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UNIT
1
Semi Conductor Components
Learning Objectives
• To study the atomic models, inter automic bonds, conductivity of
conductors, semiconductors and insulators.
• Semiconductors, doping, formation of P N junction diode, Zener diode,
Transistor, and the study V I characteristics and applications.
• To observe data sheets specifications of semiconductor devices and
applications.
1.0 Introduction
1.0
This chapter deals with physical phonomina involved in semi conductor
devices operation. The material in nature, they are divided into 3 categories.
they are
1. Conductors
2. Semi conductors
3. Insulaters.
The above materials are classified by based on the conduction of current.
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Conductors
The materials, which conducts electrical current through it, is known as
conductors
Examples - Cupper, Aluminium, Silver, brass, iron etc.,
The conductors possesses low resistivity, hence the resistance affected by
it is low, conducts, allows to flow more current through it.
Semiconductors
The materials, which offer high resistance at a low termperature. These
type of materials different behavior, with rise in each degree of temperature
resistance decreases. These materials have negative temperature co-efficient of
resistance, resulting semiconductors offers very high conductivity. At particularly
high temperature high conductivity offers more current to flow through it.
Example - Silicon, Germanium etc.
The semiconductors resistivity is lies 0.038  cms to 0.04  cms.
Insulators
These materials consists of high resistance hence practically no current flows
through it. No conductivity. Example Paper, Wood, Plastic etc.,
1.1 Properties of Solid State Semiconductor
The semi conductor devices had resistivity between 10-4 to 104  m. The
semiconductor materials are Silicon and Germanium.
These semiconductors family lies 4th group in periodic table. Those
elements had 4 free electrons in outer orbit.
3rd Orbit
2nd Orbit
1st Orbit
Fig. 1.1
Nucleus
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Fig. 1.2 Germanium automic structure
Inter atomic bonds are three types. they are
(i) Ionic Bond
(ii) Covelent bond
(iii) Co-ordinate covelent bond.
(i) Ionic Bond
Two atoms having mutually opposite charges, when they are in exited
state combine chemically sharing electrons with opposite charges form a bond
between opposite ions is known as ionic bond.
Example : Sodium atomic number, Na11, electron configuration
Na11 = 1S2, 2S2, 2P6, 3S1.
The outer most orbit of sodium is 3rd orbit and it gives one outer electron
it becomes as a donar i.e. Na+.
The chlorine atomic number is Cl17. The electronic configuration of Chroline
is
Cl17 = 1S2, 2S2, 2P6, 3S2, 3P5
In the P-orbital only electron is needed to fill P-orbital. It takes one electron and it becomes as acceptor. When it is in exited state Cl- i.e. anion. When
these combines together chemically forms a bond between donar and acceptor.
forms a bond is known as ionic bond.
Na
Cl + e-
Na+ + eCl-
---- doner
---- acceptor
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Adding Na + Cl + e-
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Na+ + e- + ClNaCl.
(ii) Covelent bond
It is the bond, which forms between same type of atoms, by sharing of
electrons equally in outer most orbit. Example : Formation of Hydrogen, Chlorine molecules, which were diatomic in state.
H-H
H2
When hydrogen is in exited state one electron in outer most oribit, the
same type of atoms comes together nearer and to get stable state forms as H2.
Similarly Cl2 molecules also forms.
(iii) Coordinate Covenlent Bond
In this type of bond three hydrogen atoms shares three electrons with Px,
Py, Pz of Nitrogen atom. When these are in exited state, to get stable condition
NH3 molecule is formed. the formation co-ordinates covelent bond figure as
follows.
H
N
H
NH3
H
Fig. 1.3 Formation of Ammonia Molecule
1.1.2 Energy Band Diagram of Material
The material are three types. They are
(a) Conductors
(b) Semiconductors
(c) Insulators
(a) Conductors
In these materials the conductor band energy and valence band energy
overlaps on each other. Hence there is no forbidden gap in between the bands,
electric current flows, the materials allows low resistance hence more conductivity.
Example : All metals are conductors, Copper, Silver, Aluminium etc.,
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Conduction Band
rgy
Band Ene
Forbidden Band
Valence Band
Fig. 1.4 Conductors Energy Band Diagram
(b) Semiconductors
Band Energy
In this type of materials there is small energy gap. It posseses negative
resistance characteristics and it changes with change of temperature. Particularly
at higher temperature, the semiconductors conducts heavily. Hence these
materials used as active devices.
Conduction Band
Forbidden Gap 0.7 eV
Valance Band
Fig. 1.5 Semiconductor Energy Band Diagram
(c) Insulators
In these type of materials forbidden energy gap is more in between
conduction and valance bands. These materials there is no current flow. Hence
the materials has very high resistance.
Examples : Wood, plastic, glass etc.,
Band Energy
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Conduction Band
Forbidden Gap 6.0 eV
Valance Band
Fig. 1.6 Insulators Energy Band Diagram
1.2 Formation of P Type and N Type Material
The semiconductors alloys are in earth. To get pure of semiconductors
purification chemical process is done.
Classification of Semi Conductors
The pure form of semiconductors are known as intrinsic semi conductors.
The intrinsic semiconductors are two type
(i) P-type intrinsic semiconductor.
(ii) N-type intrinsic semiconductor.
(i) P- Type Intrinsic Semiconductor
It is pure form of P-type semiconductor. It consist of only positive charge
carriers.
++++++++
++++++++
++++++++
Fig. 1.7 P-type Intrinsic Semiconductor
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(ii) N-type Intrinsic Semiconductor
In this type almost all carriers are electrons. No positive charge carriers.
-- -- -- -- -- -- -- --- -- -- -- -- -- -- -Fig. 1.8 N-type Intrinsic Semiconductor
The pure form of semiconductors does not conduct electricity. To conduct
electric current impurities are added.
Doping :The process of adding impurities to the pure form of intrinsic
semiconductor is known as doping.
For 106 to108 atoms one impurity atom is added. For P type Intrinsic
semiconductor, a trivalent impurity is added. For N type Intrinsic semiconductor,
a tetravalent or pentavalent impurity is added. While adding such impurities the
semiconductor becomes as extrinsic semiconductor. At room temperature its
conductivity increases ten times. Every rise in 1oc of temperature the conductivity
of extrinsic semiconductor rises ten times.
1.3 Extrinsic Semiconductor
Extrinsic Semiconductor : The impure form of semiconductors is known as
extrinsic semiconductors. These are two types. They are
(i) P-type extrinsic semiconductor : In this type the semiconductor consists
of majority carriers are positive charge carriers and minority charge carriers are
electrons.
-
++++++++
++++++++
++++++++
-
-
Fig. 1.9 P Type Extrinsic Semiconductor
+ve, holes or positive
-ve, electrons negative charge carriers
(ii) N-type Extrinsic Semiconductors : In this the semiconductor consists
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majority carriers are electrons and minority charge carriers are holes i.e. positive
charge carriers.
+ve holes, -ve electrons
-- +-- -- -- -- --+-- --+ - -+- - +- - -------Fig. 1.10 N-Type Extrinsic Semiconductors
1.4
PN Junction Diode Formation
P Type semiconductor : The pure form of intrinsic P-type semiconductor is
added trivalent impurity such as Gallium, Indium and it becomes extrinsic
semiconductor which carries majority carries as holes (positive charge carriers)
minority charge carriers are electrons.
Fig. 1.11 PN layers
Fig. 1.12 PN layers in
placed
liquid form
Fig. 1.13 P N Junction Formation
N-type Semiconductors
When small amount of pentavelent or tetravelent impurity is added N-type
intrisic semiconductor a N-type extrinsic semiconductor is formed. In this it
carries majority carriers are electrons and minority charge carriers are holes.
Pentravalent impurities are Arsanic and Anti many.
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Anode
Cathode
Fig. 1.14 PN Junction diode Symbol
PN Junction : When a piece of P-type semiconductor a piece of N-type
semiconductor being together and heat is applied in special container up to
5000 oC. At high temperature pores the material one upon other. separated by
a junction known as PN junction.
Anode
Cathode
No External field is applied
Fig. 1.15 P N Junction diode without exteral field
A N type semiconductor in the form crystal, a P-type indium piece is kept,
which position shows in the fig. 1.15. Then the whole combination is kept in side
the puddle. The puddile is heated upto 5000oC. The indium part of P-type
layer is becomes in the liquid form on the N-type crystal shown in the fig1.12.
Finally heat is withdrawn from the puddle a cooling process takes place. The
whole system is cooled form PN junction diode.
PN Junction Diode Working : At room temperature the majority carriers
cross the both layers in the P, N-type layers on the semiconductors without any
external supply shown in below.
P, N type layers an electrodes are connected known as Anode, Cathode
respectively.
Potential Barrier : It is the charge carrier, which cross across the junction.
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The charge carriers potential on the junction is known as potential barrier or
barrier potential.
The minimum cut of voltage required to conduct Garmanium diodes are
0.2 V, . In the case of Silicon diodes the minimum cut off voltages are 0.7 V.
Work function : The minimum amount of additional energy required to lift
electrons from cathode surface is known as work function. Generally this work
function refers to cathode materials such as Tungsten, Thoriated Tungstun and
oxide coated materials. These materials are used as filaments in electrical bulbs
or other heating elements to the electrodes. These are two types.They are
directly heated / indirectly heated cathodes.
1.5 PN Junction Diode Forward Bias
As soon as switch S is closed, the diode anode is connected to positive
terminal of the power supply and cathode is connected to negative terminal of
the power supply, the PN junction diode is said to be connected in forward
bias mode. To measure cut off voltage a volt meter is connected across the PN
junction diode and the ammeter is connected in series , to measure forward
current.
Biasing : It is the process in which supply giving to the semiconductor device
is known as biasing.
As soon as switch closed, increase the dc power supply form 0-1V, slowly.
We can take / observe corresponding readings in Volt/ meter reading Ameter.
At cut off voltages particularly in thecase of Ge is 0.2V and Si is 0.7V the
anode current starts to increase and reaches the diode into saturation region. i.e.
maximum current it reaches, while changing slightly from 0.6V to 0.8 V, reaches
upto 12 to 25 mA of anode current. The forward V.I. characteristics are shown
in the figure.
V
A
K
Ammeter
+
-
S
Switch
DC Power Supply
Fig. 1.16 PN Junction diode connected in Forward bias
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In this region the majority carriers attracts the interelectrodes of the dc
power supply. Hence the junction offers low resistance. It behaves just like
down fall of water from upto down.
Fig. 1.17 PN Junciton diode Forward Bias VI Characteristics
From the forward bias V.I. characteristics, the diode forward resistance is
calculated. Draw the tangent at sharp curve say the point P (x,y), which is point
of tangency.
Diode Forward Resistance rf : It is the ratio of difference of change of
anode to cathode voltage to change in difference in anode current is known as
PN junction diode forward resistance.
Diode forward resistance = Difference of Anode to Cathode voltage
Difference of Inode current.
= VAK /  IA = 0.7V - OV / 2mA - omA = 0.7V / 2mA
rf = 350 
Diode Reverse Bias VI Characteristics
Anode of the diode is connected to negative terminal of the dc power
suppply and cathode is connected to positive terminal of the dc power supply,
the connection is said to the diode is connected in reverse bias mode.
As soon as switch is closed, Increase the VAK insteps of OV, 5V, 10V,
15V, 20V, 25V. A small amount of current flows at high voltage in mA.
The diode reverse bias V-I characteristics is drawn in third co-quardrant,
because VAK , IA are nagative.
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V
A
K
Ammeter
-
+ S
Switch
DC Power Supply
Fig. 1.18 PN Junction diode reverse biased circuit
Fig. 1.19 VI Characteristics
In reverse bias the diode affects reverse resistance more than 100 k., the
graphical representation as follows.
Fig. 1.20 PN Junction diode VI Characteristics
At particular point Q in reverse bias mode suddenly a small amount rises
and stops with incresae in VAK . The break down point is known as Avelounch
break down point. The correspoinding voltage current are known as Avelanch
voltage, current respectively. Then it is represented as VAB, IAB. A tangent is
drawn at sharp curve. The point of tangency gives us diode reverse resistance.
The Diode Reverse Resistance : It is the ratio of difference in change in VAK
to change IA is known as PN junction diode reverse resistance.
