminal device would have?
9.
What
is the relationship between power
supply size and weight versus energy ef-
ficiency?
10.
As more and more
current
is
drawn from the
secondary winding by the load, the voltage remains constant until the oscillation quits When
the oscillation quits, the secondary voltage
vanishes and this is how the paraformer limits its
own
current.
this. If
All
sine
wave
the load takes too
the
Zener diode?
1 1.
oscillators are like
much
current out
ot
1
the
slops. There are so
paraformers that they may
become very popular in the future. Hopefully
someone will invent a good way to explain them!
oscillator,
oscillation
many advantages
Why might an energy gap voltage
reference be used instead of an ordinary
to
2.
What
is
What are the advantages of a switching
power supply? What are some disadvantages? For example, what problem
would you expect to have if you used a
switching power supply in a radio
receiver
21
J
a varistor?
13.
In the voltage-reducing switching power
in Fig. 12-16, what is the purpose
supply
of the free-wheeling diode?
14.
15.
What
is
Why
are high frequencies like 20
a pulse width modulator?
kHz
usually used for switching power supplies?
16.
17.
What makes up the error signal in a
pulse width modulator for a switching
power supply?
In the pulse width modulator circuit in
Fig. 12-17, the error signal can never be
zero volts.
pen
18.
if it
Why
not?
What would
hap-
did?
What advantages does a full-wave supply like the one in Fig. 12-19 have over a
half-wave switching supply like the one
in Fig. 12-18?
19.
In
general,
used
what
are
photo-isolators
for?
20.
Why are power supplies in the laboratory often isolated from ground?
21.
What
is
a ferro-resonant transformer?
How does it limit the current
that can be
drawn from the transformer secondary?
22.
Why is it misleading to refer to a
paraformer as a "transformer?" What
advantage(s) does a paraformer have
that a ferro-resonant transformer does
not have?
215
GLOSSARY
This glossary of terms
meaning
reference to the
with which you
tionaries,
AM
analog circuit: A circuit that deals with a continuous range of voltages or currents. In contrast, digital or binary circuits deal with nonlinear, full on or full off circuits which are never
part way on except while switching.
may
A
An array of a large number of
operational amplifiers which can be wired to
analog computer:
linear amplifier biased like a
B
amplifier that detects and amplifies
radio signals. Since only one polarity of
waveform
the
ready
words
differ
active detector:
class
some
not be familiar. These definifrom those of standard dicbut are more in line with shop usage.
may
tions
a
of the
give
to
is
of
is
amplified, the output signal
simulate algebra or calculus equations. This
linear calculation methods. It
can not calculate using binary arithmetic or
computer uses
is
rectified.
digital logic
active
filter:
A
frequency
filter
made by
analog switch: An integrated circuit that can
turn analog signals on and off at very high
speeds under the control of voltage pulses.
Other than the switching action, the switch
does not attenuate or distort the signal being
porating an operational amplifier into a filter
network so that desired frequencies are amplified while unwanted frequencies are attenuated.
AF:
Audio frequency, 20 Hz
methods.
incor-
to about 20,000
controlled.
Hz.
AND
AGC: Automatic
gain control
alloy transistors:
A
circuit:
A logic or digital circuit that gives
a high output signal in response to a certain
number of simultaneous high input signals.
bipolar transistor
made by
An electrode or element of an electronic
device which normally has a positive voltage
anode:
diffusing dots of impurity into both sides of a
semiconductor wafer to make the three layers,
on
P-N-P or N-P-N.
it.
An amplifier with an
output equal to the anti-logarithm of the input.
When used with logarithmic amplifiers, they
are used to multiply, divide, take square roots
and perform other mathematical operations on
In bipolar transistors, alpha is a number that represents the fraction of the emitter
current that is the collector current, a = lute-
anti-logarithmic amplifier:
alpha, a:
in other words, the emitter current less the
base current equals the collector current.
voltages.
alpha numeric display: Numbers or letters displayed by means of LED lights, liquid crystals,
or gas discharge tubes. These are used in
calculators, watches, gasoline pumps, etc.
AM: Amplitude
amplifier:
voltage
signal
A
argon:
is
Amplifier
implies
that
it
cannot
astable multivibrator:
coupled
is
A
flip-flop or multivibra-
remain long in either of its
quasistable points, but switches back and
forth in a square wave oscillation.
tor that does not
in-
amplitude modulation: Impressing a relatively
low frequency signal onto a radio frequency
sine wave by varying the RF sine wave
amplitude so it follows the low frequency
signal.
transformer
which the LC tuning
the
increased. For example, a trans-
former cannot amplify, because
crease the power.
A
in
accomplished on the input side of the amplifier
instead of the output side.
circuit that enlarges a current or
power
but ionizable, gas that is used
and voltage regulator tubes.
inert,
Armstrong oscillator:
sine wave oscillator
modulation
signal.
An
in thyratrons
The basic unit of matter. Each kind of
atom has a positively charged nucleus surrounded by a specific number of negatively
atom:
217
charged electrons.
audio amplifier (AF AMP): An amplifier designed to amplify frequencies between 20 and
20.000 Hz. Usually AF amplifiers are wide
band amplifiers and amplify most or all of the
frequencies in this band equally well.
beat frequency is the intermediate frequency
which is the difference between the local
oscillator frequency and the radio signal fre-
quency.
beat frequency oscillator (BFO):
used
multiplication:
breakdown
A mechanism
for
of
be decoded more
thermally generated electron can
atom so violently that it
ionizes. This produces more holes and electrons
strike
in radio receivers to
a
a
beta,
crystal
which ionizes more crystal atoms
until the
An
oscillator
modulate morse code
signals with a musical tone so that they
P-X junction voltage barriers in
transistors and diodes. Because of the high
voltage,
difference in frequency be-
tween two radio or audio signals being mixed
together. In a superhetrodyne receiver, the
automatic gain control (AGC): A circuit in a
radio receiver or other instrument which tries
to hold a signal at a relatively constant level.
avalanche
The
beat frequency:
A number
ft:
may
easily.
which
is
the current gain of a
transistor. Current gain is the collector current
divided by the base current.
P-N
barrier collapses.
bias:
avionics:
The
backward
bias:
A DC
of its
In diodes, a voltage
is
is
is on the
on the anode, and
binary numbers:
numbers.
current cannot flow through the diode.
balanced
A
operational
amplifier:
An
bipolar:
amplifier
A
are represented by the two different
on and off.
filter
while attenuating
all
Having two pathways. In bipolar transmeans a 3 layer transistor. X-P-X or
pathways are used.
which is designed to
pass a certain band or range of frequencies
A
system based on two
In digital circuits these
P-N-P, which conducts current through both
type X and type P semiconductor using both
holes and electrons. In other words, two
volts.
bandwidth:
0.
istors, it
from an operational amplifier which has
zero volts on the output when the input is zero
filter:
A number
and
tunnel rectifier
built
bandpass
1
numbers
states,
backward diode:
make
placed
across a diode so that the positive side
cathode, the negative side
voltage placed on an electronic deit operate in some desirable part
volt-ampere characteristic.
vice to
electronics used in aircraft.
circuits:
A circuit which will remain
turned on or turned off for indefinitely long
other frequencies.
bistable
certain range or region of frequen-
periods. Also, a tiro-state circuit.
cies that a device or filter is able to process.
This implies that frequencies outside the band
width will not be amplified or passed.
bar graph voltmeter:
blanking pulse:
m
the tran-
bridge
in a
A graph
ol
sets
beam
plot:
A graph of circuit output versus frequency. Bode plots are used to describe the frequency response of filters, amplifiers, and
other devices that must respond to some input.
sistor
base characteristic:
sus base voltage
TV
Bode
In bipolar transistors, the control element
and central semiconductor layer
pulse generated in
to turn off the electron
when the beam is returning to the beginning of
a new scanning line or a new picture frame.
This eliminates the unwanted retrace line.
A
voltmeter made from a
column of lights. Each light is controlled by a
separate comparator circuit. As the voltage being measured rises, each comparator threshold
is exceeded in turn and the lights turn on one
by one indicating the level of the voltage.
base:
A
and oscilloscopes
rectifier:
Four
diamond-shaped
wave
base current ver-
for a bipolar transistor.
rectification
secondary center tap.
218
diodes arranged
produces full
without a transformer
rectifier
circuit that
cadmium sulfide: Semiconductor material used
to make photo-resistors. When light shines on
these resistors, the resistance drops dramat-
They are commonly used in automatic
door openers and camera light meters.
The element
class
of an electronic device at
a
which electrons enter the device. In normal
operation the cathode has a negative voltage
while the anode has a positive voltage. In
vacuum tubes, the cathode is heated by a fila-
ment
An
amplifier:
D
A
amplifier:
switch
transistor amplifier used as
generating pulses,
for
The
relays, light, etcs.
on or
full
when
to cause electrons to leave the cathode
class
and enter the vacuum.
E
transistor
but not
full off,
in
A vacuum
amplifier:
used
operated
between except
A
very
efficient,
switch and
sharply tuned
The
transistor
is
turned full on or full
off. Resonant circuitry converts the current
pulses to RF sine wave AC.
tube
which accelerates electrons from a cathode onto a phosphorescent screen for displaying pictures, maps, or graphs.
Picture tube.
controlling
is
switching.
transistor or tube amplifier.
cathode ray tube:
sharply tuned
efficient,
transistor or tube amplifier biased so that the
transistor is turned on for only a small fraction
of the total sine wave cycle time.
ically.
cathode:
C
class
like a
is
A diode or transistor circuit designed to conduct current when an input signal
voltage exceeds or drops below some specified
clipper circuit:
In field effect transistors, the current
channel:
that
being controlled passes through a
piece of semiconductor called the
is
uniform
level. The effect is to "clip off" the voltage
waveform above or below that level. Also call-
channel.
An
chopper:
electronic switch that turns a
ed a
DC
signal on and off rapidly to make an AC signal
with an amplitude proportional to the DC
level. They are used in DC-to-DC inverters and
in chopper stabilized DC amplifiers.
clamp
circuit:
A
circuit
CMOS: Complementary
ductor
field
effect
Metal Oxide Semicon-
transistor integrated
cir-
A
very energy efficient digital circuit
design used in digital watches, calculators, etc.
cuits.
resembling a clipper
Largest of the three semiconductor
collector:
that prevents a voltage from exceeding a limit.
layers of a bipolar transistor.
used to protect some devices,
such as a power transistor that cannot tolerate
voltage above a certain limit.
Clamps
slicer or limiter circuit.
are often
The
collector
gathers the majority carrier current from the
emitter after it has passed through the base
layer.
A
class
An
amplifier:
untuned, linear transistor
tube
is
point in the center of a linear portion of
erating range.
band
some
amplifiers
are
bipolar transistor
op-
turned
wide
C tuned
it
is
supposed to be
breakdown voltage (BV ce
The collector-to-emitter voltage required to
breakdown a bipolar transistor and make it
pass current when it is supposed to be turned
A tuned amplifier which has
wide band linearity of a class A
some
of the efficiency of class
B
):
off.
amplifiers. It is often used as an in-
A sine wave oscillator which
used a pi network consisting of two capacitors
and one inductor as the feedback phase shift
amplifier stage in transmitters
not practical to retune the amplifier
every time the frequency is shifted slightly.
termediate
where
when
off.
collector-to-emitter
amplifier:
of the
amplifier and
or
A
small leakage cur-
rent that flows from collector to emitter in a
amplifiers.
AB
class
Class
its
A
collector cut-off current:
which the transistor or
biased to have its quiescent operating
or tube amplifier in
Colpitts oscillator:
it is
network.
class
B
A
amplifier:
amplifier which
linear transistor or tube
common
biased with its quiescent
Usually clasc B amplifiers are made from pairs of transistors so that
each transistor amplifies one polarity of the
is
point just at cut
AC
tor
off.
base amplifier:
amplifier
A
basic bipolar transis-
configuration
which has high
voltage gain but no current gain. That
is,
the
output current essentially equals the input current.
signal.
219
common
A
amplifier:
collector
basic
conduction angle: When thyristors are used to
control AC power, the thyristor can be made to
turn on only during certain angles of the sine
wave voltage waveform. The on-time angle is
bipolar
with high
current gain and no voltage gain. The input
voltage and output voltage are essentially the
same. Also called an emitter follower amplifier.
transistor
common
amplifier configuration
A
drain amplifier:
basic
FET
called the conduction angle.
conductor:
amplifier
configuration with high current gain but no
voltage gain. Also called a source follower
A
gate amplifier:
basic field effect tran-
which has high
sistor amplifier configuration
rejection ratio:
amplifier's
differential
to reject
of a
com-
of flip-flops
a divider.
crystal:
A
between
voltage:
number
by the common
amplification gain.
common mode
counts pulses of current
can be wired in series so that each flip-flop triggers the next in the series. The state of each
flip-flop, on or off, records the number of input
pulses counted in binary numbers. Also called
voltages. It equals the gain of the
differential amplifier divided
mode
A measure
ability
circuit that
or voltage. For example, a
base or grounded grid amplifiers.
common mode
A
counter:
voltage gain but no current gain. Analogous to
mon mode
a good electrical
is
constant voltage transformer: A ferro-resonant
power transformer holds the AC voltage on its
secondary winding(s) constant. The "paraformer" is a transformer-like device which also
regulates the AC voltage on the secondary.
emitter amplifier: A basic bipolar transistor amplifier configuration with high current gain and high voltage gain.
common
common
material that
voltage.
amplifier.
common
A
conductor allows electrons to travel through it.
from atom to atom, with little application of
Voltage that
is
common
piece of quartz crystal
two
electrodes.
mounted
Electrically,
bethis
device resembles a high Q. LC filter network.
In "crystal sets," the crystal is a piece of semi-
to both inputs of a differential amplifier.
conductor used to make a crude diode.
common
source amplifier:
A
field effect transis-
tor amplifier with high current gain
and high
crystal oscillator:
comparator: A circuit used to compare one voltage with another. When the voltage connected
to the positive input is greater, the output
voltage goes high. When the voltage connected
to the negative input is greater, the output
goes low.
MOSFET
complementary
MOS
(CMOS):
A
effect
integrated
circuit
made from both
P-channel
and
field
FKTs. These
sine
wave
oscillator
which
digital
circuits
Obsolete radio receiver constructed
around a diode detector that is built from a
semiconductor crystal such as galena and a
metal contact whisker. Crystal sets were usually powered by the radio signal itself which was
rectified and passed to a pair of sensitive ear-
crystal set:
phones.
type of
that
is
A very accurate current
regulator built from a junction FET and a re-
current regulator diode:
N-channel
use very
tittle
These devices hold the current passing
through them constant over a wide range of
sistor.
power.
complementary transistor
amplifier:
A
voltage.
transis-
amplifier built from matched
and P-N-P transistors. This amplifier
can be operated class M without signal inverting transformers ^m\ is relatively inexpentor
A
uses a quartz crystal as part of the phase shift
feedback path.
voltage gain.
push-pull
VI'A
current
sensing resistor:
A
low resistance in
which is used to
sense or measure the current by monitoring the
voltage drop across the resistance.
series with a large current
sive
220
current source:
deliver the
A
circuit
same current
Theoretically
load.
or
device
that will
a perfect
A
DIAC:
into any resistance
current source
AC
5 layer
for triggering
power control device used
It makes a pulse of cur-
TRIACs.
rent whenever a threshold voltage
would force the constant amount of current
through an infinite resistance— a practical im-
in either direction. It is the
P-N diodes
is
exceeded
equivalent of 2 P-N-
in inverse parallel.
possibility.
An
differential amplifier:
The attenuation or suppression of a
wave oscillation by electrical resistance in
damping:
sine
fies the difference
amplifier that ampli-
between two voltages.
an oscillator or amplifier.