Diode Reverse resistance = Change in VAK / Change in IA
= VAK / IA
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From the graph = 18V / 25mA. = 0.72 x 106 = 720 k
Applications : 1) Used as Rectifier
2) Switch
3) Used as clipper
Types of Diode :
a) Zoner Diode
b) LED
c) Photo Diode
d) Varacter Diode
e) Tunnel Diode
(a) Zener Diode
The doping concentration is 10% more compare to PN junction diode .
The V.I characteristics differ change in doping concentration.
Anode
Cathode
Fig. 1.21 Zener Diode Symbol
Forward bias V.I characteristics is identical to PN junction diode
characterisitics. In reverse bias at constant voltage current starts to increase
from minimum to maxium corresponding point is known as Zener break down
point. Corresponding voltage, currentt is known as Zener voltage current. Due
to this V.I. characteristics zener is used as a voltage regulator.
(b) Light Emiting Diode (LED)
This is made with Gallium Arsanide (GaA) or Gallium Phosphate (GaP).
LED when it is given forward bias the junction heats just like filament, it glows
light energy comes. Hence it is known as Light Emiting Diode.
.
Anode
Cathode
Fig. 1.22 Light Emitting Diode Symbol
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(c) Photo Diode
This diode is made with Cadmium Sulphide. When light falls on the junction
of the photo diode it conducts. Hence it is known as photodiode.
Applications :
1) Used in light flashes system.
2) Cameras.
Fig. 1.22 Photo Diode
(d) Varacter Diode
This diode is designed to work at high frequencies. It works depends on
the voltage applied across the junction resulting junction capacitance changes
in high frequencies.
Application : Used in high frequency tune in T.V. tuners.
Anode
Cathode
Fig. 1.23 Varacter Diode
(e) Tunnel diode
More doping is used in Tunnel diodes. This diodes offers positive / negative
resistance characteristics.
Anode
Cathode
Fig. 1.24 Tunnel Diode
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Fig. 1.25
1.6 Diode Manufacturers Specification
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Fig. 1.26
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Fig. 1.28
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Fig. 1.27
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Fig. 1.30
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Fig. 1.29
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Fig. 1.31
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Fig. 1.32 P N Junction Diode
Epoxy lens/case
Wire bond
Reflective cavity
Semiconductor die
Anvil
Post
Lead frame
Flat spot
Anode
Cathode
Fig. Photo Diode 1.33
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Fig. 1.34 Zener Diode VI Characteristics
Fig. 1.35 Photodiode VI Characteristics
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Anode
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Cathode
Fig. 1.36 Varacter Diode
Fig. 1.37 Tunnel Diode Symbol
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1.7 Transistor
A transistor is a three layer semiconductor, three electrodes device is known
as Transistor. The transisters are two types.
(1) P NP Transistor
(2) NPN Transistor
Collector
Collector
Base
P
N
Base
N
P
N
P
Emitter
Emitter
Fig. 1.40 PNP Transistor
Fig. 1.41 NPN Transistor
In a transistor base is thinly doped, collector is moderately doped and
emitter is heavily doped. The PNP transistor is used where the current gain is
less than 10. the NPN transistors are used where current gain is more than
40.-200.
A transistor is three semiconductor layers, three electrodes, two junction
device is a transistor.In the transister transformation of resitance takes place
from input to output.
In a transistor generally base to emitter junction is forward biased and
collector to emitter junction is reverse biased.
Based on transistor three electrodes commanly connected there are three
types of configurations.They are .as follows
(1) Common Base Transistor amplifier Configurations - CB
(2) Common Emitter Transistor amplifier Configurations - CE
(3) Common Collector Transistor amplifier configuration - CC
In the transistor the arrow mark shows, current carrying direction. In PNP
transistor the arrow mark is inside the junction. In NPN transistor the arrow
marks is outside direction which shown in the figure.
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1.8 Working of PNP and NPN Transistor
In a transistor emitter is heavily doped, collector is moderately droped and
base is thinly doped. A PNP transistor emitter to base junction is forward biased
and collector to base junction is reverse biased. Its emitter is connected to
positive and collector is connected to negative terminal of the dc power supply.
A small amount of negative cvoltage is appled to the base.
Emitter Junction
Collector Junction
Fig. 1.42 Common Base Transistor
In PNP transistor, due to majority carriers in P- type layer, the holes drift
from emitter to collector moves, consequently the equal number of free electrons
moving from emitter to collector thrugh dc power supply in the external circuit.
The number of holes drifting from emitter region to collector region is controlled
by the base bias and the transistor is used as an amplifier.
Working of a NPN Transistor
The NPN transistor emitter is connected negative terminal of the dc power
supply and the positive terminal of the dc power supply is connected to base.
Hence emitter base junctions is connected in forward bias mode. The collector
is connected to positive terminal conneted collector and negative connected to
base.
Fig. 1.43 Common Base Transistor
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Fig. 1.44 Common Base Transistor
The free electrons emitter attraction negative terminal. The emitter to
base offers low resistance and collector to base offers very high resistance.
Transistor Input / Output V-I Characteristics
In common base amplifier with changing emitter to base voltage slowly
corresponding IE mA changes. Draw the relation between VEB and IE mA in
graphical method.
Emitter to base Voltage
Fig. 1.45 CB Input VI Characteristics
It is shown in the curve with changing VEB keeping constant VEB = OV,
10V corresponding change is IE mA is noted and the curve between those two
parameters. The emitter current mA is taken on Y-axis and VEB is taken on Xaxis approximate readings are noted.
(i) The mitter current I increases rapdily with small increase in emitter base voltage VEB. It means the input resistance is small.
(ii) The IE is almost independent of VCB. The IE is independent of VCB.
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1.9 Current Amplication Factor 
It is the ratio change collector current to change in emitter current as small
signal current gain “ “ or CB amplifier current gain.
 =  IC / IE at constant VCB.
= IC / IE.
The values lies between 0.9 to 0.99 in a transistor.
Output V-I characteristics : The curve is drawn between VBE on X-axis and
IcmA on Y-axis keeping constant IE in the base to emitter junction. The following
points are noted.
(i) The ICmA varies with VCB only at very low voltages less than 1V.
(ii) When the VCB is from 1-2 V the Ic remains constant, which is horizontal
line.
Collector Current
(iii) A large change in VCB produces small change in Ic. Hence output
resistance is very high.
Base to Emitter Voltage
Fig. 1.46 Common Base Output VI Characteristics
Large Signal
Current amplification factor  : It is ratio of collector current to base
current.
IC / IB
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Reverse Saturation current ICEO : It is knwon as collector to emitter
reverse saturation current plays role when minority carriers comes into role.
Common collector amplifiers current gain r = It is the raio of emitter
current IE to IC collector current.
r = IE / I C
It is generally more than 1.
Relation between  and  : At junction of the transister current equation
as follows.
I E = I C + IB
(1)
or IB = IE - IC
CE amplifier current gain  is given by substituting
 = Ic / IB = IC / IE - IC
Dividing by IE both Numerator and denominator. right side of the above
equation
 = Ic / IE = IE / IE - IC / IE
 =  / 1-  .
 lies in a transistor from 40 to 200 depend upon current applicaiton factor.
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Fig. 1.47
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Fig. 1.48
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Fig. 1.49
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Fig. 1.50
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Fig. 1.51
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Fig. 1.52
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Fig. 1.53
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Fig. 1.54
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Fig. 1.55
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1.11 FET AND MOSFET WORKING
FET - Field Effect Transistor is a unipolar solid state semiconductor device.
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Fig. 1.56
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Fig. 1.57
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Drain
Gate
Gate
N Channel
P Channel
Source
Fig. 1.59 N chanel FET
P-Channel FET
The FET can fabricated N-Channel, P-Channel both channels. A narrow
N-type semiconductor is taken and diffused on opposite sides on mid points of
either sides of channels. The P layer behaves as PN junction diodes. Gates
and the area remained between two gates is known as Channels. These layers
are inernally shorted. These leads are joined called Drain “D” and Source “S”
are opposite sides of the channel. this devices posses very high input impedence.
FET : V-I Characteristics
In a N-channel FET, the source terminal is connected to the negative and
drain is connected positive of the dc power supply. The gate is always reverse
biased.
The following points are to be noted.
(i) At first, the drain current ID raises rapdily with VDS, but then becomes
constant. The VDS above which ID becomes contant is known as Pinch-off
Voltage. The path OA is pinch off voltage.
(ii) Increase in ID is very small with VDS above pinch off voltage. Consequently ID remains constant.
(iii) The V-I characteristics are identical to pentoid valve.
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Fig. 1.60 VI FET V-I characteristics
1.12 ADVANTAGES OF FET
(1) High input impedence.
(2) Small size, rugged and long life.
(3) Low noise, good high frequency response.
(4) Better thermal stability.
(5) High power gain.
(6) It is unipolar device.
Applications:
(1) Input stage is amplifiers, CRO’s and other electronic instrucments.
(2) In logic circuits.
(3) As unixer stage is Fm radio and T.V. receiver.
(4) In computers for large scale integration LSI and memory circuit.
1.13 Understanding the Naming and Convention of Semi
conductor Components
The semiconductor devices namings of electrodes are taken from
manufactureres data sheets that are taken as convention to identify the various
electrodes in active devices.
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Fig. 1.61
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Fig. 1.62
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Fig. 1.63
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Fig. 1.64
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Fig. 1.65
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Fig. 1.66
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Fig. 1.67
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Fig. 1.68
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Fig. 1.69
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Fig. 1.70
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Fig. 1.71
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Fig. 1.72
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Fig. 1.73
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Fig. 1.74
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Fig. 1.75
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Fig. 1.76
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Fig. 1.76
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Fig. 1.77
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Fig. 1.78
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Fig. 1.79
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Fig. 1.80
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Fig. 1.81
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Fig. .182
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Fig. .183
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Fig. 1.84
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Fig. 1.85
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Summary
Materials are three types
1. Conductors : The material, which passes electrical current though it is
known as Conductors.
2. Semiconductors : The materials, which conducts electricity partially at
lower temperatures, with rise in every degree temperature conductivity
increases in Semiconductors.
3. Insulaters : The materials, which does not conduct eletric current is known
as insulaters.
4. Intrinsic Semiconductors are two types, P-type intrinsic semiconductor,
N-type intrisic semiconductors.
5. Intrinsic Semiconductors : The pure form of semiconductor is known as
intrinsic semiconductors.
6. P-type Intrinsic semiconductor : It carries only holes, no electrons is
known as P-type intrinsic semiconductors.
7. N-type Intrinsic Semiconductor : It carries only electrons, no holes is
known as N-type intrinsic semiconductors.
8. Doping : The process of adding impurities to pure form of semiconductors
is known as dopoing.
9. P-Type Extrinsic Semiconductor : It carries majority carriers are holes,
minority charge carriers are electrons is known as P-type intrinsic
semiconductors.
10. N type extrinsic semiconductor : It carries majority carrieres are
electrons and minority carriers are holes is known as N-type extrinsic
semiconfuctors.
11. PN Junction Diode : A piece of P - type material , a piece of N-type
material joined at higher temperature 5000oC and electrodes are connected to
each layer is known as PN junction doide.
12. Transistor : Three layers, three electronodes device is a transistor. The
terminals are base, collector and emitter. Two types (a) PNP Transistor b)
NPN transistor.
Application : - Used as switch and amplifiers.
13.  = IC / IE,  =IC / IB, b =  /1- - Relation
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14. FET : Field Effect Transistor is a Unipolar, 3 electrodes device, had
high input resistance and it is voltage sensitive device.
15. MOSFET : Metal Oxide Semiconductor Field Effect Transistor. Two
types N-Channel, P-Channel MOSFET - High input impedence upto 1,00,000
M Ohms ,Voltage sensitive device - Used in memories.
16. Data Sheet of Semiconductors : It describes, physical and V-I
characteristics, operating temperatures, power ratings, band width. Easily
identification standard numbering - Alternative numbers, which were equivalents.
Short Answer Type Questions
1. Write classification of material ?
2. Define ionic, covelent, coordinate covelent bond.
3. Write number electrons in an atom ?
4. Define intrinsic semiconductor.
5. Define doping .
6. Define extrinsic semiconductor.
7. What are applications of PN junction diode ?
8. What are the applications of Zonar, Photodiodes ?
9. What are the applications of Varacter, Tunnel diodes ?
10. What are the uses of semiconductor data sheet ?
11. Draw the symbol of Transister.
12. Define , , r ?
13. Write relation between  and .
14. What is converter ?
15. Mention the types of rectifier circuits ?
16. Define average, RMS value of an a.c ?
17. Define efficiency of a rectifier ?
18. Define ripple factor.
19. Mention the ripple value of half / full wave rectifier.
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20. Mention the type of filters used in rectifiers ?
21. Define voltage regulation.
22. Mention the types of regulator circuits ?
23. What is transister biasing ?
24. Mention the types of transister biasing methods.
25. Define stabilization of an amplifier.
26. Mention the different regions of output transister V-I characteristics.
27. Mention the transistor configurations.
28. What are the types of power amplifiers ?
29. Define efficiency of power amplifier.
30. What are advantages of class B push pull amplifier.
31. Write applications power amplifiers.
32. Mention the IC nos used in power amplifiers.
Long Answer Type Questions
1. Draw and explain energy band diagrams.
2. Draw and explain formation of PN junction diode V-I characteristics
with neat graph.
3. Explain transister CB input / output - V-I characteristics with neat
diagram.
4. Draw and explain CE amplifier input / output V-I characteristics.
5. Derive relation between , and .
6. Draw and explain FET - VI. Characteristics.
OJT /Practical Questions
1. Study of PN Junction diode, VI characteristics and applicaitons.
2. Study of applications of Zener diode, LED, Photo diode, Verector diode
and tunnel diode. Identification of terminals and applicaitons.
3. Study of Transistor, VI Characteristics, Datasheets and applications.
4. Study of FET, MOSFET, VI characteristics, data sheets and applications.
UNIT
2
Power Supplies and Filters
Learning Objectives
• Definitions of AC , DC power supply.
• Block diagram of DC power supply.
• Study of half wave rectifier with filter and applicaiton.
• Definitions of RMS , average, ripple factor and efficiency of half wave
rectifier.
• Study of full wave rectifier with filter and applicaitons.
• Study of bridge rectifier with filter and its applications.
• Study of comparisions of rectifiers circuits.
• Study of filters, Capacitor Inductor Filter, Capacitor Inductor Capacitor
filter.
• Definition of voltage regulations, Series and Shunt regulator by using
zener diode and its applicaitons.
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2.0 Introduction
Every electronic circuit is used DC power supply. The DC power supply is
obtained from converting AC supply into DC supply. The process of conversion
of AC supply into DC power supply is known as Convertors. The AC supply
is used for electroplating, electro typing, electro metal refining, arc lamp, battery
charging, electromagnetic, valve or transistorized electronic equipments. The
main blocks of DC power supply are a step down transformer, a rectifier circuit,
a filter circuit and regulator.
2.1 DC Power Supply
AC power is converted into dc power is known as converters. Every
electronic circuit consist of 30% circuit is dc power supply circuit with different
current /voltage / power ratings as per the requirement and specifications of the
customer needs and specific applicaitons. The dc power supply consist of the
following blocks. They are as follows.
AC
Mains
230V / 50HZ
Stepdown
Transformer
Rectifiers
Filters
Regulators
L
O
A
D
Fig. 2.01
Blocks are (1) AC main supply 230 V / 50 Hz.
(2) Step down transformer.
(3) Rectifiers
(4) Filters
(5) Regulators
(6) Loads
(i) AC Main supply 230V / 50 Hz
World wide accepted standard line voltage specifications are 115V /
60Hz and 230 V / 50 Hz. In India adopted ac power transmission is 230 V /
50 Hz. as per the our power requirement of application, is drawn from the ac
mains.
Paper - II Electronic Devices and Circuits
217
(ii) Step - Down Transformers
The transformer main function is to draw the ac power from primary to
secondary at constant. Transformers are desinged depend up on current
transfered from the windings frequency.
Fig. 2.2 Transformer Symbol.
Transformer primary consists of
Inductance
= Lp - H
No. of turns
= Np
Voltage
= Vp - V
Current
= Ip - A
Transformer secondary consists of
Inductance
= Ls - H
No. of turns
= Ns
Voltage
= Vs -V
Current
= Is - A
Fig. 2.3 Bawin used for Copper Winding
Working : As soon as ac supply is given to the primary winding of the
transformer, flux induces in the first turn of the primary windings links, to second
turn, process continuous and flux links entire turns of the winding.
Faradays First Law : Whenever flux links the winding an emf is induced.
Faradays Second Law : The rate of change of flux linkages is equal to the
induced emf.
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The emf is induced in the primary winding of the transformer. By mutual
induction flux flows from winding to the secondary winding through magnetic
core.
Fig. 2.4 Transformer Core and Windings
The flux induces in the secondary winding of the transformer from first turn,
continuous process links to second turn and entire turns of secondary winding.
As above Faraday’s laws of electromagneic induction emf and current induces
which is an a.c. power. The voltage ratings are 24V, 20V, 18V, 12V, 9V, 6V,
4.5V, 3V, 1.5V with different current ratings 1A, 500mA, 300mA, 250mA,
200 mA, 150 mA, 100mA etc.
(iii) Rectifiers
It is the circuit in which the a.c. sine wave rectifies as pulsating positive half
cycles are rectified and it depends upon rectifier circuits. The rectifiers are three
types, they are
1. Half Wave rectifier
2. Full wave rectifier.
3. Bridge rectifier
above rectifiers are discussed in separate is rectifier circuits.
(iv) Filters
A filter circuit is one which converts / filters the pulsating positive half cycles
Paper - II Electronic Devices and Circuits
219
into dc voltage. The dc voltage obtined is output is with ripple. Now the types
of filter circuits are
1. Capacitor Input filter
2. Capacitor, Inductor filter or LC -type filter.
3. Capacitor, Inductor, Capacitor filter or T-type filters II- type filters.
(v) Regulator
The regulater is one in which it gives output constant dc voltage.
Defination : It is ratio of difference of no load voltage tothe load voltage to
no load voltage is known as voltage regulator. The types of voltage regulaters
are
(a) Zener regulaters (series and shunt type)
(b) IC regulaters.
(vii) Load
It is an electronic system in which it takes a dc power in generally known as
load. The loads are three types.
(a) Resistive load
(b) Capacitive load
(c) Inductive load.
b,c are used in communication networks.
2.2 Halfwave Rectifier
It is the rectifier circuit in which half of the ac sine wave conducts and
produces, remaining negative half cycle could not conduct is known as half
wave is known as half wave rectifier. The circuit is as follows.
Fig. 2.5 Half Wave Rectifier
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The half wave rectifier conducts only positive half cycles of ac sine wave
because forward biased. Whenever a negative half cycle receives the diodes
goes to reverse bias mode hence there is no conduction.
Fig. 2.6 Half Wave Rectifier wave forms
The electrolyte capacitor 1000 mF/ 25V filters the half of the wave gaves
dc voltage with 120% ripple.
This type rectifiers are used in atomic power units.
Input voltage to the rectifier
Vi = Vm Sin = Vm Sin2  ft
Im = Vm / Rs + Rl.
Irms = Im / 2
rms value of half wave.
Average value or dc value half wave rectifier
Idc = Im / 
Ripple factor =
=
Irms
Idc
=
Irms2 - I2dc
Idc
Im/2
Im/ 
Idc
2
-1
Irms
=
2
=

2
2
-1 - 1.21
-1
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221
Efficiency of half wave rectifier.
= Pdc / Pac = (Vm / (Rs+RL))2 RL / Vm2 4(Rs+RL)
= Pdc / Pac = 4 / 2 (1+Rs/RL) x 100 = 40.6 / 1 +Rs/RL
2.3 Center Tapped Fullwave Rectifier
Center tapping of the transformer, it inverts the polarity at tappings. Hence
it doubles the voltage and frequency.
Working : At point A, the positive half cycles receives the diode D, and it is
in forward bias mode conducts positive half cycle between time period 0 to 
the negative half cycle receives the D1 goes to reverse bias mode. Hence D1
wont conducts at the same time, at the time period 0 - , D2 is in forward bias
mode, conducts  - 2 period. Hence in first cycle D1 conducts 0- times
and D2 conducts  - 2 time. Hence full wave conduction takes place. This
process repeats remaining cycles. The circuit operation is one cycle full two half
conducts called fullwave rectification. With two positive half cycles of operations.
even
1/2
AC
Input
cycles
Current flows when D1
conducts
odd
1/2
cycles
Current flows when D2
conducts
Resultant Output Waveform
Fig. 2.7 Full Wave rectifier with wave forms
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2.4 Bridge Rectifier
In bridge rectifier circuit, the center tapping is eliminated. Conduction of
this circuit is identical to fullwave center tapped transformer rectifier circuit.
In working the time period 0-, two diodes D1 and D2 conducts ‘t’
receives the +ve half cycle of the wave. The D1, D2 moves into forward bias
mode and it conducts. During time period  -2 the D1 and D2 goes to reverse
bias mode, no conduction. at the same time D3, D4 conducts because it moves
into forward bias mode. Conducts  - 2, period in 1st cycle. This operation
repeats entire cycles during the operation. Hence the circuit is known as full
wave bridge rectifier.
In calculations such as Irms, Idc, Pdc, Pac, ripple, efficiency calculations
are identical to full name rectifier.
AC
Input
230V
50HZ
Ripples
C Charges
C Discharges
V
Resultant Output Waveform
Waveform with
capacitor
Waveform
without
capacitor
Fig. 2.8 Bridge Rectifier
2.5 RMS Value of Fullwave Rectifier Irms = Im / 2
Average value Idc = Im /  for half wave rectifier
Irms / Idc =  / 2 = 1.57 and Im / Idc =  = 3.14
Ripple factor F.W rectifier
Idc = 2 Im /  ; Irms = Im / 2
Irms / Idc = / 2 2 = 1.11
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Ripple Factor =
223
(Irms / Idc)2 -1 x 100
= 1.112 - 1 x 100 = 48%
Efficiency of FW rectifier = Pdc / Pac x 100
= (4 Vm2 / 2( Rs + RL)2) / Vm2 / 2 (Rs + RL)
= 81.2 / 1 + Rs/ RL = 81.2
2.6 Comparision of Rectifier Circuits
Circuit
Vdc
Vrms/ Vdc Vm/ Vdc
Average dc rms volts Peak Volt
Volt Output Output
Output
Heat Wave
PIV
Ripple
factor
Efficiency
Maximum
1
1.57
3.14
1.57
121%
40.6
Full wave
Rectifier
C.T
1
1.11
1.57
3.14
48%
81.2
FW Bridge
1
1.11
1.57
1.57
48%
81.2
Rectifier
Rectifier
2.7 Filters
Defination : It is the circuit in which it converts the pulsating the positive half
cycles into dc voltage is known as filter circuit. The filter circuits are follows.
(i) Inductor fulter - L type filter
(ii) Condenser Inductor input filter - CL input filter
(iii) Inductor Condenser filter - LCL or CLC , filter T-type and P - type
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2.8 Operation of Rectifier Circuit Using
Capacitor Input.Series Inductor and clc Filters
(i) L-type Filter
This type of filter is used, when it required large currents with low voltages
power supplies this filter circuit is used. For better regulation a shunted capaciter
is used. Hence it is called LC type filter.
Fig. 2.9 L-Type Filter
(ii) Capacitor input filter
This type filter is used for high voltage, low dc current requirement
applications.
Fig. 2.10 Capacitor Input Filter
(iii) LC Filter
For heavy current loads an LC filter is used. It also gives better regulation
at high load current. Followed C shunted capacitor provides high output voltage
and reduced ripple.
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225
Fig. 2.10 LC Filter
Fig. 2.11 LC Filter
2.9 Voltage Regulation
It is ratio of difference of load to Vdc full load to Vdc full load multiplied
by 100 is known as voltage regulation.
Voltage Regulation = Vdc no load - Vdc full load / Vdc full load x 100
2.10 Zener Regulation
A zener-diode is used to get constant output DC voltage to the load from
source. The zener diode is connected shunt path across the load, because a
zener diode breaks at constant voltage is reverse bias mode. The Rs is a series,
resistant connected in the circuit to absorb load fluxations. Hence the output
voltage does not change kept constant as Vz.