Some
differential outputs:
differential amplifiers
are equipped with
two outputs, each of which
represent the difference between the two input
signals, but the outputs have opposite signs.
Darlington transistor: A way of wiring two or
more transistors together so that the resulting
circuit will act like a single, super high gain
transistor.
differentiator:
DC
amplifier:
An
amplifier that can amplify
electronic circuit
which pro-
proportional to
is
how
fast a current or voltage is changing. This con-
and slowly changing input voltages.
cept
is
These
DC-to-DC inverter: A circuit used for converting
a DC voltage level to another, higher or lower
DC
DC
An
duces a voltage which
DC
voltage. It can also be used to produce a
basic to the mathematics of calculus.
circuits are basically high pass filters.
A
diffusion transistor:
transistor which
is
made
by diffusing impurities into a semiconductor
wafer in layers. The diffusion is controlled by
masks. These are also called planar transistors.
voltage of opposite polarity.
plates: Electrodes in some cathode
ray tubes which steer the electron beam and
control its point of impact on the phosphor
deflection
digital
A
circuit:
with
screen.
full
circuit
on and
full
works
entirely
bistable
circuits.
that
off
Digital circuits never use continuous ranges of
deflection yoke:
An assembly
of four coils
mounted around the neck of
ray tubes. The yoke steers the
is
and controls
phor screen.
its
which
voltages or currents as in analog circuits.
larger cathode
electron
beam
computer: A programmable calculating
circuit that performs all operations using
binary numbers in bistable circuits.
digital
point of impact on the phos-
When thyristors are used to control
power, the thyristor can be made to delay
turning on for a variable amount of time during each half cycle of the sine wave half cycle.
This delay is measured in degrees of angle.
delay angle:
diode:
AC
MOSFET: A MOS
dipping the
C
detector:
A
circuit
used
for
is
ter-
to
The procedure
maximum
of tuning a class
efficiency using an
measure the average
DC
collector
current.
direct coupled amplifier:
A
transistor amplifier
connected to other amplifier stages by
resistors or wires so that DC current can pass
from one stage to the next. The significance is
that these amplifiers can amplify DC voltage
which
zero.
demodulating the
final:
amplifier for
ammeter
field effect
on when the gate to source voltage
two electrodes or
minals.
transistor that uses both depletion and
enhancement mechanisms to turn the transistor on and off. These transistors are turned
half
electronic device that allows electric
electronic device with
demodulation: Extraction of information which
has been encoded onto an RF radio signal.
depletion type
An
current to flow in only one direction. More
generally, "diode" can refer to any non-linear
in-
formation impressed on radio signals. Detector
is a general word and can mean a detector for
AM, FM, PWM, or other kinds of radio modulation. The word can also mean frequency
conversion in a superhetrodyne receiver.
is
or current levels.
distortion:
Changes
in a signal that
ferent from the original signal.
221
make
it dif-
A
divider circuit:
from a series of
the
number
counter
flip-flops,
circuit, usually
energy gap voltage standard: An integrated circuit which serves as a precision voltage
reference and resembles a zener diode in the
made
each of which divides
of pulses into
it
by two.
way
The addition
doping:
atoms
of
used.
it is
of impurity into
One of the two terminals at the ends of
the current channel in a field effect transistor.
The drain is the end of the channel at which the
drain:
majority carriers leave the channel.
A
epitaxial transistors:
sistors in
The percent
cycle:
field effect
uses only the enhancement
mode to induce majority current carriers into
the channel. Whenever there is no gate voltage,
the transistor is turned off.
that
transistor
holes or electrons into the crystal structure.
duty
MOSFET: MOS
enhancement-type
a semiconductor crystal in order to introduce
process for making tran-
which gases containing
silicon
and
impurities are exposed to a wafer of silicon so
of time that a circuit is
that layers of semiconductor are
turned on.
grown on the
surface of the silicon wafer.
characteristics: The response
an
amplifier
of
to fast AC signals. These characteristics do not include the DC biasing,
dymanic amplifier
leakage,
maximum DC
difference in voltage between
should
what a voltage
be and what it actually
is. Often the error voltage just represents this
current or other static
parameters.
difference.
electric deflection:
Use
of voltage
waveforms on
deflection electrodes to steer the electron
in a
The
error voltage:
A
family of curves:
beam
acteristics
for
group of volt-ampere char-
a
how
device that shows
cathode ray tube.
trolled
The negatively charged atomic
that makes up electrical currents.
electron:
A
by some variable, such as base current,
grid voltage, etc.
particle
stream of electrons passing
through a vacuum tube.
"Beam"
other
or
the device can be con-
ferro-resonant transformer:
electron beam:
tube,
transistor,
A
sophisticated
transformer that regulates the voltage on
own secondary winding(s).
implies that
AC
its
the electrons are focused into a tight ray or
FET:
stream.
An assembly of anodes and a cathcathode ray tube that accelerates a
beam of electrons toward the phosphor screen.
Electron guns are also found in TV camera
tubes and X-ray tubes.
fibrillation:
in a
field effect transistor,
istor
made from
FET:
A
monopolar trans-
a single piece of semiconduc-
The current passing through
channel can be turned on and off by an
electric field generated by a control gate
located on or around the channel.
tor,
The smaller
two outer layers of
a bipolar transistor which is built from 3 layers
of N and P semiconductor. The layer where ma-
emitter:
field effect transistor.
The disruption of the orderly beating of the heart by an electric shock.
electron gun:
ode
See
of the
the channel.
this
jority carriers enter the transistor.
filament:
emitter follower amplifier:
A
vacuum
basic bipolar transistor amplifier configuration with high current
A
resistance heater in a light bulb or
it heats the cathode
tube. In tubes,
which releases electrons into the vacuum.
gain and unity voltage gain. Also called a com-
mon
collector amplifier.
final amplifier:
The
last amplifier in a string of
amplifiers in series.
emitter resistor by-pass capacitor: A capacitor
placed across an emitter resistor so that the
biasing effect of the emitter resistor will not be
affected by (he short term changes of the
signal being amplified.
The output
transmitter, stereo, or other
amplifier in a
power generating
circuit.
AC
Another name for a mixer
superhetrodyne receiver.
first detector:
in a
2-2 -2
circuit
A
flip-flop:
multivibrator.
A
gain-bandwidth product: A measure of amplifier
frequency response. In amplifiers, the product
of the frequency times the gain at that frequency tends to be constant.
pair of transistors
or other amplifiers wired with positive feed-
back so that when one transistor turns on, the
other is turned off. These bistable circuits can
be used as square wave oscillators, memory
elements, and counters.
FM:
gallium arsenide: A semiconductor material used
in light emitting diodes, Gunn diodes, and tun-
Frequency modulation
nel diodes.
forward bias: In diodes, a voltage placed across
a diode so that the more positive side is on the
anode and the current is free to flow through
the diode. In transistors, a voltage on the base
or gate that turns the transistor on.
forward
offset
amount
voltage:
of voltage that
used
germanium:
times used
A
getter:
zero.
of
A
free wheeling diode:
control terminal of a silicon controlled
rectifier or field effect transistor.
In P-N diodes, the
must be applied across
would be
semiconductor material
in light emitting diodes.
The
gate:
a diode in the forward direction in order for the
diode to begin to conduct current. In ideal
diodes, this offset
A
gallium phosphide:
A
semiconductor
to
make
material
some-
transistors and diodes.
silvery deposit of metal on the inside
vacuum
tubes.
This
metal
reacts
with
residual traces of air and helps to preserve the
diode used in a voltage
reducing switching power supply to allow the
inductor current to flow continuously into the
vacuum.
grid:
load.
A
frequency converter: A circuit that generates a
signal of a certain frequency in response to the
presence of another frequency signal. In superhetrodyne receivers, the radio signal is "converted" to a common "intermediate frequency"
signal with the original modulation transferred to
the new, intermediate frequency.
vacuum tube is a meshplaced between the cathode and
control grid in a
like structure
the anode. Small voltages on the grid can control the stream of electrons passing from the
cathode to the anode.
grown transistors: A transistor which is made
by slowly pulling a crystal out of a molten
semiconductor while impurities are added to
the melt to provide the proper layers.
frequency modulation: Impressing a low frequency signal onto a radio frequency sine wave
by varying the RF sine wave frequency back
and forth so that the frequency change follows
the low frequency signal.
wave
diode: A semiconductor device with a
negative resistance characteristic that occurs
at high voltages. Gunn diodes can be used to
generate high power microwave signals.
Gunn
A
bridge or double rectifier
circuit that passes current to the load during
both positive and negative halves of the
voltage cycle.
full
rectifier:
function generator:
A
half-wave
current
AC
waveforms
of various
types. Sine waves, pulses, square waves, and
triangle
A
rectifier circuit in
allowed to flow only during
1
which
2 of the
cycle.
Hartley oscillator: A sine wave oscillator which
uses a pi network consisting of two inductors
and one capacitor in the feedback phase shift
network.
test instrument that gen-
erates voltage or current
rectifier:
is
waveforms are typical "functions"
generated.
A large mass of metal or other maclamped to an electronic device, such as a
power transistor, to keep the temperature of
heat sink:
gain:
Amplification.
The number
of times that
voltage, power, or current are increased
terial
by an
the device constant.
amplifier.
223
A
is a poor conductor of
have no free conduction
band electrons or valence band holes. These
energy bands are so far apart in insulators,
that holes and electrons are not easily created.
Hf e The current gain of a bipolar transistor.
One of the four "h parameters" used to make a
insulator:
:
material that
electricity. Insulators
simple model of a transistor for calculating
values for transistor circuits.
In audio amplifiers, this means
frequency response is linear over the entire
audio spectrum.
high fidelity:
A
missing electron
of an atom.
A
in the
on
A circuit that sums a current or voltage over time so that the final level reached
represents the total quantity of current or the
total effective time that voltage was applied to
the circuit. This concept is basic to the mathematics of calculus. Integrators are basically
low pass filters.
valence energy
by a
makes it possible
from atom to atom.
hole can be filled
intermediate
Schottky diode. A rectifying
diode formed by depositing a metal anode on
an N-type or pure semiconductor. The electrons moving across the Schottky junction
move faster than in P-N diodes and are said to
be "hotter."
hot carrier diode:
built
integrator:
traveling electron and this
for electrons to travel
even a computer
a single wafer of silicon semiconductor.
cies.
band
entire circuit such as an
amplifier, flip-flop, or
high pass filter: A frequency filter which passes
high frequencies and attenuates low frequen-
holes:
An
integrated circuit:
A
dyne radio
frequency
(IF):
RF
a superhetro-
In
is converted
intermediate frequency so that it
may be amplified many times without tuning
each amplifier stage for each frequency receiv-
to a
receiver, the
signal
common
ed.
impedance: The resistance and reactance inside a circuit element. This impedance
limits the amount of current that can flow into
or out of the circuit element.
internal
hysteresis:
short of
The property of lagging or falling
some expected level in the response of
circuit. In Schmitt triggers the lag is a
voltage threshold that must be overcome be-
a
semiconductor:
intrinsic
fore the circuit will respond. In transformers,
the iron core becomes partly permanently
magnetized whenever the transformer is energized. Before the flux can change to the opposite direction, the permanent magnetism
must be overcome.
Intermediate frequency as
in
semiconductor
A circuit that reverses the polarity or
sense of a signal. In digital electronics, an inverter converts a high voltage "one" to a low
voltage "zero," or vice versa. In analog circuits
in inverting a signal means dividing the signal
inverter:
into one;
IF:
Pure
crystal with no impurity added.
e.g.,
4 volts
verter can also
superhetrodyne
mean
become 1 4 volt. An ina power supply circuit
DC voltage to AC
DC voltage level.
that converts
receivers.
some other
An unwanted radio signal that can appear in an intermediate frequency amplifier
strip, if the incoming signal is not tuned selectively enough before mixing.
voltage or to
image:
inverting
amplifier
input:
Positive
voltages
applied to the inverting input of a differential
amplifier will cause the output voltage to go
down
The electrical entrance or port at which a
signal to be processed enters a circuit.
input:
input impedance:
The
input of a circuit.
isolated
a
voltage
A power
output
supply which
with no ground
reference.
impedance of the
This impedance is the load on
internal
In amplifiers, the degree to which the
output is kept separate from the input so there
can be no unintentional feedback. In power
systems, isolation means removing any reference to ground from a voltage source.
isolation:
the circuit delivering current or voltage to the
input. It there is no inductance or capacitance,
input impedance
power supply:
supplies
may
be called input resist-
ance
224
A
transformer:
isolation
moves the ground
from an
AC
transformer that
An RF oscillator used to generate a signal which can be beat against (combined with or mixed with) an incoming radio
local oscillator:
re-
or other voltage reference
voltage.
signal so that the radio signal can be converted
JFET:
Junction
to the intermediate frequency in a superhetro-
field effect transistor
dyne
Josephson junction devices: A thyristor-like device which operates at very low temperatures.
They switch super fast and efficiently and are
expected to become important in computers in
receiver.
An amplifier with an output voltage equal to the logarithm of the input
voltage. Logarithmic amplifiers are usually
made from one or more operational amplifiers
logarithmic amplifier:
the future.
and a P-N diode provides the logarithmic charjunction capacitance:
store
an electric
parable to capacitors. This fact
vantage
acteristic.
P-N junctions are able to
charge in a manner comwhich are voltage
in varactors
A
logic circuit:
used to ad-
is
only
circuit that
a specified combination of input
signals are present. Logic circuits are used to
vari-
able capacitors.
recognize events or circumstances and take appropriate action.
junction field effect transistor (JFET):
effect transistor
A
field
which uses a back biased P-N
low pass filter: A circuit that is designed to let
low frequency signals and DC pass through
unimpeded while high frequency signals are attenuated or eliminated. In power supplies, a
filter that removes AC ripple and noise from
junction formed against the channel semicon-
ductor as a control gate.
A small unwanted current that
passes through a circuit element such as a
diode, transistor, transformer, or other compo-
leakage current:
DC
voltage.
The use
beam
magnetic deflection:
nent.
to steer the electron
LED:
A
LED:
limiter:
mon
for a
clamp or clipper
pere
.
An
adjective
characteristic
cir-
meaning that the volt-amof
a
device
as
plots
pedance of the
a
straight line on a graph. In math, an equation
is linear if it contains the independent variable
memory
main
power only and plotsas a straight
could not contain X 3 vX. etc.
in the first
line. It
Any
age or current from some other circuit
resistive,
capacitive
The load
is
is
on or
A
bistable circuit that can
full off indefinitely.
Since
it
re-
can
tion.
circuit that receives volt-
to be a load impedance.
circuit:
full
load.
be set in either state at will, one bistable circuit
can be used to record one binary bit of informa-
,
load impedance:
The current carrier most comdoped semiconductor. Holes are the
in a
matched impedance: When a voltage source is
delivering power to a load, the most power (but
not necessarily the most energy) will be
transferred to the load when the impedance inside the voltage source is matched to the im-
cuit.
linear:
larger picture
majority carrier in a P-type semiconductor.
Electrons are the majority carrier in a N-type
semiconductor.
colored alpha-numeric displays.
Another name
in
majority carrier:
P-N junction diode
which gives off light when current passes
through it in the forward direction. LEDs are
usually made from gallium arsenide or gallium
phosphide. They are used as colored lights and
make
of deflection coils
tubes.
Light emitting diode
light emitting diode,
to
produces an output
when
said
mercury vapor:
usually
An
ionizable gas used in thyra-
ton tubes.
but can also have an inductive or
component
as
is
commonly seen
microwave: Extremely high frequency radio
waves. Any radio wave above roughly 1000
megahertz.
in
antennas, loudspeakers, motors, or other loads
driven by amplifiers or power supplies.
225
mon
in a particular
A
neon bulb:
carrier least comdoped semiconductor. Elec-
The current
minority carrier:
two
light bulb consisting of
elec-
trodes in a glass envelope containing a small
amount of neon gas. In electronics, neon bulbs
are the minority carrier in a P-type
semiconductor. Holes are the minority carrier
in a N-type semiconductor.
trons
are used as pilot lights and as voltage triggered current pulse generators to turn on
thyristors.
mixer:
A
combining the incoming
circuit for
RF
with the local oscillator signal in a
superhetrodyne receiver. The purpose is to
generate an intermediate frequency signal for
signal
NMOS: A
Having one pathway.
In field effect
non-inverting amplifier:
transistors, the current being controlled passes
vibrator.
multivibrator:
A
fixed length
A
one shot
non-inverting
An
amplifier in which
when
amplifier
Positive
input:
volt-
in a positive direction.
pulse.
MOSFET:
for digital circuits.
ages applied to this input of a differential
amplifier will cause the output voltage to go up
of a
response to a short trigger
in
ex-
the input current or voltage increases.
multi-
makes long pulses
circuit that
made
MOS field effect tran-
the output current or voltage increases
through type-N or type-P semiconductor, but
not both, i.e.. one pathway.
monostable
Used primarily
sistors.
amplification.
monopolar:
type of integrated circuit
clusively from N-channel
Metal
semiconductor
field
field effect transistor
which
oxide
effect transistor.
A
non-linear:
multiplier circuit:
number times
A
circuit that multiplies
Analog
another.
An
adjective
meaning that a device
or mathematical function has a characteristic
uses a gate built like a capacitor with a layer of
silicon oxide (glass) insulator between the
metal gate and the channel.
that does not plot in a straight line
when
plot-
ted on a graph.
one
notch filter: A frequency filter that attenuates
one frequency while letting all others above
multipliers
multiply voltages or currents. Digital multimultiply binary numbers using arith-
and below
it
pass.
pliers
metic methods.
A
multivibrator:
flip-flop.
A
N-P-N
pair of transistors
or other amplifiers wired with feedback so that
when one
transistor turns on, the other
is
transistor:
bipolar
transistor
made
-
ductor in the pattern "'N-P-N.' The collector
biased positive in N-P-N transistors.
turn-
ed off. These bistable circuits are used as
square wave oscillators, counters, and as
memory elements.
is
N-type semiconductor: Pure semiconductor
which has had impurity atoms added to in-
N-channel FET: A field effect transistor with
the current carrying channel made out of an
N-type semiconductor. The drain is biased
positive in N-channel FETs.
troduce electrons into the crystalline structure.
N-type impurities have valences of +5.
one-shot
negative feedback:
A
from 3 layers of N-type and P-type semicon-
Using a signal from the out-
multivibrator:
A
circuit
that
makes
long pulses of fixed length in response to a
short trigger pulse. Also called a monostable
put of an amplifier to cancel or attenuate the
input signal so that the output signal is
multivibrator.
decreased.
OP- AMP:
negative resistance: \ property of tunnel diodes,
(iunn diodes, and certain other tube and transistor circuits
amplifier^
which
is
useful in oscillators and
operational adder: A circuit used for adding two
voltages together made with operational
\^
voltage increases, the current
th
Ugh the resistance decreases instead of inceasing as n normal resistors
.
Operational amplifier
amplifiers.
j
226
A complex transistor amattempts to achieve infinite
voltage gain, zero output impedance, infinite
input impedance and other attributes of a
perfect amplifier. Op-amps are used in much
the same way as individual transistors and can
be used in practically any circuit that does not
involve high RF frequencies.
A circuit designed to capture and
hold the highest voltage of a signal waveform
reached during a time interval. It could also be
a circuit that switches on at the occurrence of a
peak detector:
operational amplifier:
which
plifier
peak voltage, but does not actually save the
peak voltages.
A vacuum
pentode:
tube with five electrodes.
These are usually arranged concentrically from
circuit:
A logic (digital) circuit that gives a
high output signal in response to a high input
signal on any one of a number of input lines.
control grid, screen grid, suppressor gird, and
A circuit that generates an AC signal
with no outside pattern to amplify. In contrast, an amplifier can generate an AC signal
only by amplifying a signal provided by some
"perfect" diodes, or other circuit elements: A
fictional circuit element with no imperfections.
A way to explain what the circuit element is
OR
the center in the following order: the cathode,
the plate or anode.
oscillator:
supposed to do without getting bogged down
other circuit.
in explaining all of its short-comings.
An
oscilloscope:
which
instrument
displays
phase shift oscillator: A sine wave oscillator
which is usually built from a class A inverting
linear amplifier which drives a 180° phase shift
network. The phase shifted signal is fed back
voltage waveforms on a screen. The display is
usually a graph of voltage or current versus
time.
output:
The
to the amplifier input as positive feedback.
electrical exit or delivery port at
which a signal that has been processed is sent
on to the load or next stage of the circuit.
photo-conductor: A semiconductor device that
changes its conductivity or resistance in re-
output impedance: The internal impedance of
the output of a circuit or voltage source. This
impedance is in series with the load impedance
sponse to
photo-isolator:
and power is best transferred to the load when
load impedance is matched to output impedance. If there is no inductance or capacitance,
this may be called output resistance.
A
paraformer:
AC
transformer-like device that
voltage by
secondary winding.
gulates
means
parallel voltage regulator:
light.
A
device
made from
which
a
photo-transistor
is
re-
photo-resistor:
of an oscillating
A
semiconductor device usually
made from cadmium sulfide that
resistance when light falls on it.
A
a light bulb
used to
transmit information by light beam from one
voltage level to another with no electrical connection between the two circuits.
and
decreases
its
voltage regulator
system which uses a dynamic resistance, such
as a zener diode, in parallel with the load and a
fixed resistor in series with the load. The two
resistances form a variable voltage divider to
A bipolar transistor with the
base exposed to light so that light can create
current carriers in the semiconductor and turn
photo-transistor:
the transistor on.
hold load voltage constant.
Pierce oscillator:
pass transistor: The dynamic resistance in a
series voltage regulator that varies its
resistance to hold the load voltage constant.
A
sine
wave
oscillator
which
uses a quartz crystal as part of the feedback
phase shift network.
P-channel FET: A field effect transistor with
the current carrying channel made out of
P-type semiconductor. The drain is biased
negative in P-channel FETs.
piezo-electric effect:
Trapped ions
in a rigid cry-
produce a voltage across the crystal when
it is bent. This phenomenon is used in quartz
crystal frequency filters.
stal
227
PIN
A diode designed
A name used
vacuum tube.
plate:
for
PMOS: A
is
made
for the positive
anode
two transistors
PWM:
in a
A
A packet of energy of light or other
electromagnetic energy released during the
change of energy state of an electron.
quiescent point: The operating point of a transistor or other device when it is not processing a
signal. The resting voltage and current of a
device in a particular circuit.
four
semiconductor
layer
radio frequency amplifier
switch used to trigger thyristors. This device
turns full on and stays on whenever a certain
voltage threshold is exceeded.
The
point contact transistor:
now
obsolete.
ductor, the base,
A
An
ampli-
in this range.
coupled amplifier: An AC amplifier in which
each amplifier stage is separated from preceeding and following stages by a high-pass
RC
form of
earliest
(RF AMP):
designed to amplify signals with frequencies between about 20 kHz and thousands of
megahertz. Usually RF amplifiers are tuned
and amplify a narrow band or single frequency
fier
P-N-P transistor: A bipolar transistor made
from three layers of N-type and P-type
semiconductor in the pattern "P-N-P." The collector is biased negative in P-N-P transistors.
transistor,
See pulse width modulator.
quantum:
conducts.
diode:
tran-
pyrometer: A temperature measuring instrument consisting of a thermocouple and a sensitive galvanometer.
P-N junction: A boundary in a diode or transistor where P-type semiconductor is joined to
N-type semiconductor. The P-N junction is a
diode and only conducts in one direction:
P-N-P-N
made from
which each
cycles.
effect transistors.
P
amplifier
or tubes in
on during alternate half sine wave
sistor turns
type of integrated digital circuit which
exclusively from P-channel MOS field
positive to
An
push-pull amplifiers:
use as a voltage
RF amplitude
in AGC circuits and other applications. It has
three construction layers: P-type. mtrinsic,
and TV-type semiconductors.
diode:
variable resistor for controlling
RC
piece of semicon-
was contacted by two metal
filter.
Converting
rectification:
electrodes to form emitter and collector junc-
rent pulses
by means
AC
current to
DC
cur-
of diodes.
tions.
rectifier:
Using a signal from the out-
positive feedback:
put of an amplifier to reinforce the input signal
so that the output signal is increased.
precision diode:
actly zero volts
can
be
made with
operational amplifiers.
pre-regulator:
some
A
An
amplifier used in mic-
rowave receivers which is installed at one end
of a dead-end microwave waveguide pipe.
Radio signals transmitted down the waveguide
are "reflected" off the amplifier and made
diode which rectifies at exand closely resembles an ideal
diodes
diode or circuit acting like a diode
converting AC to DC.
for
reflectance amplifier:
A
Precision
diode.
A
and used
stronger in the progess.
current source circuit used in
A
regenerative detector:
series regulator circuits to generate the
fier
self-oscillating
detector that detects
AM
ampli-
radio signals and
turn-on current for the pass transistor.
amplifies them.
semiconductor: A pure semiconductor
which has has impurity added to introduce
coupling, called regenerative coupling, the
P-type
By
holes into the crystalline structure. P-type imhave a valence of +3.
relaxation oscillator:
A
cir-
begins to oscillate and modulates the
received signal with an audible whistle. This is
useful for receiving morse code signals.
cuit
purities
pulse width modulator:
increasing the feedback
An
oscillator
made from
a
produces
capacitor and an electronic switch such as a
a pulse train with a duty cycle proportional to
a voltage level, i.e., the higher the voltage, the
P-N-P-N diode. The switch turns on and shorts
out the capacitor every time the voltage across
longer the pulses.
the capacitor reaches
circuit that
228
some threshold
voltage.
A
reset trigger:
pulse or input to a bistable
SCR:
cir-
such as a flip-flop that resets the output
voltage back to zero. If the output was already
zero, the reset pulse will have no effect.
See silicon controlled
rectifier
cuit
AM
second detector: Another name for the
detector in a superhetrodyne receiver. The mixer
is called
The return of a scanning beam in a cathode ray tube to the beginning of a new scanning line or a new picture frame. If the electron
beam is not blanked out, the retrace will produce an unwanted "retrace line" on the screen.
retrace:
RF:
Radio frequency
RFC
or
RF
A
choke:
radio frequency choke.
semiconductor:
way
be-
sulator or conductor.
An
A heavily doped semiconductor maused to make a temperature measuring
device. Unlike thermistors, these devices increase their resistance with increasing temper-
inductor which presents a high impedance to
DC current with little
sensistor:
terial
resistance.
An unwanted AC
signal riding on the top
voltage from a voltage supply. In full
wave circuits, the ripple is usually 120 Hz. In
half-wave rectifier circuits, ripple is 60 Hz.
of a
Materials that are half
tween good conductors and good insulators.
Semiconductors resemble insulators in that
there are no holes or free electrons available for
conduction. But the two energy bands are very
close together so small amounts of energy can
easily convert a semiconductor to either in-
radio signals but passes
ripple:
the first detector.
ature.
DC
The conditions
area (SOA):
safe operating
i
series voltage regulator: Voltage regulator
of
current and voltage that are within the maximum power dissipating capability of a tran-
set trigger:
such as a
sistor or other device.
sample and hold
circuit:
A
command
that sets the output voltage
The semiconductor element most widely
silicon:
level is usual-
used
in transistors
and integrated
circuits.
small capacitor until needed.
(SCR): A thyristor
power.
This is a half
used
wave device that conducts current in only one
direction. It has a gate or trigger lead which
turns it on.
silicon
In transistors, the condition of hav-
more magnetic
saturation voltage
rectifier
AC
The rate at which an analog circuit,
such as an amplifier, can change its output
slew rate:
flux.
V ce sa t): The
controlled
for controlling
ing the transistor turned on as much as possible so that additional base current will not
cause more collector current to flow. In transformers, the iron core is saturated when additional primary current will not induce the core
to generate
flip-flop,
ger will have no effect.
circuit that takes a
from a control pulse. The voltage
saturation:
A pulse or input to a bistable circuit,
high. If the output is already high, the set trig-
voltage reading or voltage sample on
ly stored in a
which
uses a transistor or other dynamic resistance
in series with the load to hold the voltage
across the load constant.
voltage or current.
lowest possible
(
collector-to-emitter voltage of a bipolar transistor
when
fully
slicer:
Schmitt trigger: A comparator with positive
feedback to give the circuit hysteresis. This
gives the comparator different voltage thresholds for rising and falling voltages and results
in good immunity from high frequency noise.
Schottky diodes: A diode made
metal anode onto a piece of
semiconductor. These diodes
capacitance and low forward
Also called hot carrier diodes.
A
clipper circuit that "clips off" a voltage
waveform when the input voltage exceeds
falls below some threshold.
turned on.
SOA: Safe operating
or
area of a transistor or other
device.
A P-N junction silicon
cell, silicon type:
diode designed for generating electricity from
solar
sunlight.
by depositing a
N-type or pure
have very low
Circuits having only transsemiconductor diodes, and other devices
solid state circuits:
istors,
offset voltage.
which are not vacuum tubes.
229
switching speed: The rate at which an electronic
switch can turn full on or full off. Switching
speed is often defined as the time needed to go
from less than I0 c"c on to 90 c "c on. or vice versa.
In field effect transistors, one of the two
ends of the current carrying channel. The
source is the end where the majority carriers
source:
enter the transistor.
source follower amplifier: A basic FET amplifier
configuration with high current gain and unity
voltage gain. That is, the output voltage
equals the input voltage. Also called a
drain amplifier.
speed-up capacitor:
A
the center: the cathode, control grid, screen
common
grid,
faster.
A
and the anode or
plate.
thermal runaway: Transistor gain increases with
temperature and rising temperature can make
a transistor turn more on. As the transistor
turns more on, it heats itself and turns on still
more. This process can cause the transistor to
'run away" and turn full on.
transistor amplifier
stages that makes the following stage switch
stabistor:
tube with four electrodes,
usually arranged concentrically starting from
capacitor across a cou-
pling resistor between two
A vacuum
tetrode:
voltage reference diode which is
or more silicon P-N junction
made from two
diodes
series.
The voltage across the
must exceed the combined forward
in
stabistor
offset voltages of the diodes before
it
A
thermistor:
made from
will con-
temperature
measuring
a piece of semiconductor.
device
The
re-
sistivity of thermistors decreases dramatically
duct.
as the temperature increases.
static amplifier characteristics:
The behavior
an amplifier over long periods as
DC or slowly changing signals.
it
of
A
thermocouple:
amplifies
made by
temperature measuring device
joining two dissimilar metals and
measuring the tiny voltage that appears across
the junction.
ode similar to a
A
P-N junction silicon divaractor which is used for fre-
step recovery diodes:
terminal voltage
grated circuit voltage
three
quency multiplication.
substrate:
The semiconductor wafer
three external
that inte-
leads
regulator:
regulator
An
inte-
which has
and usually physically
resembles a transistor.
grated circuits are built on.
An AC power control tube that resembles a triode vacuum tube except that a
superhetrodyne: Most common radio receiver
design which converts the incoming radio
signal to an intermediate frequency. Most
amplification takes place at the intermediate
frequency so that each amplifier stage doesn't
need to be retuned every time a new station is
thyratrons:
thyratron contains small amounts of ionizable
mercury vapor or argon. In function, thyratrons resemble silicon controlled
gas, usually
rectifiers.
selected.
thyristors:
sweep circuit: A circuit that generates triangleshaped waveforms for steering electron beams
across the screen of cathode ray tubes.
switch:
Any
device, whether mechanical or elec-
tronic, that turns electrical current
on and
A
semiconductor
AC
power control
Thyristors resemble transistors, but
they are bistable and once turned on, they cannot be turned off until the current which is being controlled returns to zero. Types of thyris-
device.
tors are
off.