Fig. 2.12 Zener Regulater
The zener diode, when it is connected in reverse bias mode, at constant
voltage Vz, the zener current only changes from minimum to maximum, without
change in Vz the load resistance may change. However, shunted zener gives
output constant voltage irrespective of intenal current changes. Hence the zener
is used as voltage regulator.
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Let V1 = Input voltage to Regulator
Vo = Output voltage
Current through R is = Iz + IL
R can calculated as from ohms law.
R = E1 - Eo / Iz + IL.
Switched Mode Power Supply
By using switched mode diodes such IN 4001 and IN4007 connecting in
the form of bridge dc power may be generated.
The diodes basically two types.
1. Rectifying diodes - Conducts heavy current in forward and a low current
in reverse bias mode. By using these diodes series regulated power supplies
(SRPS) are assembled as per the customer ratings requirements. Later the
positive half cycles are filtered and regulated.
2. Switched mode Diodes - These diodes work in forward bias mode. In
reverser bias mode does not work and act as switch off mode. By using switched
mode diodes bridge ac line voltage is rectified, to obtain pulsating positive half
cycles. (b) The pulsating the positive half cycles are filtered by using suitable
filtering capaciors. (c) The filtered dc voltage is equal to 230 2 = 324 V. Then
it is given switching transister. This device convers dc voltage in to square wave.
(d) The square wave is given to pulse transformer primary winding. (e) One part
of the primary winding is given error amplifier to get constant voltage secondary
of the pulse transformer. Irrespective input variations a constant output is obtained
from the secondary of the pulse transformer. (f) Taking number tappings from
the secondary of the pulse transformer output voltages are rectified and taken
as dc supply voltages.
Fig. 2.13 SMPS
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227
Summary
The vocational EET student note that any electronic circuit consists of mainly
depends on dc power. how it is generated. The main types conversion from ac
to dc power circuit is known as converters. The converter circuits are mainly
two types. they are
(1) Series regulated power supplies (SRPS)
(2) Switched mode power supplies - (SMPS)
To generated higher power SRPS is costly and more weight, but SMPS is
light weight low cast. Hence in consumer eletronics smps is used.
Powre Supply : It is the electornic circuit which converts a.c. supply into
d.c supply.
Rectifier : Rectifiers are three types (a) Half wave rectifier (b) Full wave
rectifier (3) bridge rectifier.
RMS Value : It is the value is taken some of the square root of squares of
the voltages or currents divided number of parts average value in RMS value
D.C. Value : It is average value of a.d.c quantity for half wave rectifier
average value = maximum value /  .
Ripple Factor : It is the ratio of a.c. content presenting d.c. output to d.c.
output multiplied by 100 is known as Ripple factor.
Efficiency : It is the ratio of output power to input power. For half wave, full
wave rectifiers the efficiences are 40.6%, 81.2% respectively.
Filter : By using an electrolytic capacitor the pulsating half waves are
convered into d.c. voltage.
Voltage Regulator : It is the ratio of difference of no load voltage to load
voltage to no voltage nultiplied by 100. Generally for small power applicaitons
zener diode using as voltage regulator.
Short Answer Type Questions
1. Define (dc) power supply or converter ?
2. Mention the types of rectifier circuits.
3. How many diodes are used in HW, FW, BR ?
4. Write average values of HW, FW rectifiers.
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Electronics Engineering Technician
5. Define ripple factor.
6. Define efficiency of rectifier (HW / FW) ?
7. Mention the types of filter circuits.
8. Define voltage regulation.
Long Answer Type Questions
1. Draw the block diagram of power supply. Explain working of each block.
2. Draw and explain working of half wave rectifier.
3. Draw and explain working full wave rectifier.
4. Draw and explain working of bridge rectifier.
5. Write comparisions of rectifier circuits.
OJT / Practical Questions
1. Study of power supplies(AC / DC).
2. Study of halfwave rectifier with filter and applications.
3. Study of full wave rectifier with filter and regulators applications.
4. Study of Bridge rectifier with filter and regulator applications.
5. Study of Filter circuits.
6. Study of regulator ( by using Zener diode).
UNIT
3
Small Signal Amplifiers
Learning Objectives
• Definition of transistor biasing
• Study of types of transistor biasing, - base emmitter biasing,
Collector feedback resistor biasing, Voltage divider biasing.
• Definition of stabilization
• Comparision of amplifier circuits.
• CC , CB and CE
• Study of transistor output to VI characteristics drawing load line,
operating point, fixing - cut of region, saturation region, and active region
identification.
3.0 Introduction
A properly biased transistor raises the strength of a weak signal to strengthen
signal and thus acts as an amplifier. Almost all electronic equipments must include
means for amplification of electrical signals. For instance radio receivers amplify
very weak signals sometimes a few millionth of a volt at antenna until they are
strong enough to fill a room with sound. The transducers are used in the medical
and scientific investigations generate signals in the microvolt ( V) and milli volt
(mV) range. These signals must be amplified thousands and million times before
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Electronics Engineering Technician
they will be strong enough to operate indicating instruments. Therefore
electronic amplifiers are a constant and important ingredient of electronic systems
3.1 Proper Biasing in Amplifier Circuit and List of Biasing
The transistor basic formation is top strength weak signal into strengthen
signal. In the transistor current transfer from low resistance input to high resistance
output. The transformation resistance takes place when current flows from input
to output, hence device is known as transistor. The small signal amplifier are
working input low voltages up to 2V and output voltages maximum up to 10V.in
amplifier input signal is very low. These low voltage signal are also amplifier.
The transistor amplifier circuit taking on of the terminal is common to both
input and output is three types. They are
(1) Common Base Amplifier – CB amplifier
(2) Common emitter amplifier- CE amplifier
(3) Common collector amplifier-CC amplifier.
Transistor Biasing : The process of giving supply to both input and output is
known as transistor biasing. The transistor biasing it as to fulfill the following
condition.
(1) Minimum VBE = 0.7 V Silicon transister
= 0.3 V Germanium Transister
(2) Minimum VCE = 0.2 V Both Si, Ge
(3) Zero signal Collector.
When above three biasing conditions are fulfilled by the transister works as
an amplifier.
There are three types of transistor biasing. They are as follows.
(1) Base Emitter Biasing
(2) Collector feedback resister biasing
(3) Voltage divider biasing
Base Emitter Biasing: In Base Emitter Biasing transistor amplifier circuit is
used two de power supplies are Vec,Vbb,
Keeping Vcc =OV, 5V, 10V Vbb is valid from OV to 2V for OV =Vec, there
is no change in IB from 0 A
Paper - II Electronic Devices and Circuits
231
For change Vec=5V constant vary the VBB from OV to 2V corresponding Ic
and VCE is noted. the VBE =0.7V Si, 0.3 Ge transister ,the device comes from
cut off condition to condition region. Vce also exceeds0.3V and reaches zero
signal collector current, when these three conditions are fulfilled, the transistor
works as an amplifier.
Fig. 3.1 Transistor Biasing
The conditions are
i) Minimum VBE = 0.7V for Si transistor
=0.3V for Ge transistor
ii) Minimum VCE= 0.3V for both side Ge transistor
iii) Fulfill zero signal collector current IC.
2. Collector feedback Resister biasing: This type of amplifier needs only
single dc power supply. The circuit is shown in Fig.
As soon as supply Vcc is given to the circuit, base current IB comes through
Rcb to the base of the transistor. Transistor VBE= 0-7V reaches comes into
conductor region. At the junction current Ic flows consider as zero signal
collector current. The Ic flows across the collector to emitter voltage Vce
exceeds 0.3V comes the junction into conductor region .The transistor fulfilled
biasing conditions, works as an amplifier.
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Fig. 3.2 Collector Feedback Resister Biasing
While designing Rcb, the base flow is taken into account.
3.Voltage Divider Biasing :
In shown is said to be voltage divide biasing. The R1 R2 resistors are designed
1:10 ratio. Most of the current passes through R2 small amount of current flows
through R, The difference of the current i.e. IR2 – IR1 flows to the base of the
transistor.
Fig. 3.3 Voltage Divider Biasing
IB reaches base of the transistor the base junction voltage VBE exceeds
07V comes the input junction into conduction region. Ic flows through the collector
to emitter junction comes into conduction region and exceeds VCE =0.3V. The
Paper - II Electronic Devices and Circuits
233
transistor fulfill biasing conditions i.e. VBE=0.7,VCE=0.3V and zero signal collector
current. In this biasing mode the transistor works as an amplifier.
3.2 Stabilization
In small signal amplifier the transistor fulfilled biasing condition i.e. VBE=0.7V
VCE=0.3V and zero signal collector. supply ratings should be remaining constant.
Keeping constant supply ratings, junction temperature plays important role. The
variation IB,IC,VBE,VCE. influences change of temperatures. The large signal
current gain < variation takes place. In this context transistor parameters are to
be keeping in our mind. And V-I output characteristics.
In small signal amplifier operating point Q, Q signal swing varies from Q1 to
Q2.At Q1 operating allows low voltage more current At Q2 low current, more
voltage. In this context  value changes 10% same as value. Silicon transistor
operates -65oc to 120oc. Generating transistor operates -65oc to 65oc. at
particular higher temperatures reverse saturation junction current flows in between
two layers of the transistor.
Fig. 3.4 Voltage Divider Biasing
Stabilization
It is the point in which a small variations at input and output ratings, small
change in temperature, gains also varies but operating swing should not exceed.
Transistor acts as amplifier.
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The transistor output V-I characteristics are divided into three regions. They
are
(1) Cut-off region
(2) Active region
(3) Saturation region
In cut-off region the transistor acts as off-switch. Base to emitter, collector
to emitter Junctions acts as reverse bias mode.
In active region Base to emitter Junction is in forward biasing and collector
to emitter junction acts as reverse bias mode. In this mode the transistor works
as an amplifier.
In saturation region both junction are forward biased, a high saturation
current reaches a small amount VCE voltages.
3.3 Classification of Amplifier
According to frequency, mode of operation, type and methods of coupling,
R,C. coupled ,Transformer coupled and directly coupled.
Classification of Amplifier
Various type of amplifier circuit can be classified on the following four bases.
1.On The Base of Frequency:
(i) A.F Amplifier
(ii) R.F Amplifier
(iii) I.F Amplifier
(iv) VideoAmplifier
2. On the base of ability (mode of operation)
(i) Class ‘A’Amplifier
(ii) Class ‘B’Amplifier
(iii) Class ‘AB’Amplifier
(iv) Class ‘C’Amplifier
3. On the base of coupling
(i) RC Coupled Amplifier
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235
(ii) LC Coupled Amplifier
(iii) Transformer Coupled Amplifier
(iv) Direct Coupled Amplifier
4. On the base of Power.
(i) Voltage Amplifier
(ii) Power Amplifier
3.4 Know the Frequency Response,Gain of The Above
Amplifier
When a single amplifier stage cannot produce sufficient amplification, two or
more stages are coupled together for this purpose. The method of applying the
output signal of the first amplifier stage of the input circuit of second stage is
called coupling.
Fig. 3.5 1.R.C.Coupled Amplifier
In the R-C coupling the signal is performed by employing two resistors and
a capacitor that is why it is called R-C coupling. This method is most economical
and amplifies a wide frequency. The resistor Rc is the load resistor which acts as
collector load resistor for the first transistor. The ac component of first transistor
reaches to the base of second transistor through coupling capacitor Cc. The
capacitive reactance of the coupling capacitor should be lesser than the load
resistance, otherwise, the ac component of the signal will also pass through the
load resistor. The coupling capacitor also prevents the d.c. voltage to reach the
base of the second transistor and does not allow the later to become over loaded.
DIAGRAM:
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Electronics Engineering Technician
Hence, it is also known as blocking capacitor. An R-C coupled Amplifier is
shown in the above fig.
Frequency response FIG:
Fig. 3.6 Frequency Response
The fig shows the frequency response of a typical R-C coupled Amplifier. It
is clear that the voltage drops at low (<50Hz) and high (>20KHz) frequencies
whereas it is uniform over mid - frequency range (50Hz to 20KHz).