SCRs, TRIACS. P-N-P-N diodes, and
DIACS
switching power supply: A high efficiency power
supply that changes voltage levels and
regulates load voltage. The "switchers use inductors or transformers to change the voltage
level. Feedback controls the energy content ot
the current pulses passing through the inductors by means ot pulse width modulators
A curve or an equation
which explains how voltage or current, which
transfer characteristic:
is
applied to the input of an amplifier or other
circuit is related to the output. In bipolar transistors,
it is
a graph of base current versus col-
lector current.
230
A
transistor:
solid state control device in
which
The number of electrons lost or gained
by an atom during a chemical reaction.
valence:
a small current (or small voltage in field effect
transistors) can control a large current. Except
for the non-linear UJT, transistors can control
current in either linear or non-linear applica-
valve:
tions.
varactor:
British
word
for
vacuum
tube.
A P-N junction silicon diode designed
use as a voltage variable capacitor. When
diodes are back biased, their capacitance decreases with increasing back bias voltage.
for
TRF
A
receiver:
radio
Tuned Radio Frequency
receiver
stages of
RF
receiver.
which has several tuned
amplification prior to detection
and conversion to the audio signal or sound.
The disadvantage of this design is that each
varistor
RF stage must be retuned separately to tune in
another station on another frequency.
A
TRIAC:
thyristor used for controlling
video amplifier:
for amplifying
AC
SCR
wide band amplifier designed
TV
picture signals.
of the high gain
drives the negative input terminal to the same
voltage as the positive input. The impedance
A
between the two inputs is nearly infinite, but
the two inputs act as though they were connected by a "short circuit."
or a flip-flop.
three electrodes. Starting from the center these
electrodes are arranged concentrically.
are a heated cathode, the control grid,
VMOS:
Oxide Semiconductor
These are power transistors which have extra power dissipation
capability due to vertical current flow down
through the silicon wafer on which they are
They
and the
tuned amplifier: An amplifier designed to amplify one frequency and attenuate all others.
characteristic. It
oscillators
may
Metal
built.
A high current gain, unity
voltage gain amplifier made with an opera-
voltage follower:
tunnel diode: Heavily doped P-N junction diode
with a negative resistance region in its volt-
ampere
Vertical
field effect transistors.
plate or anode.
tional amplifier.
be used to make
and amplifiers.
voltage regulator tube:
tunnel
Because
of an operational amplifier, negative feedback
The basic amplifying vacuum tube with
triode:
RF
A
virtual short circuit:
voltage or current pulse used
to change the state of a bistable device such as
an
semiconductor device used
AC voltage. Also
called metal oxide varistor.
power. This is a full wave, non-linear device
that can conduct current in both directions. It
has a gate or trigger terminal that turns it on.
trigger pulse:
A
(MOV):
for clipping noise spikes off
rectifier:
A
form of tunnel diode used to
AC signals. The forward conduction voltage is zero volts. They are also called "backward diodes" because what is normally the forward offset voltage region,
to = 0.6
tube-like de-
an evacuated tube containing a small amount
When the gas is conducting,
the voltage across the tube remains constant.
rectify very small
volts, is
A vacuum
vice used for regulating voltage. It consists of
of ionizable gas.
used as the back bias region.
voltage source:
A
circuit that is
supposed to
de-
constant voltage across any resistance
load. Theoretically, a perfect voltage source
would hold a constant voltage across zero reliver a
unijunction transistor (UJT): A switching transistor that resembles a JFET in construction
and
symbol. It is used
and pulse generators.
circuit
oscillators
unipolar transistors:
vacuum
tube:
An
sistance, a practical impossibility.
in relaxation
volt-ampere characteristic: A graph of the voltage across a device versus the current through
the device showing its performance.
Field effect transistors.
electronic control device in
which current flow across a vacuum
ed by voltages on electrodes.
is
wave guide: A metal pipe used
wave radio signals.
controll-
231
to conduct micro-
An inert, but ionizable, gas used in
high intensity white lights, strobe lights, and
Xenon:
camera
flashes.
When a P-N diode is reverse
biased with an increasing voltage, eventually a
negative voltage, the zener voltage, is reached
zener breakdown:
at
which the diode abruptly yields and allows
If
the cur-
restricted,
this is
current to flow at the zener voltage.
rent through the diode
reversible
zener
diode:
is
and does not destroy the diode.
A
voltage
reference
diode
made
from a back biased P-N junction diode. The
diode is specially doped to provide a calibrated
zener breakdown voltage.
zero crossing detector:
tect
when
a voltage
A
circuit that
can de-
waveform passes through
A comparator with one input connected to zero volts can be used this way.
zero volts.
•>;v>
Answers
SECTION
1.
The atom has
to
Study Questions
5.
I
a dense, positively charged
nucleus which is surrounded by shells, or
layers of negatively charged electrons. For
every positively charged proton in the nucleus, there is a negatively charged electron
orbiting the nucleus. These electrons are
held in orbit around the nucleus by the attraction between the positive and negative
charges. The chemical and physical properties of
atoms depend on the numbers
The
essential difference between insulators
and semiconductors is that the insulators
have a huge energy difference between the
valence band and the conduction band. In a
pure insulator the conduction band has no
electrons for conduction and electrons can
not readily rise from the filled valence band
up to the conduction band. In a semiconductor it takes very little energy to raise electrons from the valence band up to the conduction band. This produces holes and elec-
trons so that conduction can begin.
of elec-
trons present in orbit around the nucleus.
One would expect
Conduction band electrons and holes can be
induced into a semiconductor by the application of heat or light energy. Another way is
weight, silver colored, electrically conductive metal. It isn't used to make things in
matrix. These impurities have valences of 5
or 3 so that when they combine with the
6.
2.
lithium to be a metal
because it has one electron orbiting outside
the outermost filled shell. Lithium is a light-
the everyday world because
ly
with water and
is
it
to
7.
8.
that the conduc-
heated a few
slightly
Semiconductors are so useful because they
can change their conductivity dramatically
with small changes in energy input. Amorphous carbon behaves like a metallic conductor. All of the conductivity in
band electrons have more energy. In
order for a valence band electron to become
a conduction band electron, the valence
band electron must gain energy. However, it
tion
warm semiconductor
may overlap.
is
the resistivity increases
fore.
electron shell around another atom. Another
difference between valence band and con-
true that in a
an ordinary metal
move faster and collide more often. When a
semiconductor is heated equally, the resistance drops dramatically because electrons
are forced out of the valence band and put
into the conduction band. This produces opportunity for conduction in both bands
where there had been little conduction be-
Electrons in the conduction band are free to
move around throughout the mass of material. In the valence band, electrons are locked
into tight orbits around their atoms. They
can move from atom to atom only by moving over into an opening or hole in another
two energy
crystal
because the conduction band becomes more
congested as the conduction band electrons
it could either give away 4 electrons or accept 4 electrons.
is
When
degrees,
it were a typical semiconductor, one
would expect it to have a valence of 4 so that
is
the
tron or a hole, respectively.
bet you thought of krypton as a green,
glowing rock (its not). The outer shell of
krypton is filled. Therefore, krypton is a
chemically inert gas. Krypton is not a semiconductor, and one would not expect it to be
I'll
duction band electrons
into
of 4, there will either be one too
one. If
4.
impurities
semiconductor atoms which have a valence
many or one
few
electrons
in
crystal
matrix.
too
the
This
will produce either a conduction band elec-
reacts violent-
about as strong as but-
ter.
3.
introduce
amorphous
carbon occurs in the conduction band.
Heating the carbon does not bring extra
conduction electrons up from the valence
band. Therefore, subtle application of
energy cannot convert amorphous carbon
these
levels
233
an insulator to a conductor
thereby turn electricity on and off.
from
9.
In
order
for
electrons
to
flow
and
4.
from the
cathode to the viewing screen of a picture
tube, there must be a positively charged accelerating anode to attract electrons
Characteristics of the semiconductor diode
which are not
electrons are free to leave the cathode. Final-
the cathode must be heated so that the
conduction band electrons can be driven
A
forward resistance
B.
A
forward offset voltage
C.
Zener breakdown
D.
Reverse leakage current
E.
Change
ly,
right off into the
10.
The
resistance
When
between the cathode and
the cathode
is
5.
positively charged with
respect to the anode there
is
with
tem-
F.
Other discrepancies
the next section.
A.
120
B.
60 ohms
C.
0.006 watts of power
A.
30 watts of power
B.
1.6
C.
0.033
will
be discussed in
ohms
an absence of
conduction electrons on the cathode. Even
though the cathode is heated, there is no accelerating electrode with a positive charge
to attract them away from the cathode. As a
result, no current flows between the cathode
6.
and anode. What we are describing is a vactube diode which only conducts in one
7.
uum
ohm
left
at 0.5
ohm
Jones was
the
direction.
amperes
at 30
fired
amperes
because the two diodes on
side of the diode bridge are shorted
two halves of the transformer secondary. This means that during
the half cycle of voltage when the diodes
conduct, they will appear as "short circuits''
directly across the
SECTION
1.
characteristics
in
perature
vacuum.
anode of an electron tube decreases as the
cathode is made hotter and hotter. This is
because more and more electrons are kicked
off the cathode as it becomes heated.
1 1
"perfect diode"' are:
A.
away
from the negatively charged cathode. The
picture tube must be evacuated so that the
like the
II
Increased temperature generates electron-
They
draw huge
hole pairs which increase conductivity. This
across the transformer.
decreases the forward resistance and increases the backward leakage as well. Since
the forward resistance at a given current
currents during this half cycle because there
is only the small forward resistance to at-
will
tenuate the currents. If hundreds of amperes try to flow through diodes that are
only rated for the heat from a few amperes,
level is equal to the
forward offset voltage
divided by the current, a lower resistance
must mean that the offset voltage is lower
too. Manufacturers' data sheets confirm
the diodes
will
literally
burn up and be
destroyed.
this.
8.
2.
It is
possible that
when
tion 18 are destroyed, they will be, in effect,
proportional to the cross-sectional area of
the conductor. Since large diodes have more
material, they also have a larger cross-
cut out of the circuit. This would convert the
circuit to a conventional full wave rectifier
using a transformer center tap as shown in
sectional area. Another
Fig. 2-14. Provided the voltage from the
transformer is correct for this circuit, it
should work fine. It is possible, but very
is
that
more material
way
offers
to look at this
is
more hole-electron
pairs for current to travel across the diode.
unlikely, that the wire
A.
the diodes in ques-
Conductivity through any conductive solid
Both the relatively constant forward offset
voltage and the zener voltage provide a constant voltage over a wide range of currents
passing through the diode.
between the
trans-
former center tap could have burned out
before the diodes burned out. That would
have converted the circuit to a conventional
diode bridge rectifier like Fig. 2-15.
2;;
i
Jones is right. It will not burn up because
the upper and lower pairs of diodes conduct
in opposite directions. There is no way lor
current to flow out of the secondary winding. On the other hand, it won't work either
because both sides of the pseudo "bridge'
just
direct positive
current
to
the
14.
10.
Jones
load.
is
TV
AC
that
its
signal has to be small
enough so
maximum
negative peak voltage is
less than the positive half of the switching
square wave. Otherwise, when the two signals are added together during the "on"
time period, the AC signal negative peaks
would dip so low that they would be clipped
off by the diode. This would distort the
no way for negative current to get
to the load. Saying it another way, there is
no way for positive current to travel from
one side of the secondary, through the load,
and back to the opposite side of the secondary to complete the circuit.
There
The
AC waveform
by the diode because, when it
waveform. The
ON
is
not rectified
is
added
to the
sum
of the
part of the square wave, the
two waveforms
always positive with
is
re-
spect to the diode cathode.
commercial detector.
VOUT
15.
A.
An LC
tenuates,
V -
B.
tuned circuit shorts out, atall but the desired radio signal.
The detector
the signal. This
rectifies
produces a series of
DC
pulses occuring
at the radio frequency.
C.
An RC
filter
smooths the
quency) pulses into a
varies
up and down
DC
RF
(radio fre-
voltage which
at the audio frequen-
cy.
D. The
11.
to
12.
V|N
headphones
TV
AUDIO
VOLTAGE
REGULAR
PROGRAM
r
^
>-*H
BATTERY
VOUT
O
ELECTRONIC
SWITCH RESETS
CAPACITOR TO
ZERO AFTER
-\
COMMERCIAL
>
>
235
DC
the
Vqut
filter
current
sound energy.
COMMERCIAL
V|N
13.
discharge
capacitor and convert the
COMMERCIAL
EXCEEDS
V THRESHOLD
in-
16.
AM
An
SECTION
detector and diode frequency con-
III
verter are similar because:
1.
They both
A.
rectify
an
AM
modulated
trons from passing through the diode until
the electrons acquire enough energy (volt-
radio signal.
The outputs from both
B.
passed to
filters
age) to climb over the barrier.
are
rectifiers
fre-
AM
barrier
shown in a series with a "perfect" diode. The
diode is back biased by the battery, so current could not flow even if the battery were
quency.
An
The
contains no energy. In the equivalent
circuit shown in Fig. 3-1, the "battery " is
itself
which eliminate the
radio signal and save the modulation
The "battery" in a normal diode is the forward offset voltage barrier. It prevents elec-
detector and diode frequency con-
real.
verter as used in a superhetrodyne differ
because:
A.
2.
The modulation extracted by the
AM
forward offset voltage, 0.6 volts. This imseven P-N junctions in-
placed on the signal by the
broadcast station, not the receiver itself.
In the frequency converter the modula-
detector
is
tion is applied to the radio signal
mixing the radio signal with a
Since the stabistor diode is made from a
stack of several P-N silicon diodes, we can
assume that 4.2 volts is some multiple of the
plies that there are
side this stabistor.
by
3.
locally
Stabistors, high voltage rectifiers, and the
panel of silicon solar cells (Fig.
generated radio sine wave.
of silicon diodes in
silicon solar cells
and light emitting
sist of a large
B.
In
the
which
is
while in
AM
series.
modulation
extracted is an audio frequency
the frequency converter it is a
detector,
the
4.
The purpose
ward conduction produce
in the
silicon solar cells, the
of the frequency converter
superhetrodyne is to provide easy
tuning for a receiver with several high
local oscillator is
In
on the P-N junction raises electrons
from the valence band on the P side up to
the conduction band on the N side. These
electrons are trapped there by the forward
offset voltage barrier and therefore are
available for use in an external circuit.
ting the particular radio signal.
The
visible light.
energy of visible light
falling
AM
Q, sharply tuned, amplifiers. The
detector does not participate in selec-
17.
Both
diodes are an application of the forward offset voltage barrier. In the case of LEDs,
electrons falling off this barrier during for-
radio frequency.
C.
3-4) all con-
number
needed to beat with
the incoming radio signal to produce a constant difference frequency which can be de-
5.
tected by the "mixer" and amplified by a
string of high gain amplifiers. The receiver
is
In order to have a reverse (zener) breakdown
voltage in excess of 200 volts, several silicon
diodes must be connected in series. The forward offset voltage is multiplied by the
tuned by shifting the frequency of the
number
local oscillator. Since the difference frequen-
cy that can be amplified by the IF amplifiers
is constant, the radio frequency that can be
received is shifted along with the local
oscillator frequency. Some antenna tuning is
needed to exclude signals that have the
same difference frequency with respect to
the local oscillator hut are above the local
oscillator frequency instead ot below it. This
tuning is not critical and some receivers just
use tixed band pass tillers that do not have
to he changed tor each station.
6.
of
P-N junctions present.
The zener breakdown voltage in a zener
diode is controlled by the amount of impurities added to the semiconductor. The
higher the impurity concentration, the lower
the zener breakdown voltage.
7.
236
The voltage regulator
will use a 5 volt zener
diode rated at greater than 2.27 watts, say 3
watts. The dropping resistor should be
about 9 ohms.
8.
When
reverse biased, varactor diodes store
charge in a
way comparable
The reverse voltage
13.
barrier prevents con-
14.
When
varactors are used in tuned
they are back biased with a DC
voltage through a very high resistance.
circuits,
device that
is
discussed
nonlinear could be used
in
this
section
SECTION IV
could
theoretically be used for this purpose.
Varactors are especially good for this
because they have low forward resistance
and the capacitance stores energy over most
1.
10.
PIN
tion.
tor;
diodes are
named
resistance.
quite high.
for their construc2.
N + semiconducused for voltage variable
resistors and as high voltage rectifiers.
11.
12.
a kind of
By
using the voltage across a
itself,
the
In amplification the original signal is not increased in size. The original signal is used to
a large stream of current to produce a new,
larger signal
3.
Zener diodes and tunnel diodes are both
heavily doped so that the zener breakdown
voltage is below what it would be if the
diode were optimum for conventional ap-
5.
down
is
controlled by a
control a tube or transistor which modulates
are
Schottky barrier diodes are made from a
metal anode bonded directly to a semiconductor cathode. The P-N junction results
from metal atoms diffusing into the semiconductor forming a very small P region.
These diodes have very low capacitance,
high switching speed and a very low forward
offset conduction voltage.
plications.
is
output can be a large voltage controlled by a
very small voltage.
conductor, and a layer of
They
vacuum tube
load resistor or across the device
They have three layers of semiconduca layer of P-h a layer of intrinsic semi-
tor.
transistor or
small voltage or current. Using these devices, a very small voltage or current can
control a large current passing through the
Energy is dissipated in the forward resistance for only a small fraction of
is
A
variable resistor which
of the cycle.
the cycle so the efficiency
a transition
Tunnel diodes and Gunn diodes both have
as a frequency multiplier. Therefore every
device
for rectifying very
They have
negative resistance over a part of their forward conduction volt-ampere characteristic.
Tunnel diodes use very little energy and extremely low power supply voltage. Their
principle drawback is that the output signal
is very small, hundredths of a volt. Gunn
diode oscillators can produce high power
microwave pulses, but large power supplies
and cooling equipment needed. The decision
would probably be based on how much power was needed.
and electrons in a P-N
diode are very limited. As more and more
voltage is applied, the amount of charge
that can be stored, the capacitance, de-
Any
signals.
diodes.
capacitor, the holes
9.
used
rectifiers are
AC
between conduction and non-conduction at
zero volts. They are used with the direction
of the P-N junction reversed from normal
duction, just as the dielectric prevents conduction in a capacitor. Charge is stored
because electrons are pushed into holes in
the P side of the junction and electrons are
removed from the N side. Unlike the conduction band electrons in the metal plates of a
creases.
Tunnel
small
to capacitors.
The
which resembles the
3 kinds of gain are voltage gain, current
gain,
and power gain. The power gain can be
large while the current gain
if
4.
original.
the voltage gain
When
is
is
an amplifier amplifies
the amplifier
may
output must be
less
than one
very large.
its
own
output,
However, the
partly in phase with
oscillate.
at least
the input.
A
good switching transistor dissipates very
power when it is turned on or turned
little
because the current is large when the
voltage is small and vice versa. However,
while the transistor is actually switching
from on to off, the current and voltage are
off
Tunnel diodes have a zener breakbut their S-shaped for-
of zero volts,
both high simultaneously and the power disThe longer the time in-
ward conduction characteristic includes a
segment of negative resistance which allows
them to be used as oscillators and reflec-
sipation will be high.
tance amplifiers.
will
terval required to switch, the
237
more heat that
be dissipated during the interval.
6.
When amplifying an analog signal like
music, the amplifier output is a waveform
which has a continuously changing amplitude. Since the transistor is rarely turned
the
way on
or
sipating power
music signal
7.
is
the
all
way
the entire
off,
time
all
it
dis-
as
that
the
sible.
For each control voltage (or
control current) the vacuum tube pentode
(or transistor) outputs behave like current
sources because the output current flowing
through the devices are independent of the
power supply voltage and load over a wide
operating range. This is an advantage because it makes the amplifier performance insensitive to changes in power supply voltage
current from the emitter as pos-
much
Also, the emitter usually has
more doping than the collector. This results
in a low reverse breakdown voltage between
the base and emitter.
present.
12.
Ie
or the resistance of the load in
series with
much
is
it
and
of the collector
emitter are quite different. The collector-tobase junction is usually large so that it can
dissipate large amounts of heat and collect
A current source is a device that delivers a
constant current regardless of the voltage
across
The physical design
11.
=
Ic
+
lb-
=
<*
Ir
I,
P
=
lb
it.
or load resistance.
Divide the
h = h + Ib
Ic
Ic
one at 100
MHz
should be two, because
MHz, by
the current gain
(50
(2)
MHz) =
ft
equation through by
Ic
.
lb
Ic
ft
^-
is
+
1
Ic
a
^-1
a
=
l
ft
The current gain
definition. At 50
=
first
=
1
-
—
=
a
a
—
1
a
a
a
a
a
100
MHz.
13.
An
ideal
resistance
would
transistor
when turned
full
have
zero
Since the
on.
resistance would be zero, the voltage across
9.
The voltage across the load
resistor will be
it
180° out of phase with the voltage across
is
sum
of these two
must be small
large. Since the same
the transistor. Because the
voltages
Even though
(V ce would be zero.
)
would
the col-
no power dissipation
lector current is large,
result.
constant, one
whenever the other
is
current passes through both the load and
the transistor collector-to-emitter, these cur-
14.
The voltage
across the transistor will be out of phase
rents are obviously in phase.
ther increases in collector current.
with the current through the transistor because the collector to emitter resistance is
changing while the load resistance
is
A saturated transistor is one that is turned
on as much as possible. That is, further increases in base current will not produce fur-
(nor-
mally) fixed.
15.
The
that
safe operating area of a transistor
is
area of the collector-to-emitter volt-
ampere characteristic which can be used
10.
Transistor gain results from a very thin
base region which can be bridged by minority carriers when the proper control current
is on the base. A "transistor" made by wiring two P-N junction diodes together would
have no gain because there is no way to
"turn on" the reversed collector to base
without exceeding the
power dissipation
16.
An ohm meter
maximum
continuous
for the transistor.
can be used to check whether
the two P-N junctions in a bipolar transistor
are intact. This test does not tell you
junction. This junction can be bypassed by
wiring the collector to the base, but this is
anything about the transistor gain, switching speed, breakdown voltages and a host
not very useful.
of other characteristics.
17.
The best way
to test a transistor
to install
is
9.
which you want
it
in the particular circuit in
it
to operate, then test the circuit.
sine
wave that
SECTION V
2.
is superior to reattenuating AC current because a perfect switch does not dissipate
any energy and therefore runs cool.
for
Thyristors
sistors
better
are
than
bipolar
tran-
AC
power because
not damaged by reverse bias and
for
controlling
they are
they are either fully turned on or fully turned off. There is no situation where a thyristor remains half on and dissipates large
amounts
3.
10.
An SCR
5.
the resistance
The
is
it
relaxation oscillator triggering circuits
to build
share the
current
is
sudden pulse
is
11.
it is
DC
inverter
are
commonly used
voltage to
proportional to temperature, there
thryristor
will
is
a
AC
to
DC
converter.
They
to convert a low
1.
DC
DC
DC
makes the
SCR
in a
DC
circuit, the
2.
to cathode current will eventually turn itself
This is necessary because the SCR cannot be turned off by the gate once it has
been triggered.
off.
trigger circuit which fires the
by means
A TRIAC can be triggered by positive and
negative gate pulses that are applied to the
same lead. With two separate SCR's wired
in inverse parallel, the gate pulses for each
must be electrically isolated from each other
and therefore must be generated separately.
FET
gates
control
the
of
an anode gate
polarity
instead
of
a
a
is
is
of
current
virtually zero.
transistor, the
at least the forward
drop,
minimum
P-N junction
no matter how
collector-to-emitter current
may
small
the
be.
positive
because "positive to P conducts" and so does "negative to N."
An SCR
With a bipolar
voltage
voltage
SCR
requires
flow
Since there are no functioning P-N junctions
on the output side of a FET, there is no fixed
minimum voltage drop across a P-N junction. The output voltage from drain to
source can approach zero if the current is
low.
polarity. This is
8.
relaxation
firing angle
through the output side of the transistor,
just as bases do. In bipolar transistors the
control variable is current. In an FET the
control variable is voltage and the current
must be designed so that the anode
negative
The
fire.
SECTION VI
voltage.
In order to use an
An SCR
what angle
at exactly
that flows into the gate
7.
on.
it
up slowly and, because the leakage
is
oscillator therefore
on. It has only
An
circuit
the
to
independent of temperature.
voltage from a battery up to a high
voltage. Sometimes they also connect
6.
of current
sure to turn
circuits allow the gate current
no way to predict
the
not possible to turn it
two states, full on
and full off. Also, once the SCR is turned on,
the gate loses control and cannot turn the
anode-to-cathode current off. With a characteristic like this, the SCR cannot follow a
randomly varying analog signal.
way
TRIAC
can't be used as a Hi-Fi or analog
amplifier because
part
if
When
thyristor gate which
property of negative resistance. It is this
property that makes thyristors suitable for
building relaxation oscillators.
4.
wave,
Other control
all
terminals, the
triggered before the peak of the sine
deliver a
of heat.
Tunnel diodes and thyristors
is being triggered by a
phase with the sine wave
going to be triggered at all.
is very low, the TRIAC
will conduct for nearly the complete 360 cycle. When the resistance is very high, the
TRIAC will not trigger at all so the conduction angle will be 0°. At intermediate
resistances the TRIAC conduction angle for
both positive and negative half cycles will
range between 90 ° and nearly 180 °, if it conducts at all.
High speed switching
sistance
is in
TRIAC
across the
must be
1.
TRIAC
Since the
with a large gate current has a
3.
volt-
ampere characteristic that resembles a P-N
JFET
between the
P-N juncno
gate and channel. There are
drain and
the
tions along the path between
of makind
one
source. Since there is only
The P-N junction
in a
is
jority carrier in the channel, the
unipolar device.
silicon diode.
239
JFET
is
a
4.
A
simple, accurate 2 volt reference can be
made with
7.
and a precision
resistor as
You would probably connect
the gate to the
source with a depletion transistor so that it
would be turned "half on." Connecting it to
just a current regulator diode
shown below.
the drain would turn
>
8.
MOSFET
it full
produce much less
do bipolar transistors.
transistors
electronic noise than
mA CURRENT
REGULATOR DIODE
on.
2
UNREGULATED
DC VOLTAGE
10
This noise would be heard as hiss or static in
a sensitive radio receiver and using MOSFETs can reduce this. Dual gate MOSFETs
>
VOLT
are ideal for receiver circuits that require
VOLTS
DC REGULATED
2
one signal to control the level of another
signal or where one signal must be mixed
with another signal. Three common applica-
1000Q
1* TOLERANCE
tions are mixer, local oscillator,
>
5.
6.
*
AGC
and
amplifier.
of a MOSFET is insulated from the
channel with a thin layer of glass. Since
glass is an almost perfect insulator, the input resistance for a MOSFET is almost infinite. The gate of a JFET is a back-biased
P-N diode and even the best diodes have at
least 0.1 microampere of leakage current.
Therefore the input impedance of a JFET is
lower than that of a MOSFET.
The gate
9.
You
will also
the
bias
need two resistors to properly
bipolar
transistor.
resistor will be necessary
A
turn-on
to prevent the
base from shunting too much current from
the input signal to ground. Remember that a
MOSFET gate draws almost no current and
so it can be directly wired to the outputs of
preceeding stages. In contrast, a base is a
P-N diode shorted to ground.
An enhancement MOSFET
depletion
redesigned
MOSFET
so
that
cannot replace a
unless the circuit is
the gate bias is ap-
propriate. For example,
when
There
is
a family of bipolar logic circuits
(DCTL) that
the depletion
tors
gate is zero (Vg S = 0), the transistor is half
turned on. When the enhancement gate voltage is zero, the transistor is turned off.
of
the
directly couples bases to collec-
preceeding stages.
However,
these circuits are run on very low voltage
(1.5 volts) and have other severe restrictions.
+ SUPPLY
LOAD
RESISTANCE
BASE CURRENT
LIMITING RESISTOR
1
1
1
1
1
OUTPUT
INPUT
N-P-N
t
TURNOFF
RESISTOR
>
HI
240
10.
This question was deliberately misleading.
Neither transistor is acting like a resistor
and both are acting like inverters. The transistors clamp the output to ground or to the
positive supply as appropriate.
SECTION
1.
VII
The load resistance should equal the internal
resistance of the voltage source for optimum
transfer of power. This
is
true of any voltage
source, including amplifiers. In batteries
11.
A CMOS inverter draws
is
a small spike of curswitches from high to low
or from low to high. It draws very little cur-
rent whenever
rent
when
it is
the best
way
to get
tery right now, but
it
timum method
total
quiescent.
it
power out
may
it
of the bat-
not be the op-
for getting
maximum
the
energy out of the battery
in
the long
run.
12.
The
VMOS
bottom
transistor has its drain on the
2.
of the silicon chip. This forces the
current to flow vertically from top to bottom
and therefore it must traverse a large volme
of semiconductor. This disperses the heat
while a conventional MOSFET confines the
heat to the surface of the silicon.
VMOS
FETs would
be awkward to print in an integrated circuit with other components
because wiring would have to be attached to
the bottom of the wafer. At present, IC's are
manufactured entirely on one side of a sil-
3.
Install a
RL
MOSFET
device using the follow-
Make
sure that the circuit in which you
intend to install a device is turned off.
Touch a grounded chassis or bench with
your hand.
4.
Pick up the IC and remove
protective foam.
it
from the
one holding the
Install the
In
the
The disadvantage
tection on a
of
collector
(emitter
is
a portion of the input resistance. This
is
Rl
increases the total input
if
we make Rl
bigger, the transistor will be less likely
to turn on so the transistor resistance
device.
circuit
output resistance
will
be
higher.
C.
14.
common
follower) amplifier the load resistance
resistance. Conversely,
MOSFET.
MOSFET
could also
In the common emitter circuit, increasing the input resistance (increasing Ri
in Fig. 7-3) will increase the output im-
and the
5.
We
it
creasing
circuitry in
install the
to the rest of the circuit.
because the input current must go
through Ri, through the base to emitter
junction and through Rl- Therefore, in-
which you want to
IC with the hand opposite the
Touch the
simple, let's look at the out-
pedance. The transistor is turned more
toward off by smaller base currents and
therefore has a higher resistance.
B.
3.
is
to
from the point of view of "V ou t"
as seen in Figs. 7-3, 7-5, and 7-6. but this
would be more complicated.
A.
2.
way
To make things
look at
ing steps in order:
1.
the voltage source
put impedance from the point of view of the
load resistor, Rl,. That is, we are matching
icon wafer.
13.
AC, a transformer is
match load impedance
to source impedance. However, if the source
is a DC voltage and you are trying to transfer DC power to a load resistance, then there
is no easy way to change the voltage-tocurrent ratio. Complex ways to do this effeciently are DC-to-DC inverter circuits and
switching power supplies.
If
a convenient
having zener diode pro-
MOSFET
gate is that it degrades the input isolation and will increase
the input capacitance of the MOSFET. The
zener diode not only has a leakage current, it
has some capacitance which is added to the
input capacitance of the device.