At low frequencies (<50Hz), the reactance of coupling capacitor Cc is quite
high and hence very small part of a signal will pass from one stage to the next
stage.
At high frequencies (>20KHz), the reactance of Cc is very small and it
behaves as a short circuit. This increase the loading effect of next stage and
serves to reduce the voltage gain.
At mid frequencies (50Hz to 20KHz), the voltage gain of the Amplifier is
constant. The effect of coupling capacitor in this frequency range is such so as to
maintain a uniform voltage gain.
Applications:They are widely used as voltage amplifier i.e. in the initial stages
of public address system. It is cheap and provides excellent audio fidelity over a
wide range of frequency.
Paper - II Electronic Devices and Circuits
237
Transformer Coupled Amplifier
Fig. 3.7 Transformer - Coupled Amplifier
ga
in
In the transformer coupling method an interstate or a driver transformer is
employed for coupling. The primary winding of the transformer acts as an
inductive load for the first transistor and the secondary winding acts as the signal
source for the second transistor.
Frequency
Fig. 3.8 Frequency response
The frequency response of a transformer couples amplifier is shown in the
above fig. It is clear that frequency response is rather poor i.e. gain is constant
only over a small range of frequency.
The output voltage is equal to the collector current multiplied by reactance of
primary. At low frequencies, the reactance of primary begins to fall, resulting in
decreased gain. At high frequencies, the capacitance between turns of windings
acts as a bypass condenser to reduce the output voltage and hence gain.
Therefore there will be disproportionate amplification of frequencies in a complete
signal such as music, speech etc. Hence, transformer – coupled amplifier
introduces frequency distortion.
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Electronics Engineering Technician
Applications
Transformer coupling is mostly employed for impedance matching. In general,
the last stage of a multistage amplifier is the power stage. Here, a concentrated
effort is made to transfer maximum power to the output device e.g. a loud speaker.
For maximum power transfer, the impedance of power source should be equal
to that of load. Transformer coupled amplifier is used for power amplification.
Direct Coupled Amplifier
The latest method of coupling is direct coupling. Its circuit is very simple as
shown in the fig. In this method, the collector of the first transistor is directly
connected to the base of the second transistor. Hence, D.C. will be present on
the base too.
The advantage of direct coupling are – no distortion and uniform response
over a wide frequency range. These circuits are used for very low frequency
amplification purposes.
Fig. 3.9 Direct Coupled Amplifier
Applications
The transformer coupled amplifiers are used for amplifying extremely low
frequencies (as low as a fraction of a Hertz).
Frequency response of the amplifier:
Fig shows the frequency response of a typical R- C coupled amplifier. It is
clear that voltage gain drops at off low (<50Hz) and high (>20KHz) frequencies
where as it is uniform over mid frequency range (50Hz to 20KHz). This behavior
of the amplifier is briefly explained below:
Paper - II Electronic Devices and Circuits
239
(i) At low frequencies ((<50Hz), the reactance of coupling capacitor Cc is
quite high and hence very small part of signal will pass from one stage
to the next stage. Moreover, CE cannot shunt the emitter resistance RE
effectively because of its large reactance at low frequencies. These two
factors cause a falling off voltage gain at low frequencies.
(ii) At high frequencies (>20KHz), the reactance of CC is very small and it
behaves as a short circuit. This increases the loading effect of next
stage and serve to reduce the voltage gain. Moreover, at high frequency,
capacitive reactance of base-emitter junction is low which increase the
base current. This reduces the current amplification factor (beta symbol).
Due to these two reasons, the voltage gain drops off at high frequency.
(iii) At mid-frequencies (50Hz to 20KHz), the voltage gain of the amplifier is
constant. The effect of coupling capacitor in this frequency range is such
so as to maintain a uniform voltage gain. Thus, as the frequency increases
in this range, reactance of CC decreases which tends to increase the
gain. However, at the same time, lower reactance means higher loading
of first stage and hence lower gain. These two factors almost cancel
each other, resulting in a uniform gain at mid frequency.
Advantages
(i) It has excellent frequency response. The gain in constant over audio
frequency range which is the region of most importance for speech,
music etc.
(ii) It has lower cost since it has employs resistor and capacitors which are
cheap.
(iii) The circuit is very compact as the modern resistors and capacitors are
small and extremely light.
Disadvantages
(i) The R-C coupled amplifier have low voltage and power gain. It is because
the low resistance presented by the input of each stage to the preceding
stage decreases the effective load resistance (RAC) and hence the gain.
(ii) They have the tendency to become noisy with age, particularly in moisture
climates.
(iii) Impedance matching is poor. It is because the output impedance of R-C
coupled amplifier is several hundred ohms whereas that of a speaker is
only a few ohms. Hence, little power will be transferred to the speaker.
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Applications
The R-C coupled amplifier excellent audio fidelity over a wide range of
frequency. Therefore, they are widely used as voltage amplifier e.g. in the initial
stages of public address system. If other type of coupling (e.g. transformer
coupling) is employed in the initial stages, this results in frequency distortion
which may be amplifier in next stages. However, because of poor impedance
matching, R-C coupling is rarely used in the final stages.
3.5 Comparision of CB,CE, and CC Amplifiers
S. No
Characteristics
CB
CE
CC
1.
Voltage Gain
100-200
300-600
<1
2.
Current Gain
<1
20-100
20-100
3.
Power Gain
Medium
High
100
4.
Input Impedance
5.
Output Impedance Very High
6.
Phase Inversion
7
Applications
Very Low
00
HF
Low
Very High
Medium
Very Low
1800
00
AF For Impedance matching
Methods of Transistor Biasing
The following are the most commonly used methods of obtaining transistor
biasing from one source of supply i.e. Vcc
(i) Base resistor method
(ii) Biasing with feedback resistor
(iii ) Voltage divider bias.
Stabilization
The collector current in a transistor changes rapidly when
(i) The temperature changes
(ii )The transistor is replaced by another of the same type.
This is due to the inherent variations of transistor parameters.
When the temperature changes or the transistor is replaced, the operating
point Ic and VCE also changes. However, the faithful amplification , it is essential
that operating point remains fixed.
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241
The process of making operating point independent of temperature changes
or variations in transistor parameters is known as stabilization. A good biasing
circuit always ensures the stabilization of operating point.
Summary
Amplifiers: There are transistor configurations. They are common base
amplifier, common collector amplifier, and common emitter amplifier.
Biasing: The process of giving supply to the transistor is known as biasing.
They are three types biasing (1) Base resistor biasing (2) Collector feedback
resistor biasing (3) Voltage divider biasing.
In a transistor biasing a transistor should fulfill following requirement to acts
as an amplifier.
(1) Zero signal collector current
(2) Minimum collector to emitter voltage VCE = 0.3 V
(3) Minimum VBE in the case of Silicon transistor 0.7 volts, Germanium
transistor 0.3 volts.
Stabilization: The proper operating point, reverse saturation currents and
large signal current gain important role in stabilization.
The Coupling Networks: In the transistor the coupling components used as
RC, LC, RL, Transformer and Direct Coupling.
Short Answer Type Questions
(1) What are the applications of CB amplifier.?
(2) What are the applications of CE amplifier.?
(3) What are the applications of CC amplifier.?
(4) What is stabilization.?
(5) Define stabilization.?
(6) Name the different types of coupling networks.?
Long Answer Type Questions
(1) Compare the characteristics of CB, CE, CC amplifiers.?
(2) Explaining biasing and stabilization of transistors.?
(3) Draw the two stage RC coupled amplifier explain working.?
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Electronics Engineering Technician
(4) Draw the transformer couple amplifier explain working.?
(5) Draw the direct couple amplifier explaining working.?
(6)Draw the frequency response curve of an RC coupled amplifier and explain
AF and RF frequency response.?
OJT / Practical Questions.
1. Study the transistor biasing methods and its applicaitons.
2. Study the stabilization
3. Study the transistor output VI characteristics, mark cut off region , active
region and saturation region, fixing of operating point on load line.
4. Study of comparision of CB, CC, CE configurations.
5. Study two stage RC coupled amplifier, working with gain versus logitherm
of frequency characters.
6. Study the two stage transformer couple amplifier with its applications.
7. Study the direct couple amplifier and its application.
UNIT
4
Power Amplifiers
Learning Objectives
• Study defination of Voltage,Power amplifiers.
• Study the differences between Voltage and Power amplifiers.
• Study the types of Power amplifiers-Class A,Class B,Class C,
Class B push pull .
• Study the Power amplifiers applications.
• Study the different ICs used in Power amplifiers.
4.0 Introduction
The circuit in which raises the strength of a weak signal is kown as amplifier.
Almost all electronic equipment must include means for amplifying electrical
signals. For instance, radio receivers amplify very weak signals. A practical
amplifier always consists of a number of stages that amplify a weak signal until
sufficient power is available, to operate to loudspeaker or other output devices.
The first few stages in the multistage amplifier have the function only voltage
amplification. However the last stage is designed to provide maximum power.
Therefore the final stage is power amplifier.
In some applications, feedback technique is used to alter some of the
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properties like gain, bandwidth, input and output impedances of the amplifer.
The amplifier which employs the feedback technique is known as feedback
amplifiers. An opreational amplifier is basically a direct coupled high gain amplifer
with feedback available in form of integrated circuit.
The object of this chapter is to study the different types of power amplifiers,
feedback amplifier and different applications of operational amplifier.
Definition of Power Amplifier:
A transistor amplifier in which raises the power level of the signals that
have audio frequency range is known as transistor power amplifier. In general
the last stage of multistage amplifier is the power stage. A power amplifier differs
from the voltage amplifier. A transistor that is suitable for power amplification is
generally called as power transistor.
Difference between Voltage and Power Amplifier
A voltage amplifier is designed to have maximum voltage amplification.
However, there is no importance of power amplification. On the other hand
power amplifier is designed to achieve maximum power output.
4.1 Voltage Amplifier
An electronic circuit whose function is to accept an input voltage and
produce a magnified, voltage as an output voltage. The voltage gain of the
amplifier is the amplitude ratio of the output voltage to the input voltage.
Voltage amplifiers are distinguished from other categories of amplifiers whose
ability to amplify voltages, or lack thereof, is of secondary importance. Amplifiers
in other categories usually are designed to deliver to power gain or to isolate
one part of a circuit from another. Power amplifiers may or may not have voltage
gain, while buffers and emitter followers generally produce power gain without
a corresponding voltage gain.
To obtain high gain, cascades ofsingle amplifier circuits are used, usually
with a coupling network, actually a simple filter, inserted between the stages of
amplification. One such filter is a high-pass network formed by a coupling
capacitor, the output resistances of the driving stage, and the input resistance of
the driven stage. Since dc voltages are blocked by the capacitor, this ac coupling
permits independently setting dc bias voltages for each amplifier stage in the
cascade. The coupling network also rejects signal with ac frequency components
below a cutoff. The capacitor must be sufficiently large not to attenuate any of
the frequencies that are to be amplified. If dc is to be amplified, a direct-coupled
amplifier is required., and the design is some what more complicated since dc
Paper - II Electronic Devices and Circuits
245
bias voltages on each transistor now cannot be set independently.
The amplifiers discussed above are called single-ended amplifiers, since
their input and output voltages are referred to a common reference point which
by convention is called ground. These single-ended circuits, while satisfactory
for most non critical applications, have several weaknesses which degrade their
performance in high-gain, weak-signal applications. Their unbalanced
construction and their use of a common ground point for return currents makes
them susceptible to noise pickup.
To minimize noise on sensitive signal lines, special balanced differential
amplifier circuits are often used in critical amplifier applications. Differential
amplifiers are designed to have equal impedances to ground for each side of the
signal line and to have a output voltage proportional to the differece of the voltages
from each signal line to ground. This symmetry cancels common-mode noise
voltages, voltages which tend to appear on each of the signal lines as equal
voltages to ground. Proper circuit design, which attention to the symmetry of the
input circuit construction, can ensure that the majority of undesred noise pickup
will be common-mode noise and, hence, will be attenuated by the differential
amplifier.