In the common base configuration the
input voltage source must provide all
the current that appears in the load resistance. Therefore, the output resistance includes not only the transistor but
also the input source resistance. As the
input
source
resistance
increases,
so
does the output resistance, as seen by
241
the load resistor.
4.
The common base configuration can be used
as a voltage step-up transformer. The com-
mon
used as a cur-
collector amplifier can be
The common
rent step-up transformer.
7.
Class C or class E amplifiers must be
retuned each time the frequency is changed, even by a few kilocycles. On the other
hand, a class B amplifier can cover a wide
range of radio frequencies with fair efficiency without retuning. The class B (or A
or AB) amplifiers will need bigger power
emit-
has both voltage and current gain and
therefore can be used as either. However, if
impedance matching is the main goal, then
the common emitter amplifier is not the best
ter
better cooling for the amplifier,
bigger transistors, and bigger power bills
supplies,
choice.
from the power company
5.
In order for the transistor to use the
range of
its
transfer function,
load resistor small enough to
imum
I
c
must have a
pass the max-
must
In addition to
Ic
for
the
same
of transmitted power.
it
current encountered on the load
MAX.
rent
amount
full
MAX, some
8.
If a sine
wave
signal
is
amplified by a sup-
posedly linear amplifier which has a curved
transfer characteristic, then the signal on
the output is a distorted sine wave. According to Fourier's Theorem, this signal is
no longer a pure sine wave and contains
more than one frequency. The straighter
line,
cur-
also be provided to drive the se-
in Fig. 7-4. An approximate equation for the minumum load resistance in this circuit only is as follows:
cond amplifier stage
the transfer characteristic, the more accurately a sine
wave
(or
other waveform)
be reproduced in the output. Classes
AB, C, or E just produce power at one
will
desired
cc
RL =
Ic
MAX
frequency.
Distortion
in
+
lout
can actually be useful by generating har-
monic frequencies. In a
linear amplifier,
distortion can be greatly improved
V cc
Ic
MAX
these
classes does not degrade the efficiency and
by
a
push-pull amplifier configuration since the
distortion in the
+f.
T
n a
/ Vc.c. - 0.6 V^olt
two transistors tends to
method we did not
cancel out. Another
is the use of negative feedback
which tends to make transfer character-
discuss
R;
istics less tilted (less gain),
9.
A
but straighter.
P-N-P and N-P-N transistors used to build
very high resistance like 200 megohms is
completely unable to supply the current
needed for the transistor to turn on and
follow its load line and supply current to the
should
following stage.
distortion.
complementary
push-pull amplifier
matched, not only for gain,
power dissipation, and other attributes,
they should also have equal amounts of
a
be
In other words, their transfer
characteristics should be mirror images of
6.
The common base
amplifier
is
each other.
often used to
drive high impedance loads since this configuration has the highest output imped-
10.
the collector circuit
ance. Even if the output impedance is not
exactly 300 ohms, it will be most efficient
to match a high impedance output to a
high impedance load rather than rely on a
imum DC
may
is
of the
LC
circuit in
adjusted for min-
current in a class
C
amplifier.
This can be done by changing the inductance, the capacitance, or possibly even the
impedance
transformer to correct a large mismatch. It
may turn out that the amplifier stage driving this final amplifier
The resonant frequency
1
not be able to
produce enough current to drive the common base amplifier. Some sort of transformer or matching circuit may be needed
on the input side of the final amplifier.
1.
of the load.
The adult pushing the swing represents the
transistor. The pushes that the adult gives
to the child
on the swing represent the low
voltage, high current pulses imparted to
the
on.
242
LC
circuit
when
the transistor turns
12.
Most
so
power consumed by a transconsumed by the final amplifier
of the
mitter
is
pays to have
it
Design a bias system that generates
that
this stage very efficient.
The penalty
is that it must be tuned each
time the transmitter frequency is changed.
It would be very inconvenient to tune each
stage of the amplifier in the transmitter
each time the frequency is changed because there may be four or five stages in a
high frequency transmitter. Since little
power is lost, it is easiest to use wide-band,
linear amplifier stages everywhere except
is
The four emitter
gain and one
the
resistors insure that
14.
16.
same
may turn on or off before the
may overload and destroy one
class
C
is
this is
The
is
difference
is
C
that the class
much
must be much
17.
15.
is
as though
amplifier
some threshold
you turned
off a
A
cheap amplifier as described would be no
The Darlington configuration that
is
described has extremely high gain,
10,000,000, and the slightest change in the
circuit will cause the quiescent point to
zoom to full on or full off. The slightest
change in temperature would upset the
quiescent point. Even the sound level of the
music would change the power dissi-pated in
the transistors and would change the
bargain.
that an input signal
greater than
The circuit shown in question 100 is another
example of negative feedback. When the
transistor.
temperature. The emitter resistance will
help to stabilize the amplifier by decreasing
the gain. Unfortunately, if enough feedback
before the transistor can begin to turn on.
It
no signal to amplify.
transistor turns on, the drain voltage goes
amplifier is biased so that the
biased full off. On the load line
the same place shown for class B.
biased so
the
down. This decreases the bias current
through Rj and R2 and tends to turn off the
transistors.
transistor
The
that
all
may
others
so
whenever there
sources. Voltage sources must' be provided
to produce this bias voltage for each tran-
Once one fails, the
soon follow. In a class D amplifier
the quiescent point is usually
located where the transistor(s) are turned
full off. This point is not critical, so thermal runaway is not usually a problem. It is
more likely with class A where the transistor is already half turned on.
of
state
sistor.
transistors have exactly the
others. This
current
JFETs or depletion MOSFETs. the gate
voltages needed to just turn off the transistors will be below the voltages on the
four transistors will share the load equally.
No two
or
is in this
For
the final amplifier.
13.
voltage
amplifier
water
used
such
faucet with a wrench. Before you can turn
is
on the water again with you bare hand, you
will need the wrench to loosen the handle.
have enough
gain to amplify from the mic-rophone level
to the loudspeaker level in one stage. This
feat requires about 50 decibels of power
gain. Another problem is that the bias
resistors and load resistance are connected
Steps
biasing
for
a
class
B
transistor
amplifier:
a.
maximum
stabilize
rent
possible collector or
b.
Read
what base current or gate
needed to just barely turn off
off
voltage
is
gain
same power supply. The slightest
in the power supply voltage or curwill be instantly sensed by the input
through the bias
resistors.
power supply leads may
V/R L
high
change
drain current on the current axis.
IMAX =
a
to the
Plot the load line. Plot the supply
voltage on the horizontal voltage axis.
Plot the
to
amplifier, the amplifier will not
Inductance
in the
re-sult in positive
feedback through the bias resistors and the
amplifier will probably oscillate all by itself.
In summary, a properly designed Hi-Fi
amplifier should achieve this much gain in
several small steps rather than all at once.
the transistor.
243
18.
SECTION
Question 101 was an example of a potential
thermal runaway. As a transistor gets hot.
its gain increases which tends to turn on the
transistor more on for a given set of circumstances. When the transistor turns on,
it may become hotter still and turn on even
1.
phase
2.
more likely to do
Negative feedback and
transistors are
than
this
silicon.
good cooling are ways
combat
to
amplifier characteristics
parameters
pertain
establish
that
to
DC
the
The dynamic
what happens
changing its state
resting state of the transistor.
characteristics
when
pertain
the transistor
rapidly; that
is,
is
when
We stated
is
correct
the entire oscillator loop.
to
the transistor
is
ac-
3.
tually amplifying a signal.
20.
the positive feedback so that only one
passed back to the input with
phase shift. Sine wave oscillators always settle down to a frequency
c
that gives a total of 360 phase shift around
the
those
and a tuned
amplifier
The phase shift network inverts the sine
wave signal or delays the signal 180° in
phase. The phase shift network also tunes or
frequency
Static
oscillator usually consists of a
inverting
shift network.
filters
this pro-
blem.
19.
wave
sine
voltage
more. Eventually the transistor may turn
full on and may even be damaged. Ger-
manium
A
VII
that high efficiency
was
Positive feedback
necessary to sustain the
is
oscillation.
The
amplifies
derived from
is
the ampli-fier
signal that
its
own
output.
a goal, so
the best three amplifier classes are C, E, and
C and E require an AC signal, usually
RF, to drive them and elaborate resonant
output circuits. These two approaches are
probably too complex to be worth the expense. A class D amplifier is simple and effiD.
4.
The only complicated part is that the
input signal must be de-veloped so that the
amplifier is pulsed full on and full off so that
the average heat produced achieves the
desired temperature. This approach resembles the way we used SCRs and TRIACs in
Section 5. But here there is no AC voltage to
turn off the SCR, so transistors are much
easier to use than an SCR. The emitter
follower configuration has the lowest output
resistance and so is best for the low
cient.
resistance heaters.
Germanium power
have a lower saturation voltage than
silicon
transistors,
less
power and
oscillating.
be more efficient.
is shown.
will
A
perfect 360° phase shift.
AVERAGE ON TIME
ON
y
q
ON
OFF
+12 VOLTS
NPN GERMANIUM
V,r
POWER TRANSISTOR
>
OFF
change
be greatly exaggerated at the amplifier
The magnified noise will be immediately redelivered to the input and the
oscillation is started. No matter what the
frequency of the noise, the phase shift network will soon force the oscillation to conform to the particular frequency that gives a
' DETERMINES
ON
cause a tiny disturbance
output.
possible amplifier circuit
Vjn
will
at the input to the amplifier. This
so these will dissipate
will
Slight variations in the power
supply voltage
tran-
sistors
This brings to mind the old question about
which came first, the chicken or the egg.
Yes, it is possible that an oscillator might
not be self-starting. Occasionally you might
actually find an oscillator that sits at its
quiescent point and refuses to begin
oscillating. However, if the amplifier has a
large voltage gain and the phase shift network delivers a large positive feedback
signal,
the circuit is bound to begin
9>>
OFF
rl heater
INPUT SIGNAL. TRANSISTOR
IS RAPIDLY TURNED FULL ON
AND FULL FF.
>
24
i
5.
You would expect
the detector is much larger than if the
detector were just a diode. When selfoscillating,
the regenerative detector
to find that the phase
change very abruptly with very
small changes in frequency as the frequency
is shifted away from the actual oscil-lator
shift will
makes Morse code
signals sound musical
so that they are easy to read. This circuit is
largely obsolete, but until recently it was
operating frequency.
6.
7.
RC phase shift oscillators are commonly used for low frequencies because they do not
require inductors. To operate at low, audio
frequencies, a Hartley or Colpitts oscillator
would need a very large, expensive inductor.
used
one ILS marker beacon
The three functions are
better performed by a superhetrodyne receiver with a separate oscillator for a beat
frequency oscillator (BFO).
an RC phase shift
one specific frequency
because the RC network only shifts one
specific frequency exactly 180°. Other frequencies are shifted more or less than this
and this drives the oscillation back to the
frequency where there is a perfect 360° of
total phase shift.
The
oscillation
in
13.
oscillator occurs at
8.
in at least
receiver design.
An
active detector
is
AM
an
an amplifier used as
if it were
it will am-
detector. It is biased as
half of a class B amplifier so that
one polarity of the RF signal. If
really does have a linear
transfer characteristic like one half of a
class B amplifier, then the detector may be
plify only
the
amplifier
called a "linear active detector."
SECTION IX
Three separate RC phase shift circuits are
needed in an RC phase shift oscillator because it is only practical for each circuit to
shift the sine wave about 60°. If you were to
try to produce a 90 ° phase shift with each of
two RC circuits, the output feedback
voltage would be vanishingly small.
1.
Bistable,
non-oscillating
commonly used
are
multivibrators
for counters (also called
and for temporary memories in
computers. They are also used in phase detectors and a host of other minor circuit apdividers)
plications.
9.
you want
phase shift
If
to vary the frequency of an
RC
oscillator, ideally you should
vary all three resistors simultaneously,
perhaps with a triple-decker potentio-meter.
10.
2.
The Hartley, Armstrong, and transformer
ly until
coupled oscillators can be tuned with a
trigger pulses.
single
variable
oscillator could
capacitor.
pitts could be
The
shut
Colpitts
Alternatively,
tuned by a single slug-tuned
3.
of a quartz crystal oscillator
that the frequency
The regenerative detector performs
functions.
It
detects
an
AM
changed by new
the power supply
is
the information stored in flip-flop
is lost.
The
trigger pulse tries to turn on both tran-
4.
transistor that
is
Just after the 13th pulse the
puts would be thirteen: Qi =
=
already turn-
1,
Q4 =
1.
flip-flop out-
Q> = 0, Qa
During the 13th pulse the
1,
counter is still registering twelve because
the counters respond to the falling edge of
the pulses. Therefore, the outputs will be
three
signal.
The
ed on tends to remain on. However, the
transistor that is off, turns on and this action turns off the previously on transistor.
This is because the gain of the transistor
that switches produces a bigger signal
than the input trigger pulse.
is extremely stable.
This is because the phase shift of this circuit
element varies dramatically whenever the
frequency starts to drift away from its resonant frequency. Saying it another way, the
crystal has very little resistance in comparison with its reactive impedance and
therefore is an extremely high Q circuit.
12.
When
the Col-
coil.
is
off,
sistors.
The advantage
deliberately
it is
memories
be tuned by a dual or ganged
variable capacitor.
11.
A flip-flop stores information by maintaining the state of its Q or Q output over a
long period of time, that is, the output remains high or low more or less permanent-
It
Qj
amplifies the signal so that the output of
245
=
o,
Q2 =
0.
Q3 =
1.
and Q,
=
1.
5.
The bases of astable multivibrators are
usually turned off by charge stored in the
capacitors so there
is
no need
for a
1 1
6.
12.
resistor.
The load
resistances, the faster the capacitors are
13.
recharged, the more swiftly the collector
If
will be.
the set trigger pulses are slower than the
14.
The advantage
of the multivibrator
inverter circuit
is
that
it
make DC.
AC
is
and
15.
power
be dissipated
Monostable
make
10.
One
multivibrators
pulses longer.
16.
on or
is
full
off
The comparator turns
whenever one input
higher than the other.
A zero crossing detector with positive feedback is a Schmitt trigger. The positive
feedback increases the gain and makes the
square wave output more square. Also, it
changes the switching threshold so that it
does not switch until the input voltage exceeds the thresholds above and below the
makes the
trigger
The output
of a Schmitt trigger looks like
the output of a zero crossing detector, but
the square wave output is delayed in phase
because the switching thresholds are no
longer at zero. Although the average of
these two thresholds is zero, it is no longer
actually detecting zero crossings.
17.
Hysteresis is a lag or delay in the response
some device like an electronic circuit. In
the case of the Schmitt trigger, the input
voltage must overcome the effect of the
of
positive feedback before the Schmitt trigger responds. This lag in response is an ex-
in the drop-
ample
are
a high gain differential
filtered
ping resistor alone.
9.
is
cy noise.
All these steps are efficient
will
comparator
zero voltage point. This
version efficiency of the zener diode circuit
will be only about 30% or 40% because half
power
that
insensitive to low amplitude high frequen-
and a 90% overall efficiency is typical. The
advantages of the zener diode voltage conversion approach are that it is cheaper,
simpler, and physically smaller than the
multivibrator. Moreover, the zener diode
will regulate the voltage so that the output
will be 6 volts DC even though the input
voltage may vary from, say 15 volts DC
down to 7 volts DC. Assuming that the input voltage is 12 volts DC, the power con-
of the
A
voltage
can be an efficient
rectified
are often
a very high gain
turns full on or full off
whenever the input signal goes above or
below zero volts.
full
to decrease or increase voltage without wasting power. First the power is converted to square wave AC, then it is passed
through a transformer to reduce the volt-
to
They
A zero crossing detector is
non-inverting input.
way
age. Finally the
digital circuits.
amplifier with an inverting input and a
frequency of the astable multivibrator, the multivibrator will switch
before the trigger pulse arrives. This
means that it will have already been set by
the time the set pulse arrives. Therefore,
the faster astable frequency will predominate. In fact, the multivibrator will ignore all the set trigger pulses unless they
occur while the Q output is low, zero.
natural
8.
clock pulse is a very frequency stable
square wave that is used to synchronize
amplifier
voltages will rise and the more square the
7.
A
and control
smaller than the capacitor discharging resistors, Ri and R2. The smaller the load
square wave
needed to count
derived from sine wave oscillators because
these are usually more frequency stable
than square wave oscillators.
must be very much
resistors
flip-flops are
to 10,000.
base to
The resistors Ri and R2 do
not pass enough current to the bases to
keep them turned full on.
ground
Seven more
up
used
shot multivibrators arc usually
of hysteresis.
to
18.
If
a Schmitt trigger has too
much
positive
feedback, the input signal would not be
able to overcome the feedback and the cir-
made
from one half <>t an astable multivibrator
and one half of a bistable multivibrator.
would be permanently locked into
on or full off states.
cuit
2 Hi
full
19.
The "S"
drawn on the Schmitt
symbol is a picture of the
sign
-like
trigger circuit
3.
V
m
vertently
.
A
unijunction transistor
built
how
of rating
well dif-
common mode
the
affects
inad-
output voltage to
some degree and
this error is called the
voltage gain. The desired
voltage gain is the voltage gain caused by
the difference voltage between the two in-
common mode
cuit.
20.
way
a
The common mode voltage
voltages.
This
ut graphed versus V
symbol illustrates the hysteresis of this ciris,
is
ferential amplifiers ignore
transfer characteristic of a Schmitt trigger; that
This ratio
something
a transistor
is
like a junction
FET. They
puts.
The common mode voltage
rejection
are largely obsolete but are occasionally
ratio is the ratio of the desired voltage gain
used as oscillators to generate voltage or
divided by the
common mode
voltage gain.
current pulses.
4.
21.
The NE555 timer integrated
circuit is
used
The output voltage
of
amplifier will
the differential am-
plifier
more
reliable,
and more
5.
versatile.
Direct coupled amplifiers, also called DC
amplifiers, can amplify very slowly chang-
DC
amps
The inner workings of ICs are usually
measurements made with
cessible to
over, there is
Knowing
all
grated
circuits
closely
matched
the details of
circuit.
SECTION X
6.
limitation of
op-amps
is
Opinte-
and the transistors are
in their
temperature
drift
is
designed so that as one transistor drifts,
another transistor will drift equally to compensate for the change. Also, the transistors often have base-to-emitter diodes
which help control the gain over a wide
temperature range.
The block diagram of the IC will tell you
what the IC as a whole is supposed to ac-
The greatest
signals.
they are
and gain characteristics. The op-amp
their inner construction is rarely helpful.
complish in the
stable because
volt-
no way to repair defective
parts in an IC.
AC
signals as well as
are
inac-
meters, ammeters, or oscilloscopes. More-
1.
to.
is
ing
23.
differential
has differential outputs, one output
referring
presently cheaper than a
single unijunction transistor. It is more
precise,
the
will
including: monostable multivibrator,
square wave generator, linear voltage
ramp generator (sawtooth wave generator)
and missing pulse detector.
The NE555
If
go up while the other will go down. The
plus and minus names on the inputs are
based on the polarity sense of the output.
If the amplifier has two outputs, either input can be plus or minus, depending on
which of the differential outputs you are
for various oscillator related applications
22.
rise.
that
The chopper stablized DC amplifier is a
method of amplifying very low frequency
or
they tend to be slower in switching time
and slew rate than individual transistors.
This is reasonable since they are composed
of several transistor amplifier stages in
series. Op-amps are not perfect in any
respect, but except for switching speed,
they greatly exceed the capabilities of in-
DC
signals with a high frequency
amplifier.
AC
The
DC
signal
is
and amplified by several
stages.
AC
each stage
AC
amplifier
amplifiers are stable because
is
the
Finally,
AC
chopped into
DC
AC
isolated from the others.
signal
filtered to restore the
DC
is
rectified
and
signal.
dividual transistors.
7.
2.
to
amplifiers
form of differential
extremely high voltage
tional amplifiers are a
amplifier
with
— V ee The
pot is used to balance the
amplifier output to zero volts when the input is zero volts. This is only important
when very small signals are being amplified, so many op-amps don't need them.
amplify the difference between two input voltages. OperaDifferential
Offset null leads are connected to a center
tapped pot with the center tap connected
gain.
247
.
8.
A
bar graph voltmeter
is
more comparators, each
an array of ten or
which is wired
= 6KQ. The output voltage would be
— 14 volts. Large power supply voltages
Rf
13.
of
to switch at a specific voltage.
much
The com-
parators respond to a range of voltages
greater than
needed.
ar-
ranged in a linear or logarithmic scale. The
output of each comparator drives an LED
or other indicator. Voltage is measured by
observing how many of the comparators
have responded to voltage above their in-
±14
volts
would be
the op-amp were a 741, the
If
power supply would have to be larger than
is permissible to have such a large output
voltage.
dividual thresholds.
Theoretically,
14.
work
if
the
would
amplifier
not
the inputs always stayed at precise-
An input voltage causes a
small difference voltage between plus and
minus inputs that is quickly cancelled by
the negative feedback.
ly zero volts.
9.
A
voltage follower
is
used to amplify cur-
rent without changing the voltage of the
input signal. In other words, it matches
high impedance voltage sources to low impedance loads without distorting the
original voltage.
The negative op-amp
in-
put has the same voltage as the output
because they are wired together. Whenever
the positive input attempts to rise above
the negative input, it makes a temporary
+6
15.
The output voltage
16.
The presence of an input offset current
means that the output will not equal zero
when the input voltage is zero.
The huge op-amp gain
up until the negative inagain equal to the positive op-amp
difference voltage.
will
be
volts.
forces the output
put
is
input.
17.
Op-amps
sometimes equipped with null
These can be connected to a
potentiometer which can adjust the output
to zero when the input is zero. Both op-amp
inputs should have equal resistance along
the pathways to ground so that the op-amp
will balance at zero. These pathways include
are
offset leads.
10.
Any op-amp circuit with negative feedback will cause the output voltage to
change until the negative input voltage is
again equal to the positive input voltage.
Therefore, the op-amp could compensate
for any diode offset voltage.
the source resistance of the input voltage.
18.
1 1.
There is a virtual short circuit between the
two op-amp inputs whenever the circuit
has negative feedback. The short-circuitlike condition will result whenever the
negative feedback coupling from output to
19.
is
a thermometer
composed
of
(or
are used to couple AC signals
which have a zero point at zero volts to the
amplifier which operates with its zero point
at
1/2 V cc
These capacitors charge to
whatever permanent DC voltage is present
The capacitors
.
We
assume that the op-amp has infinite gain which implies that any negative
feedback will do the job.
12.
pyrometer
galvanometer) and a thermocouple. The thermocouple consists of two
wires of dissimilar metals fused to-gether at
one end.
negative input is sufficiently strong so
that the negatively input voltage will be
forced to equal the positive op-amp input.
The more gain the op-amp has, the less
feedback current is required to accomplish
this.
A
a voltmeter
1 2V
CC in this case. As a result,
the amplifier can't be used for slowly changing DC signals because the ca-pacitors
would just adapt to the input signal and the
amplifier would never see the signal. High
across them,
The current flowing
into the op-amp inputs
very small and has no significant effect
on the amplifier circuit gain.
frequency signals are not
distorted by the capacitors.
is
'J
is
attenuated
or
20.
The op-amp with negative feedback behaves
5.
almost as though it were a perfect voltage
source with zero output resistance. The feedback largely compensates for the drop in
output voltage caused by load current. The
op-amp does have output resist-ance and
can't compensate for very large loads. One
symptom
of too big a load is the
op-amp
tated
may
RC
be dic-
6.
In the adding circuit it is vital that all the input currents that are being added all flow
toward a point that is fixed at zero volts.
The non-inverting amplifier does not have
such a fixed point. If the non-inverting
amplifier were used, each input current
could change the amplifier input voltage and
produce an error in the addition.
7.
active frequency filter is one or more RC
sometimes LC or LR) filter sections built
around an op-amp amplifier circuit which
compensates for the attenuation of the
An
logarithmic
amplifier
is
an
inverting
is
a
forward biased P-N junction diode. P-N
diodes have a forward volt-ampere characteristic that is approximately V = In I.
cess.
Therefore, the voltage across the op-amp
Bode
plot
is
between the negative input
a graph of decibels of at-
drove the current into the input
By changing
to 1/3, the circuit can
criminating.
calculator.
1K
loge
resistor.
3K
J>
-3lnx
>
ANTILOGe
V
rrn
^
and
the gain of the amplifier from 3
become a cube root
Several identical active filters can be put in
series to make a filter network more dis-
—
at zero volts
the output will be a voltage proportional to
the natural logarithm of the voltage that
tenuation or gain versus frequency. It is used to describe the frequency response of
amplifiers, filters, servo mechanisms, etc.
VlN = X^>
frequency except
in
amplifier in which the feedback element
desired frequencies during the filtering pro-
4.
frequencies
A band pass filter can be constructed by
putting a high pass filter in series with a low
pass filter. The cut-off frequencies are
selected above and below the pass band so
that only a narrow band of frequencies can
pass through both filters.
A
(or
A
fre-
narrow band called a
notch. The notch filter is made from a low
pass filter and a high pass filter in parallel.
The cut-off frequencies are arranged so they
are located on the upper and lower edges of
the notch band. The two filter outputs must
be combined using an operational adder to
make a composite signal.
SECTION XI
3.
shift
They
filters.
A notch filter passes every
the
must remain very
small so that the maximum voltage swing
for that load is not exceeded.
2.
phase
to
phase shift is unlikely to add up to an exact
360° at a frequency that is favored by the
will
loads, the input voltage
1.
similar
back. However, the amplifier is in the noninverting configuration and the total loop
With heavy
by the input voltage.
are
quently have a voltage gain greater than
one, a phase shift network, and positive feed-
The symptom dis-cussed in the
was that the output voltage will not be
able to reach the high levels that
filters
oscillators like the one in Fig. 8-4.
overheat.
text
These
V
VOLTAGE GAIN OF -3
249
J
= X3
SPEEDOMETER
D.C.
GENERATOR
xy
MULIPLIER
<r
IC
<r
VOLTAGE
6
MILES/HOUR
-MILES
GALLON
V^A^
^>
VOLTMETER
CALIBRATED
rrn
MILES
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IN
rrn
>
±>
-VOLTAGE 6
TO GALLONS/HOUR
FUEL
FLOW
METER
10.
Division circuit for mile-per-gallon meter.
11.
Integration and differentiation are opposite
processes. Differentiation produces a
voltage that is proportional to the rate of
change of the input voltage. Integration produces a voltage that is proportional to how
long the input voltage remains high.
12.
A
15.
series of pulses.
simple RC integrator is inaccurate
because the voltage across the capacitor
decreases the voltage across the resistor.
This prevents the capacitor from charging
at the correct rate.
An op-amp
integrator
16.
is
an inverting amplifier so the negative opamp input remains at zero. This keeps the
capacitor charging current through the
resistor
proportional
only
to
the
tains integrals, derivatives, or both.
14.
The current flowing
The
to the equation can be another
equation with no derivatives or integrals
that gives a concrete statement of how
each variable actually changed. The solution that is obtained with an analog computer is in the form of graphs which plot
how each variable changed over the time
voltage.
Differentiators tend to be impractical
because they are too sensitive to low amplitude, high frequency noise.
A differential equation is a mathematical
explanation or prediction of how a process
with changing variables will progress with
time. A differential equation always consolution
input
13.
Integrators are used in sweep circuits and
analog computers. They could also be used
as a sequential pulse counting machine.
Suppose a series of 18 equal pulses were
applied randomly to the integrator over a
period of several seconds or minutes. A
well designed integrator would produce a
voltage precisely proportional to 18 pulses.
So, in effect, the integrator is adding up a
period.
into
the integrator
should have exactly the same waveform as
the input voltage.
17.
250
They
are the same circuit except that the
inverting amplifier has a separate load.
The grounded base
SECTION
amplifier and the volt-
age-to-current converter share the following characteristics:
1
The goals
XII
power supply design often
of
in-
clude:
*
A
*
A
current gain of about
1
AC
A. Rectifying
high voltage gain
high impedance.
if
to
DC.
the load has a
B.
Holding the output voltage
level cons-
tant.
A high power gain
high resistance.
*
if
the load has a
C.
*
A
*
The input voltage and output voltage
have opposite polarities, assuming the
low input impedance.
grounded hose
is
Both
tend to have current
circuits
voltage level.
D. Limiting the current to the amount
that can be delivered safely.
E.
Filtering out
AC
ripple or other noise.
at zero volts.
F.
*
RMS
Changing the
Isolating
supply
the
output
from
ground.
source outputs.
G.
Low
supply inductance and resistance
to load current.
18.
The negative feedback through the
deflec-
H. High efficiency.
tion coil insures that the voltage across the
op-amp
compensate for the impedance
of the deflection coil and force the current
ramp through the coil.
19.
will
The inverting
amplifier configuration
comparable to the
fier.
The voltage
common
follower
the emitter follower or
is
I.
a
emitter ampli-
K.
Low
cost.
comparable to
common
collector
2.
is
100 because a 100
Q
3.
draws 100 times more current than
10K Q
Reliability.
The zener regulator
is
an example of a
parallel regulator.
The current gain
resistor
J.
size.
is
amplifier.
20.
Lightweight and small
resistor.
100 Q
WW
10KQ
Large zener diodes which are able to dissipate more than a few watts are expensive
and rarely used. It is cheaper and more effi-
10KQ
Rload
V
"V
VOLTAGE-TO-CURRENT
INVERTER WITH
VOLTAGE GAIN = 1
CURRENT GAIN = 100
CONVERTER
SOURCE.
251
IS
CURRENT
heavy heat sinks are not needed. If
an inefficient supply were allowed to run
hot, the transistors or other temperature
sensitive materials would be damaged.
cient to use zeners as voltage references for
large,
a series regulator pass transistor or other
voltage regulator design.
4.
In a parallel regulator energy is wasted in
resistive elements. In a series regulator, the load serves as part of the voltage
two
10.
temperature
is
lated
5.
Both the emitter follower amplifier and the
op-amp voltage follower can be used to amplify a voltage reference to
make
11.
a voltage
The current sense
resistor
is in
series with
the output current and the voltage across
it is proportional to this current. A zener
base-to-emitter junction,
diode,
voltage reference device
is
the sense resistor voltage.
or
3-terminal regulator
is
the out-
12.
Separate
built
for
3-terminal
a sophisticated
regulators
are
and negative supplies
usually desirable to ground
positive
because it is
the metal case to dissipate heat. A positive
voltage regulator could be used to build a
negative supply, but the case will have to
be insulated from ground.
8.
A
more
supplies.
made of semiconhas accurate, symmetrical, positive and negative breakdown
voltages which resemble two zener diodes
wired in series. When wired across an AC
voltage, the varistor switches on so abruptly that it can be used to clip very short
duration noise spikes off an AC sine wave
varistor
is
a resistor
material.
It
Switching power supplies are extremely eflightweight, and compact. Using integrated pulse width modulators, they can
be constructed with few parts, and they
often have built-in current limiting abilities. If a switching power supply were
used to power a radio receiver, you could
expect to have radio noise interference
from the high frequency AC switching
waveform. Another disadvantage is that it
requires a relatively complex circuit if an
isolated output is desired. Although switching power supplies should run cool, they
are more temperature sensitive than a
regulated supply based on a device like a
ferro-resonant transformer which contains
no transistors.
ficient,
series regulator built as an integrated circuit.
are
accurate
placed across
When
rent to the load.