In cases where a voltage amplifier is required for some special purpose,
operational amplifiers are used to fill the need. The operational amplifier is an
integrated circuit containing a cascade of differential amplifer stages, usually
followed by a push-pull amplifier acting as a buffer. The different voltage gain of
the operational amplifier is very high, about 100,000 at low frequencies, while
its input impedance is in the megohm range and its output impedance is usually
under 100 ohms. The amplifier is designed to be used in a negative-feedback
configuration, where the desired gain is controlled by a resistive voltage divider
feeding a fraction of the output voltage to the inverting input of the operational
amplifier.
Power Amplifier
Power amplifier is designed to obtain maximum output power(ie product
gains of voltage and current).
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4.2 Comparisions Between Voltage and Power Amplifier
Particular
Voltage Amplifier
Power Amplifier
High > 100
Low (20 to 50)
Rc
High (4-10) k
Low (5-20) 
Coupling
Usually R-C coupling
Transformer C coupling
Input Voltage
Low of few mV
High (2-4 V)
Collector current
Low(  1mA)
High (>100 mA)
Power output
Low
High
Output impedance
High ( 12 k 
Low (200 
Power dissipation
rating of active
device
Need not be large.
Should have large
rating

Necessity of cooling Not necessary
arrangements.
Cooling arrangements
and heat sinks are
needed
4.3 Classification of Power Amplifiers
The power amplifiers can be classified in the following ways.
1.According to the Usage of Frequency Signals:
(i) Audio frequency power amplifiers.
(ii) Radio frequency power amplifiers.
(iii) Video frequency power amplifiers.
Paper - II Electronic Devices and Circuits
247
2. According to the Period of Conduction:
(i) Class A Power Amplifiers: The period of cnduction is for total 3600
(full cycle).
(ii) Class AB Power Amplifiers: The period of conduction is for 1800
only (half cycle).
(iii) Class AB Power Amplifiers: The period of conduction is greater
than 1800 but less than 3600 (in between class A and class B).
(iv) Class C Power Amplifiers: The period of conduction is for less than
1800.
3. According to the Configuration Used:
(i) Single ended amplifier.
(ii) Push pull amplifier.
(iii) Complementary symmetry push pull amplifier.
4. Applications of Power Analysis:
1. Used in public addressing systems.
2. In audio systems like radio, tape recorders, record players.
3. In T.V. receivers.
4. In broadcast and T.V transmitters.
5. In repeater circuits.
6. In all communications systems.
7. In nuclear research centres.
4.4 Efficiency of Power Amplifier
Amplifier coverts of d.c power obtained from d.c. supply to a.c. power
delivered to the load. The conversion efficiency of an amplifier is defined as
“The ratio of the a.c. output power to the d.c. power supplied to the intput
circuit. The conversion efficiency also called as collector circuit efficiency in
case of transistor amplifier”.
Thus % of collector circuit efficiency,
=
signal power delivered to the load
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Electronics Engineering Technician
d.c. power supplied to the output circuit.
x 100
= ((1/2 VmIm) / (VCC1C) x 100%) = (50VmIm) / (VCCIC) %
4.5 Class-A, PowerAmplifier Working
Fig 4.1 shows a simple series single ended class A amplifier with resistive
load RC. The transistor used here is under fixed biasing condition. As the input
signal is applied, the transistor operates in active-region and hence the amplified
power output appears across the load resistor. The static output characteristics
along with input, output waveforms are shown in Fig 4.1.
Fig. 4.1 Series Fed Class A Amplifier
For the purpose of analysis, we assume the static output characteristics to
the equidistant for equal increments of the input excitation.
Fig. 4.2 Series Fed Class A Amplifier - VI Characteristics
When the applied input signal is a siusoidal sigal the base current varies
Paper - II Electronic Devices and Circuits
249
sinusoidally and causes the transistor to amplify these sinusoidal variations. Thus
the amplified output signals are also inthe form of sine wave forms as shown in
Fig. 4.2.
The power output of this circuit is
P = VCIC = I2C RC
....................(1)
Where VC and IC are the rms values of output voltage and current
respectively.
The magnitudes of VC and IC may be found graphically from Fig. 4.2. In the
Fig. 4.2 Im and Vm represent the peak sinusoidal output current and voltage
swings respectively.
Then
1C = Im/ 2 = ((Imax - I min) / 22)
..............................(2)
Vc =Vm / 2 = ((Vmax - Vmin)/ 22) ..............................(3)
P =VcIc= (Vmax - Vmin) (Imax - Imin) /8
Collector Dissipation and Conversion Efficiency:
Collector Dissipation: In a power amplifier, it is of significance to know,
that what fraction of the total d.c power is effectively converted into a.c output
power. In this analysis, we assume the load impedance to be pure resistor. The
average power input from the d.c. supply is VCC IC.
The power absorbed by the output circuit is I2C Rl + IC VC where IC and
VC are the rms values of output current and voltage respectively. R1 is the static
load resistance. The average power dissipated in the transistor is PD. Then, by
using law of conservation of energy.
Vcc Ic = I2c R1 + IcVc + PD
But Vcc = Vc + Ic Rc
.......................................(4)
.....................................(5)
Where VC is d.c. collector voltage. By submitting the value of VCC from
Eqn (5 ) into Eqn (4 ).
PD = VcIc - VcIc
.......................................(6)
The PD is the power is dissipated in the active device. If the input signal is
zero, then a.c. power output VcIc also zero. Therefore, in accordance with Eqn.
(6), the collector dissipation PD is maximum and has value equal to VcIc. Thus
the device is cooler when delivering power to a load tha with zero signal condition.
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4.6 Class - A, Power Amplifier Efficiency
It is the ratio of output power to input power is known as efficiency.
= (P output max / PD max)x100 = 0.5 Vec - ICQ / Vcc Icq x 100
4.7 Single Ended Class-A, Power Amplifier With
Transformer Load
The single ended class -A ,power amplifier with transformer collector load
is shown in Fig 4.3. The resistors R1 , R2 and R3 form the biasing and stabilization
network. The emitter bypass capacitor CE offers low reactance path to the
signal.
Here, we use transformer in the collector load. In the resistence coupled
stage, the quiescent current passes through the load resistance. Thus these appears
a cosiderable waste of power due to drop across load resistance due to passage
of quiescent collector current which does not contribute to the a.c. output power.
In addition to this usually many electronic systems have the loud-speaker as the
load. The output impedance of the amplifier is high. The loud-speaker voice coil
impedance is of 8 Ohms(symbol). For maximum transfer of power from amplifier
to speaker, the impedance has to be matched. This can be done by using a stepdown transformers.
Fig. 4.3 Single Ended Transformer Coupled Power Amplifer
Working:
The operating point is so selected that the transistor works only in the linear
portion of its characteristics. The input signal varies with the base current. This
produces a variation in the collector current. As the collector current in the
Paper - II Electronic Devices and Circuits
251
primary of the output transformer varies, with the induced voltage in the secondary
of the output transformer varies
4.8 Class-B, Push - pull Amplifier Efficiency
It is the ratio of power output to power input power of the class B pushpull
amplifier is known as efficiecy.
Efficiency = Po / Pdc =  / 4 x Vem / Vec x IC / IC x 100
But for practical purpose = 3.14*100 / 4= 75.50 ~ 75%
4.9 Advantages and Disadvantages of Push-Pull Amplifier
Advantages:
1. Even harmoics are absent in the output.
2. The problem of core saturation and non-linear distortions will not appear
because of cancellation of d.c. components of collector current.
3. The output is double as that offered by a single ended stage.
4. The effect of ripple voltage of the power supply due to inadequate
filtering are balanced out because of flow of ripple current in opposite direction
in the primary of the output transformer.
Disadvantages:
1. Two transistors have to be used.
2. It requires two equal and oppsite voltages at the input. Therefore push
pull circuit requires, the use of driver stage, to furnish these signals.
3. If the parameter of the two transistors are not the same, there will be
unequal amplification of two halves of the signal.
4. The circuit gives more distortion.
5. Transformer used are bulky and expensive.
4.10 Class-B, Push-Pull Amplifier
By complementary symmetry is meant a princple of assembling push-pull
class amplifier without using centre-tapped transformers. Fig 4.4 shows the
transistor push-pull amplifier using complementary symmetry. It employs one
npn and pnp transistors and requires no centre tapped transformers.
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Fig. 4.4 Complementary Symmetry Push-Pull Amplifier
Working:
During the positive half cycle of the input signal, transistor T1 (the npn
transistor) conducts current while T2 (the pnp transistor) is cutoff. During the
negative half cycle of the signal, transistor T2 conducts while T1 is cut off. In this
way, npn transistor amplifies the positive half cycle of the signal while the pnp
transistor amplifies the negative half cycle of the signal. Here output transformer
is used for impedace matching.
Advantages and Disadvantages of Complementary-Symmetry Push-Pull
Amplifier
Advantages:
1. This circuit does not require transformers. This saves weight and cost.
2. Equal and opposite input signal voltages are not required.
Disadvantages:
1. It is difficult to get a pair of transistor, that have similar characteritics.
2. Two separate collector power supplies are needed.
3. Power supply float with respect to the ground i.e., neither side of the
power supply is grounded.
Paper - II Electronic Devices and Circuits
253
4.11 Class-B, Push-Pull Amplifier Efficiency
It is the ratio of power output to power input power of the class B pushpull
amplifier is known as efficiecy.
Efficiency = Po / Pdc =  / 4 x Vem / Vec x IC / IC x 100
But for practical purpose = 3.14*100 / 4= 75.50 ~ 75%
4.12 Power Amplifier
Applications of Power Amplifier
Power Amplifier are used in
(i) Public address system amplifiers.
(ii) Radio receivers.
(iii) Radio and T.V Transmitter.
The term power amplifier is a relative term with respect to the power
delivered to the load ,from the source, by the supply circuit. In general a power
amplifier is desigated as the last amplifier,a transmission chain (the output stage)
and is the amplifier stage that typically requires most attention to power efficiency.
Efficiency cosiderations lead to various classes of power amplififer based o the
biasing of the output transistors or tubes.
Power amplifiers by Application:
• Audio amplifier,Power which is Audio power amplifiers.
• RF power amplifier, such as for transmitter final stages.
• Servo motor controllers, where linearity is not important.
Power amplifier circuits
Power can be divivded into:
• Vacuum tube/Valve, Hybrid or Transistor power amplifiers.
• Push-pull output or Single-ended output stages.
4.13 Power Amplifiers IC Numbers
1. CA 3007,Class -AB, power amplifier Power output is low up to 30mw.
2. A 1ow power amplifier system by using IC mc1554.
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Electronics Engineering Technician
3. A 20W, Class-B, power amplifier by using IC mc1533.
Summary
AF amplifier: these are used in audio frequency (20Hz to 20KHz) range.
RF amplifier: these are used in RF frequency(20KHz to 30MHz) range.
Voltage amplifier: it is an amplifier which gives only voltage amplification.
Power amplifier: It is an amplifier, which gives power amplification.
Power Amplifier:
1) Class-A power amplifier.
2) Class-B power amplifier.
3) Class-B push-pull power amplifier.
4) Class-C power amplifier.
5) Power amplifier and voltage amplifiers are used for amplification purpose
in commuication networks.
Short Answer Type Questions
1. Define voltage amplifier?
2. Define power amplifier?
3. Mention the types of coupling components used in between the amplifier
stages.
4. Draw the Class-A, Class-B, Class-B push- pull amplifier wave forms.
5. Mention max efficiency of Class-A, Class-B push-pull amplifier.
6. Write application of Class-A amplifier.
7. Write applications of Class-B push-pull amplifier?
8. Mention the IC numbers used in power amplifiers.
Long Answer Type Questions
1. Draw and explain working of Voltage amplifier?
2. Draw and explain working of Power amplifier?
3. Explain briefly amplifiers based on their mode of operation.
Paper - II Electronic Devices and Circuits
255
4. Draw and explain class-A transformer coupled amplifier.
5. Draw and explain working of Class-B push pull amplifier.
OJT/Practical Questions
• Study the Voltage/Power amplifiers types.
• Study the Class-A,Class-B,Class-C,Class-B push-pull amplifiers input/
output wave forms,efficienies and applications.
• Study the ICs numbers used for Voltage/Power amplifiers.
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Electronics Engineering Technician
UNIT
5
Feedback Amplifier & Oscillators
5.1
Learning Objectives
• Study definations of positive /negative feedback.