A
stable
voltage.
other
put current exceeds the design limit, the
voltage reference device conducts and
steals current from the base of the pass
transistor. This prevents the pass transistor from passing greater amounts of cur-
7.
A
power
ductor
regulated source. They both have voltage
gains of one, but high current gain that will
adapt to the needs of the load.
6.
and
than
zener diodes. They are preferred for use in
precision voltmeters and precision regu-
only one resistive, voltage dropping element.
divider, so there
Energy gap voltage references
3-terminal regulator can be used to build
a current source
and ground terminals across a fixed
tor.
13.
by connecting the output
The regulator
The free-wheeling diode discharges
the
charged inductor into the load so that the
energy stored in the inductor is not wasted.
resis-
will drive a fixed current
through the resistor and to other circuits
by holding
the
sistance constant.
needed,
it
voltage
If a
would be
across
the
re-
14.
large current were
inefficient
and expen-
sive to use a high voltage regulator.
The
pulse width modulator converts a
DC
the DC voltage
system. In a switching supply, the pulse width modulator
compares the output voltage with a reference voltage and generates control
pulses to switch the transistors and keep
the output voltage constant.
versely
regulator should have the lowest regula-
proportional
to
level in the Fig. 12-17
tion voltage available.
9.
A
voltage level into a pulse width modulated
pulse train. The width of the pulses is in-
An efficient power supply runs cool because it does not convert energy into heat.
Since there is no heat to be dissipated.
252
15.
High frequencies are used for switching
power supplies so that the inductor and
filter components in the supply can be as
put current rises, it produces a flux that
counters the flux from the primary winding. This negative feedback reduces the
output current. Also, as the secondary side
small as possible.
of the core begins to saturate because of
16.
The
high currents in the resonant winding and
the secondary winding, the inductance of
the secondary winding will fall. This will
error signal is an amplified difference
voltage which is the difference between a
reference voltage and the actual output
also limit the current.
voltage.
17.
The
error signal can never be zero volts
because that would turn the pulse width
modulator output on continuously. This
would turn the switching transistors on
continuously. As soon as the inductors
were fully charged, all the voltage drop
would be across the transistors and the
load. This would overheat the transistors
and load very quickly.
22.
A
paraformer is not an ordinary transforThe primary and secondary windings
mer.
do not share a common flux path. Instead,
magnetic field energy is forced to leave the
primary flux path and join the secondary
flux path as the inductance of the second-
ary winding
paraformer
own
18.
A
wave system
full
as
shown
is
suited for driving a transformer so
be used
for raising voltage levels.
it
can
full
and the inductive comsupply can be lighter and
in the core
in the
smaller than in the half-wave design. As
drawn, the full wave system can be isolated
from ground.
19.
Photo-isolators are useful for transmitting
information
which
is
across
a
voltage
difference
either unpredictable or varies in
The photo-isolator has an extremely
high impedance between the light and the
photo-transistor so that current can't pass
through it and it cannot compromise the
size.
isolation.
20.
By isolating the supply output from
ground, power supplies can be wired together in parallel or in series, or with either
polarity grounded.
21.
A
in
is
oscillating,
it
generates
its
sine
form does not appear on the secondaryvoltage waveform.
wave system demagnetizes the transformer core on every half cycle. Less iron is
needed
ponent
changed by the current
waves which are independent of
the waveforms on the primary. This means
that noise on the primary voltage wave-
better
The
is
the primary. Because the secondary of the
ferro-resonant transformer is a power
transformer which regulates its own AC
voltage and limits the current it can deliver. The output current passes through
the compensation winding which is located
on the primary side of the core. As the out-
253
FINAL EXAMINATION
3.
STUDENT
GRADE
_
Electrons are able to leave the cathode and
flow to the plate in a vacuum tube when:
A. The tube is evacuated and there is no air
to prevent conduction across the gap.
B.
The cathode
is
C.
The
positive with respect to the
plate
is
heated.
cathode.
Select the
1.
BEST
answer
for
each question.
Semiconductors are important
All of the above.
E.
None
of the above.
in electronics
because:
A.
D.
4.
The following
The valence energy band and the conduction energy band are far apart in
A.
semiconductors.
B.
is
true of
AC
They
all
They
are always
rectify
"diodes.
all
to
DC.
made from
a semicon-
ductor.
B.
Each atom always has
a valence of +4.
so they react together to form stable
C.
C.
Vacuum
D.
Small applications of energy can convert
semiconductors from insulators to conductors or vice versa.
tubes are obsolete.
all
non-linear and have
two
They
E.
They can never be used
all
have a forward offset voltage.
as the amplify-
ing device in oscillators or amplifiers.
All of the above.
Conduction by holes uses less voltage
than conduction in the conduction band.
5.
2.
are
D.
F.
E.
They
electrodes or terminals.
crystals.
Which semiconductor device depends on
P-N junction for its function?
A.
A
thermocouple
B.
A
thermistor
a
Zener diodes:
A.
Do
B.
Are never used
not have a forward offest voltage.
in series voltage regula-
tors.
C.
Have nothing
in
common
with tunnel
diodes.
C.
D.
A cadmium
A Gunn
E.
A
F.
None
sulfide cell
D.
Can never be used
E.
Are most important for their backward
breakdown characteristics.
F.
None
in place of stabistors.
diode
varistor
of the
above
255
of the above.
6.
The saturation voltage
10.
bipolar transis-
in
MOSFETs:
Depletion
tors:
A.
A. Are turned half-on when the gate-tosource voltage is zero.
lower in germanium transis-
Is usually
tors.
B.
Is
measured between the
collector
and
B.
emitter.
Occurs when the transistor
C.
is
turned
full
higher
Is usually
power
in large
is
easily punctured
by
static elec-
transis-
C.
tors.
E.
a thin silicon oxide gate insulator
tricity.
on.
D.
Have
which
Sometimes have an external substrate
lead, 4 leads total.
All of the above.
D.
Are not always shown using exactly the
same symbol.
7.
Which device
RF
A.
signals?
A triode
B.
PIN
is
not capable of amplifying
E.
All of the above.
vacuum tube
C.
diode
Varactor
D.
Common
E.
Tunnel diode
11.
A CMOS
verter,
is
digital circuit, such as a binary
extremely efficient because:
in-
base amplifier
A.
The power supply current never travels
directly from +Y<id to ~~ ^ dd- except
during switching.
8.
B.
C.
E.
Binarv arithmetic.
D.
Unlike
PMOS
CMOS
and
FET
NMOS,
no resistors
transistors.
up
to 20 times
wafer than bipolar transistors and this keeps them
circuitry takes
more area on the
An enhancement MOSFET
age that
than bipolar transistor logic
are used, only
Taking
of logarithms
Addition and subtraction
It is faster
circuits.
Division
D.
C.
9.
B.
Operational amplifiers are used for all the
following mathematical operations except:
A. Integration
silicon
cooler.
has the advant-
it:
E.
A. Can never be damaged by excess
All of the above.
volt-
age.
12.
-
B.
Is
"fail
safe'
and turns
gate-to-source voltage
C.
off
when
the
An amplifier with high voltage gain and
high current gain is:
is zero.
Can be handled with no
special precau-
A.
The source follower
B.
The voltage
C.
The common source
D.
The common
E.
The common gate
tions.
D.
Has lower gate capacitance than
FETs.
E.
F.
Can always be used as a direct
ment for JFET transistors.
None
follower
other
replace-
of the above.
256
amplifier
collector amplifier
amplifier
13.
Which
of the following transistor leads
is
17.
Which
of the following
is
not true about
dif-
normally biased with positive voltage?
ferential amplifiers.
A. The P-N-P collector
A. The output voltage equals the differential of the input; that is, the first de-
B.
N-channel
FET
source
C.
P-channel
FET
drain
rivative.
B.
An
operational amplifier
is
an example
of a differential amplifier.
D.
The N-P-N
collector
C.
14.
Which
transistor amplifier has the lowest in-
put impedance?
the output goes down.
A.
Common
emitter amplifier
B.
Common
base amplifier
C.
Source follower amplifier
D.
Common
collector amplifier
E.
Common
cathode amplifier
18.
15.
Beta
A.
A
B.
The current gain
C.
The
collector
D.
The common mode voltage is
age of the two input voltages.
E.
None
An
inverting op-amp
Greek alphabet.
of a bipolar transistor.
current divided by the
the aver-
of the above.
is
A.
-5
B.
-3
C.
-4
D.
-1/4
E.
Infinity
wired with a signal
ohm and a negative
4K ohm. What is the
input resistor of IK
feed-back resistor of
volt-age gain?
is:
letter in the
When the inverting input has a higher
voltage than the non-inverting input,
base current.
D. For most practical purposes, the same
as
Hfe
19-
.
Which
of the following
is
not true about
operational amplifiers wired with negative
E.
16.
feedback as
All of the above.
Operational amplifiers can be used to build:
A.
Sine
wave
oscillators.
in question 18.
A.
The voltage gain
B.
The input
will
be
finite.
resistance of the inverting
amplifier will be nearly infinite
B.
Schmitt triggers.
C.
Comparators.
D.
Linear amplifiers.
C.
The two inputs
same voltage.
D.
The output
will
have virtually the
resistance will act as
if
it
were zero over a wide range.
E.
Zero crossing detectors.
E.
F.
All of the above.
The output
will act like a voltage source
over a wide range.
257
20.
Which device
is
unlikely to be used in a
24.
A.
Stabistor
B.
Bipolar transistor
C.
Tunnel
D.
Energy gap voltage reference
E.
Field effect current regulator diode
N-channelJFET
B.
P-channel
MOSFET
C.
N-channel
MOSFET
D.
Enhancement type
E.
Depletion type
The following
monly used
MOSFET
high
for
A.
First class
B.
Classes
C.
Classes A,
power control
Memory elements
C.
Analog, wide-band amplifiers
D.
Classes A, C. and
D.
Digital logic circuits
E.
Classes AB. C. and
E.
Counters
F.
Class
likely
substitute for the
DIAC
control circuit
to
be used as a
in a
TRIAC power
"stereo""
and B
B.
The device most
fidelity
and third class
A
A
MOSFET
classes of amplifiers are com-
for:
AC
P-N
construction?
A.
amplifiers.
26.
23.
its
Two-state or bistable devices are not useful
A.
22.
of the following devices has a
rectifier
25.
21.
Which
junction in
voltage regulator circuit.
AB, and B
E
E
only
The following class(es) of amplifiers are commonly used for amplifying radio signals.
A. Classes A. B. C. and
D
is:
A.
A
xenon flash tube
B.
Classes C,
B.
A
neon bulb
C.
Classes
A PIN
D.
Class
D
only
C.
E.
Class
C
only
D.
A
P-N-P-N diode
E.
A
thyratron
diode
Which of the following devices cannot be
used to build a relaxation oscillator?
A.
A DIAC
B.
Unijunction transistor
C.
P-N-P-N diode
27.
When
A
a Class
Schottky diode
E.
Neon bulb
and B
C
amplifier
is
properly tuned,
you would expect:
A. The music to be distorted.
B.
The
plate to glow
red hot in a tube
amplifier.
C.
The plate
minimum.
D.
The
E.
The
or
collector
current
collector or plate voltage
to be the
D.
AB, and E
to
be
waveform
shape of perfect square waves.
collector voltage to be high while
the collector current
is
high.
28.
The load
A.
B.
line
is:
31.
A straight line drawn on the
ampere characteristics of a device.
A
way
graphical
of
Sine
oscillators:
A. Are never used in the generation of
volt-
square waves.
showing the voltage
B.
Are usually
A
built
from a non-inverting
across a device and the current through
class
the device while
back path through a 180° phase-shift
it
operates in a par-
ticular circuit.
C.
wave
A
graph
amplifier with a positive feed-
network.
that
shows
the
maximum
C.
tolerable load for a transistor.
Are most stable when the phase-shift
changes most dramatically with frequency.
D. All of the above.
D.
29.
E.
None
F.
A
of the above.
and B only
Thermal runaway
Are often stablized with
piezo-electric
galena crystals.
E.
in a class
A
amplifier can
32.
Are basically a bistable circuit with two
quasistable operating points.
Doping
is:
be discouraged by:
A.
A. Negative feedback to hold the gain constant with temperature.
B.
Using a larger
C.
Cooling the transistor.
B.
D. Using a Darlington transistor.
All of the above.
F.
A, B, and
C
C.
Class
D
only.
electronic
techni-
A refrigerant sprayed on electronic components to cool them off and make intermittant circuits easier to find.
A
process of painting transistors.
band
A
electrons.
poor performance on an
FCC
exam.
amplifiers:
33.
A. Are good for nothing.
B.
Are good
C.
Could be used for switching elements
a switching power supply.
for
None
Which
RF
amplifier
is
Common
B.
Emitter follower
C.
Common
drain
D.
Common
base
E.
Common
gate
emitter
in
amplifiers.
of the above.
259
an inverting amplifier?
A.
analog amplification.
D. Are used as tuned
E.
by
D. The addition of impurities to a semiconductor to provide holes or conduction
E.
30.
narcotic used
jobs.
transistor.
E.
A
cians to relieve the frustration of their
34.
A
37.
Darlington transistor:
Which statement about
wave
sine
oscil-
lators is false?
A.
Is
faster switching than bipolar tran-
A. Colpitts and Hartley circuits can be
modified to include a crystal in the feed-
sistors.
B.
Is
two
or
more bipolar
directly together so that they
used as a single high gain
C.
May
only be used in
back path.
transistors wired
may
be
B.
transistor.
common
Sometimes stray capacitance is used as
part of the feedback loop in sine wave
oscillators.
emitter
configuration.
FET
complimentary
D.
Is a
E.
Is exceptionally cute.
C.
There is a total of 180° phase shift
around the oscillator amplifier and feedback loop.
D.
A
transistor.
sine
wave
oscillator
can be built with
a tunnel diode as the active amplifier
ele-
ment.
35.
Several P-N junction rectifying diodes connected in series describes:
E.
Sometimes
sine
wave
transistor oscil-
lators are biased like class
A.
Semiconductor
high
voltage
rectifier
B
or
C
ampli-
fiers.
diodes.
B.
C.
Stabistor diodes.
A
28
volt
solar
38.
battery
made from
The
A.
silicon solar cells.
bistable flip-flop:
Can be used
as a counter because it produces two output "Q" pulses for every
input pulse.
36.
D.
All of the above.
E.
None
A
varactor could never be used
to:
A.
Replace a variable capacitor.
B.
Tune an LC resonant
Can be used
C.
Often has set and reset inputs which
always change the state of the Q output
with every input pulse.
D.
circuit.
as a memory element in
which each of the two transistors can
remember one bit of information for a
total of two bits per flip-flop.
B.
of the above.
Will oscillate at a frequency determined
by the speed-up capacitors.
C.
Multiply frequency.
D.
Amplify
E.
None
E.
RF
signals.
Will remain in either of its two stable
states indefinitely.
of the above.
F.
260
Onlv A, B. and E
are true.
Answers
to the Final
Exam
1.
D
20.
C
2.
F
21.
C
3.
D
22.
B
4.
C
23.
D
5.
E
24.
A
6.
E
25.
B
7.
B
26.
B
8.
E
27.
C
9.
B
28.
F
10.
E
29.
F
11.
A
30.
C
12.
C
31.
C
13.
D
32.
D
14.
B
33.
A
15.
E
34.
B
16.
F
35.
D
17.
A
36.
E
18.
C
37.
C
19.
B
38.
E
261
NOTES
NOTES
NOTES
I
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Box 36 • 1000 College View Drive
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Tel: (800) 443-9250 • (307)
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