• Study the camparions of positive and negative feedback.
• Study the block diagram and working of negative feedback types of
negative feedback.
• Study the conditions to get oscillations,block diagram of positive
feedback,derivation over all gain of an oscillator.
• Study of types of oscillators working,expressions of frequency of RC
Phase shift, Collector tuned,Heartly,Collpits oscillators.
• Study of comparisions of RC and LC oscillators.
• Study of crystal oscillators working advantages.
• Study of applications of oscillators.
Paper - II Electronic Devices and Circuits
257
5.0 Introduction of Feedback Amplifiers
The phenomenon of feeding a portion of the output energy back to the
input circuit is known as feedback. The effect results in a dependence between
the output and the input and an effective control can be obtained in the working
of the circuit. Feedback is of two types.
1. Positive Feedback
2. Negative Feedback
Positive or regenerate feedback:
When the feedback voltage or current, is in phase with the input signal, it is
called positive or regenerative feedback. The positive feedback increases the
amount of amplification.
Negative or Degenerate feedback:
When the feedback voltage or current,is out of phase to the input signal,it is
called negative or degenerative feedback. Negative feedback decreases the
magnitude of amplification. Its main advantage is the reduction in the distortion
of the amplifier.
Feedback:
The process of injecting a fraction of output energy of some device back to
the input is known as feedback. Depending upon whether the feedback energy
aids or opposes the input signal, there are two basic types of feedbacks in
amplifiers. These are.
1. Positive Feedback
2. Negative Feedback
1. Positive Feedback: In positive feedback, the feedback energy (voltage or
currents), is in phase with the input signal and thus aids it. Positive feedback
increases gain of the amplifier also increases distortion, noise and instability.
Because of these disadvantages, positive feedback is seldom employed in
amplifiers. But the positive feedback is used in oscillators.
2. Negative Feedback: In negative feedback, the feedback energy (voltage
or current), is out of phase with the iput signal and thus opposes it. Negative
feedback reduces gain of the amplifier. It also reduce distortion, noise and
instability. This feedback increases bandwidth and improves input and output
impedances. Due to these advantages, the negative feedback is frequetly used
in amplifiers.
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Electronics Engineering Technician
5.1 Comparision Between Positive and Negative Feed Back
The difference between positive and negative feedback is,
Negative Feedback
Positive Feedback
1.
Feedback energy is out phase
with their input signal
Feedback energy is in phase
with the input signal.
2.
Gain of the amplifier decreases Gain of the amplifier increases
Gain stability increases
Gain stability decreases
S.No.
3.
5.
Noise and distortion decreases. Noise and distribution
increases.
Increase the band width
Decreases bandwidth
6.
Used in amplifiers
4.
Used in Oscillators
5.2 Expression for the Gain of Feedback Amplifier
The configuration of the feedback amplifer in its shortest form in shown in
Fig 5.1 The feedback factor of the feedback network is given by  = Xf / Xo
where Xf and Xo are feedback and output signals respectively. The input to the
amplifier is Xs. The gain of the basic amplifiers is A. Therefore, Output sigal Xo
= AXi where Xi is the input signal to the basic amplifier which is equal to
difference signal Xd.
Therefore Xo = AXd
But for negative feedback
Xd = Xs - Xf = Xi
Therefore Xo = A(Xf - Xf)
We know that  = Xf / Xo or Xf =  Xo
Substituting this value is Eqn
Xo = A(Xs - Xo)
Xo + AXo = AXs
Xo (1+A) = AXs
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259
Xo = AXs / 1 + A
The gain of feedback amplifier is
Af = Xo / Xs = A / 1+A
Here, Af is less than A giving in reduction in gain. If positive feedback employs,
in deominator is - (minus) ad therefore gain increase.
Fig. 5.1 Block Diagram of Simplified Single loop Negative Feedback amplifier
Effects of Negative Feedback:
The following are the advantages of negative feedback in amplifies.
1. Gain Stability: An important advantage of negative feedback is that the
resultant gain of the amplifier can be made independent of transistor parameters
or the supply voltage variations.
Af = (A) /(1+ A)
The product of A is much greater than unity. Therefore in above relation
1 can be neglected as compared to A. Then, the expression becomes.
Af = (A / A = (1 / 
It may seen that the gain now depends only upon feedback fraction . The
feedback circuit is usually resistive network. Therefore, it is uneffected by changes
in temperature variations in transistor parameters ad frequency. Hence, the gain
of the amplifier is extremly stable.
2. Reduces Non-Linear Distortion: The negative feedback reduces,with
the non linear distortion in large signal amplifiers. It can be proved mathematically
,giventhat
Df = (D) / (1 + A)
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Electronics Engineering Technician
It is clear from the above equation that, a negative feedback reduces the
distortion by factor 1 + A.
5.3 Types of Nagative Feedback Amplifiers
The feedback amplifiers can be classified according to mixing and sampling
employed to it as follows:
1. Voltage series feedback amplifier
2. Current series feedback amplifier
3. Current shunt feedback amplifier
4. Voltage shunt feedback amplifier
1. Voltage Series Feedback Amplifier: This uses output voltage sampling
and series mixing.
2. Current Series Feedback Amplifier: This uses output current sampling
and series mixing.
3. Current Shunt Feedback Amplifier: This uses output current sampling
and shunt mixing.
4. Voltage Shunt Feedbac Amplifier: This uses output voltage sampling and
shunt mixing.
5.4 Conditions of an oscillators - Barkhausen Criteron
Oscillations produced by adequate positive feedback in an amplifier is called
a feedback oscillator. Fig. 5.02 gives the block diagram of feedback oscillator.
An amplifier is an essential part of an oscillator. Oscillations may be produced
by adequate positive feedback in an amplifier.
Fig. 5.2 Block diagram of an Oscillator
Paper - II Electronic Devices and Circuits
261
Consider an external signal Xs applied directly to the input terminals of the
amplifier shown in Fig.5.03. This results in an output signal Xo. The output of the
feedback network is.
Xf = Xo = AXs
This output of the mixing network is X1f = -Xf = -AXs
Let it be so arranged that X1f is identical with Xs. If now the external source
is removed and terminal 2 is connected to terminal 1, the amplifier continues to
provide the same output voltage Xo as before without any exteral input signal.
The system then functions as an oscillator. The condition necessary for oscillations
is that X1f = Xs. Thus the instantaneous values X1f and Xs are identical at all
times.
Since X1f = -AXs implies that -A=1 i.e., the loop gain must be equal to
unity and phase angle of -A is zero. This condition for sustained oscillations is
called the Barkhausen criterion.
Barkhausen Criterion:
1. Sustained oscillations are produced in a sinusoidal oscillators at a
frequency for which the total phase shift introduced,as the signal travels from the
input terminal through the basic amplifier, feedback network and mixing network
back to the input terminals its precisely zero or a integral multiple of 2 radians.
2. Sustained oscillations are not produced if at the oscillation frequency
the magnitude of the loop gain i.e., the product of the transfer gain A, of amplifer
and magnitude of the feedback factor  of the feedback network is less than
unity.
Requisites of an Oscillator
1. Tank Circuit: It consists of inductor connected in parallel with capacitor C.
The frequency of oscillations in the circuit depends upon the values of inductance
(L) ad capacitace (C). In RC oscillators inductor replaced by resistor(R).
2. Transistor Amplifier: The transistor amplifier receives d.c power from the
battery and changes it into a.c. power for supplying to the tank circuit. The
oscillations occurring in the tank circuit are applied to the input of the transistor
amplifier. The amplified output of oscillations is due to the d.c. power supplied
by the battery. The output of the transistor can be supplied the tank circuit to
meet the losses.
3. Feedback Circuit: The feedback circuit supplies a part of collector energy
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to the tank circuit in correct phase to aid the oscillations i.e., it provides positive
feed back. In oscillator is to satisfy Barkhausen criteria has to get sustained
oscillations.
5.5 Classification of Oscillators
The oscillators can be classified in the following ways.
1. According to the generated waveform.
(a) Sine wave oscillators.
(b) Relaxation or non-sinusoidal oscillators.
2. According to the fundemental mechanism involved
(a) Feedback oscillators.
(b) Negative resistance oscillators.
3. According to the associated circuit components
(a) RC oscillators
(b) LC oscillators
(c) Cyrstal oscillators
4. According to the frequency range:
(a) Audio frequency (AF) oscillators
(b) Radio frequency (RF) oscillators
(c) VHF or microwave oscillators.
5.5.1 | A| > 1
When the total phase shift around a loop is 00 or 3600 and |A >1, then the
output oscillates but the oscillations are of growing type. The amplitude of
oscillations goes on increasing as shown in Fig. 5.3
Fig. 5.3 Growing type of Oscillations
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5.5.2
263
| A  = 1|
As stated by Barkhausen criterion, when total phase shift around a loop is
0 or 3600 ensuring positive feedback and | A| = 1 then the oscillations are
with constant frequency and amplitude called sustained oscillations. Such
oscillations are shown in Fig 5.4
0
Fig. 5.4 Sustained Oscillations
5.5.3 |A  < 1|
When total phase shift around a loop is 00 or 3600 but | A< 1| then the
oscillations are of decaying type i.e. such oscillation amplitude decreases
exponentially and the oscillations finally cease. Thus circuit works as an amplifier
without oscillations. The decaying oscillations are shown in Fig 5.5.
Fig. 5.5 Exponentially decaying Oscillations
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Classification of Oscillators
As type of tank circuit employ to the amplifier circuit in positive feedback
the following oscillators.
1. RC Phase Shift Oscillator
2. Collector Tuned Oscillator
3. Hartley Oscillator
4. Collpitt’s Oscillator
5.6 RC Phase Shift Oscillator
Fig.5.06 shows the circuit of a phase shift oscillator. It consists of a
conventional single transistor amplifier and a RC Phase shift network. The phase
shoft network consists of three sections R1 C1, R2 C2 and R3 C3. At some
particular frequency f0, the phase shift of each section is 600, so that the total
phase-shift produced by the RC network is (3 x 60) = 1800. The frequency
of oscillations is given by
fo = ( 1 ) / ( 2  RC 6)
where R1 = R2 = R3 = R
C1 = C2 = C3 = C
Figure 5.6 RC Phase Shift Oscillator
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265
With the circuit is switched ON, it produces oscillations. The output E0 of
the amplifier is feedback to RC feedback network. This network produces a
phase shift of 1800 and a voltage E1 appears at its output which is applied to the
transistor amplifier.
The feedback factor  = E1 / E0. It can be shown that the feedback factor
of the RC network is  = 1/29.
This expression has an important significance. For self starting the oscillations
we must have A  >1. It means that gain A of the amplifier must be greater than
29. Only then the oscillations can start.
The feedback phase is correct. A phase shift of 1800 is produced by the
transistor amplifier. A further phase shift of 1800 is produced by the RC network.
As a result, the phase around the entire loop is 3600.
Advamtages :
1. It does not require transformers or inductors.
2. It can be used to produce very low frequencies.
3. The circuit provides good frequency stability.
Disadvantages :
1. It is difficult for the circuit to start oscillations as the feedback is generally
small.
2. The circuit gives small output.
5.7 Tuned Collector Oscillator
The tuned collector oscillator contains tuned circuit L1 -C1 in the collector
load .The feedback coil L2 in the base circuit is magnetically coupled to the tank
circuit coil L1.and hence the name. The frequency of oscillations depends upon
the values of L1 and C1 and is given by
f = ( 1 ) / (2  L1C1)
The figure coil L2 in the base circuitis magnetically coupled to the tank
circuit L1. In practice L1 and L2 form the primary and secondary of the
transformer. The biasing is provided by potential divider arrangement. The
capacitor C connected in the base circuit provides low reactance path to the
oscillations.
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Fig. 5.7
Circuit Operation:
When switch S is closed. Collector current starts increasing and charges
the capacitor C1. When this capacitor is fully charged, it discharges through Coil
L1, setting up oscillations of frequency.
f = ( 1 ) / (2 L1C1)
These oscillations induce some voltage in coil L2 by mutual induction. The
frequency of voltage of coil L2 is the same as that of tank circuit but its magnitude
depends upon the number of turns of L2 and coupling between L1 and L2. The
voltage acaross L2 is applied between base and emitter and appears in the
amplified form in the collector circuit, thus overcoming the losses occurring in
the tank circuit. The number of turns of L2 and coupling between L1 and L2 are
so adjusted that oscillations across L2 are amplified to a level just sufficient to
supply losses to the tank circuit.
It may be noted that the phase of feedback is correct i.e., energy supplied
to the tank circuit is in phase with the generated oscillations. A phase shift of
1800 is created between the voltages of L1 and L2 due to transformer action. A
further phase shift of 1800 takes place between base-emitter and collector circuit
due to transistor properties. As a result the energy feedback to the tank circuit is
in phase with the generated oscillations.
5.8 Hartly Oscillator
Hartly oscillator is very popular and is commonly used as a local oscillator
in radio receivers.
Fig.5.8 shows the circuit of Hartley oscillator. The tank circuit is made up
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267
of C L1 and L2. The coil L1 is inductively coupled to coil L2, the combination
functions as auto-transformer. The self bias is provided here for biasing. the
capacitor Cb blocks the d.c. component. When the power is ON, collector
current starts rising and charges the capacitor C. When the capacitor is fully
charged, it discharges through coils L1 and L2 setting up oscillations of frequency.
f = ( 1 ) / ( 2 (L1+L2) C)
Fig. 5.8 Hartley Oscillator
The oscillations across L1 are applied to the base-emitter junction and
appears in the amplified form in the collector circuit. The coil L2 couples the
collector circuit energy back into the tank circuit by means of mutual inductance
between L1 and L2. In this way, energy is being continuously supplid to the tank
circuit to overcome the losses occurring in it.
It may be seen that the phase of feedback is correct. The capacitor C and
L1 - L2 are 1800 out of phase. A further phase shift of 1800 is produced by
transistor circuit. In this way, energy feedback to the tank circuit is in phase with
oscillations.
Advantages :
1. Easy to tune.
2. Adaptability to a wide range of frequencies.
5.9 Colpitt’s Oscillator
Fig 5.9 shows the circuits of colpitt’s oscillator. The tank circuit is make up
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Electronics Engineering Technician
of C1 C2 and L. The biasing is provided by self biasing.
When power is ON, collector current starts rising and charges the capacitors
C1 and C2. These capacitors discharges through coil L setting up oscillations.
The frequency of oscillations is given by
f = (1) / (2  LCT) where CT = (C1C2) / (C1 + C2)
The oscillations across C1 are applied to the base-emitter junction and
appear in the amplified form in the collector circuit and supply losses to the tank
circuit. The amount of feedback depends upon the relative capacitance values
of C1 and C2.
Fig. 5.9 Collpitt’s Oscillator
It may be noted that the phase of feedback is correct. The capacitors C1
and C2 act as a simple alternating voltage divider. Therefore the tank circuit of L
C1 C2 produce 1800 phase shift. A further 1800 phase shift is produced by the
transistor. In this way feedback is properly phased to produce continuous
undamped oscillations.
5.10 Oscillators Frequency Equations as Follows
a) Collector tuned oscillator frequency f = (( 1 ) / (2  CTL1)
b) RC Phase Shift Oscillator frequency
fo = ((1) / (2  RC 6)
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269
where R1 = R2 = R3 = R
C1 = C2 = C3 = C
c) Hartly Oscillator frequency (formula)
where fo = ((1) / (2CL1) where L1 = L1 + L2 + - 2M
d) Colpitt’s Oscillator frequency
fo = ((1) / 2  CTL) where CT = C1C2 / C1+C2
5.11 Comparison of LC and RC Oscillators:
S.No.
Particulars
LC
Oscillators
RC
Oscillators
1.
Requirements of
Inductor / transformer
Yes
No
2.
Cost
More
Less
3.
Output Frequency
High
Low
4.
Frequency stability
Poor
Good
5.
Output voltage
More
Less
5.12 Piezo Electric Crystals
Certain crystalline materials, exhibit the piezo-electric effect i.e., when we
apply an a.c. voltage across them, they vibrate at the frequency of the applied
voltage. Conversely, if the crystals are forced mechanically to vibrate, they
generate an emf at the fundamental frequency of the crystal. This nature is found
in materials namely: Rochelle salt, quartz and tourmaline. Of the various
piezoelectric crystals quartz is most commonly used. The advantages of quartz
crystal is.
1. Optimum value of mechnical strength
2. Inexpensive
3. Readily available in nature
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The nature shape of the quartz crystal is a hexagonal prism. The useful
crystal is obtained by cutting the nature crystal. The crystal is usually mounted in
an oscillator circuit to vibrate best at one of its resonant frequencies, usually the
fundamental frequency. The formula of the fundamental frequency of crystal is
given by.
f=k/t
where
t = Thickness of crystal
k = constant that depends o its cut and other physical factors.
In order to use crystal in an electronic circuit, it is placed between two
metal plates. A crystal can be conveniently replaced by an electrical equivalent
circuit. When the crystal is not vibrating, it is equivalent to capacitance Cm
because it has two metal plates separated by a dielectric (crystal). However
when crystal is vibrating, it is equivalent to series tuned circuit RLC. Therefore,
the electrical equivalent circuit of the crystal is shown in Fig.5.10. In this figure.
Cm = Mounting capacitance
Cs = Series capacitance introduced by air gap
R-L-C : Electrical equivalent of vibrational characteristics of crystal.
Fig. 5.10
The series resonant frequency of crystal is the resonant frequency of LCR
branch is given by
fs = ((1) / (2 LCs))
The parallel resonant the frequency of the crystal is the frequency at which
the loop current il reaches the maximum value. Since C is in series with Cm the
loop capacitance CT equal to (CmC) / (C + Cm). So the parallel resonant
frequency is given by
fp = ((1) / 2 LCT))
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271
5.13 Transistor Crystal Oscillator
The Fig.5.11 shows the crystal oscillator. This circuit is same as Colpitt’s
oscillator. In this circuit the crystal is mounted to act as an inductor which forms
the tuned circuit with C1 and C2. The positive feedback is provided by the
capacitive voltage divider network. The crystal now acts as an inductor that
resonants with C1 and C2 and the oscillating frequency of the circuit now lies in
between series and parallel resonant frequencies of the crystal. The resistors R1,
R2 for biasing and RE for stabilization. The CE configured transistor provides
1800 phase shift where as the remaining 1800 phase shift is provided by the
feedback network.
Fig. 5.11 Crystal Oscillator
Advantages
1. It can produce highest oscillating frequencies.
2. The quality factor (Q) of the crystal is very high. The Q factor of the
crystal may be as high as 10,000 compared to about 100 of LC tank circuit.
3. They have a high order of frequency stability.
4. Low cost.
5. Simple in construction.
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Disadvantages :
1. They are fragile and consequently can only be used in low power circuits.
2. The frequency of oscillation cannot be changed appreciably.
Summary
Oscillator Circuit or Tank Circuit: A circuit which produce electrical
oscillations of any desired frequency is known as an oscillatory circuit.
Frequency of oscillations is given by f = (1) / (2LC)
Feedback Oscillator : Oscillations produced by adequate positive feedback
in an amplifier is called a feedback oscillator.
Barkhensans Condition for Sustained Oscillations:
1. A = 1
2. Phase angle of - A is zero.
Colpitt’s Oscillator : The tank circuit of this oscillator is made up of C1 C2 and
L. The frequency of oscillations is given by
f = (1) / (2LCT) where CT = (C1C2) / (C1 + C2)
Hartley Oscillator : The tank circuit of this oscillator is made up of CL1 and
L2. The frequency of oscillations is given by
f = 1 / (2(L1+L2) C)
RC Phase Shift Oscillator :
The phase shift network of this oscillator consists of three identical RC
sections. The phase shift of each session is 600. Frequency of oscillations is
given by
f = (1) / RC6)
Crystal Oscillator : It is used to get high frequency stability. This is possible by
employing crystal in a transistor oscillator.
Relaxation Oscillator : An oscillator which produces non-sinusoidal wavesl
like square, sawtooth, rectangular, triangular etc., is called a relaxation oscillator.
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Short Answer Type Questions
1. Define an oscillator.
2. Explain how oscillations produce in tank circuit.
3. Explain the condition for oscillation.
4. Explain the classifications of oscillators.
5. State the requisites of an oscillator.
6. Draw the circuit of a Collpitt’s oscillator and explain its working?
7. With a near diagram explain the action of Hartley oscillator.
8. Draw the circuit diagram of an RC phase shift oscillator and
explain.
9. Mention the advantages and disadvantages of phase shift
oscillator.
10. List the applications of oscillators.
11. Draw the circuit diagram of crystal oscillator and explain its working.
Also list its advantages and disadvantages.
12. Draw the circuit diagram of UJT relaxation oscillator and explain its
working.
13. What are the requisites of an oscillators?
Long Answer Type Questions
1. Write camparisions of negative and positive feedback.
2. Draw and explain positive feedback.
3. What are the requirements a transister amplifier works as an
oscillator. Explain ?.
4. Explain working of RC phase shift oscillator.
5. Explain working of tuned collector oscillator with neet diagram.
6. Explain working of Hartely oscillator.
7. Explain working of Colpitts oscillator.
8. Explain working of Crystal oscillator.
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Practical/OJT Questions
• Study the oscillators-RC phase shift,Hartely,Colpitts,Tuned collector
and Crystal oscillators.
UNIT
6
Analogic’s
Learning Objectives
• Study of different IC s used as Voltage regulators.
• Study of working of siries/shunt voltage regulators.
• Study oft the advantages of IC s regulators.
• Study of the positive/negative voltage regulator by using IC 7800
and 7900series.
• Study the operation of LM317 adjustable voltage regulator.
• Study the operation of differential amplifier.
• Study operational amplifier working and Input impedence,Open loop
gain,Slew rate, CMRR,Input offset voltage,Input offset Current and
specifications.
• Study the block diagram of IC 741 working.
• Study the operational amplifier working as summer, integrator,
diffentiator, inverter, multiplier,voltage follower,voltage to current
converter,current to voltage converter,camparitor and square wave
generator.
• Study the block diagram of IC 555 and working.
• Study the working of PLL.
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Electronics Engineering Technician
• Study the block diagram of PLL-LM565 and working.
• Study the working of operation VCO, LM566.
6.0 Introduction
The function of a voltage regulator is to provide a stable dc voltage for
powering other electronic circuits. A voltage regulator should be capable of
providing substantial output current. Voltage regulators are classified as:
Series regulator
Shunt regulator
Series regulator use as power transistor connected in series between the
unregulated dc input and the load. The output voltage is controlled by the
continuous voltage drop taking place across the series pass transistor. Since the
transistor conducts in the active or linear region, these regulators are also called
linear regulators. Linear regulators may have fixed or variable signal output voltage
ad could be positive or negative. The schematic, important characteristics, data
sheet, short circuit protection, current fold-back, current boosting techniques
for linear voltage regulators such as 78 XX, 79 XX series, 723 IC are discussed.
Switching regulators, on the other hand, operate the power transistor as a
high frequency on/off switch, so that the power transistor does not conduct
current continuously. This gives improved efficiency over series regulator. In the
principle of switching power supply and its advantages over linear type of voltage
regulator are discussed.
6.1 Series Voltage Regulator
In your robot, the energy is derived from batteries. Specifically, there are
two sets of batteries wired up to act as voltage sources,a 9V, battery, and two
1.5V batteries connected in series that act as a 3V source. Since different
circuits in your robot require different voltage sources, it is not always possible
to hook up the battery directly to power the circuits. The ICs in your robot
circuit are designed to work with a constant 5V source. Therefore, it is important
to convert the 9V source into a 5V source. Since a DC voltage (one that is fixed
over time such as a battery) is being converted to another DC voltage, the
circuit that does this is called a DC-to-DC converter or a voltage regulator. If
we were to convert 110V AC (alternatig current - like the power in a wall
outlet) into a 5V DC source, the circuit would be an AC-DC converter.
In this lab, you will build the voltage regulator circuit which converts the 9V
Paper - II Electronic Devices and Circuits
277
batteries output into a constant 5V voltage source. The circuit has already been
designed for you.You task will be to build and test its operation. You will also do
some experiments that will allow you to develop an understanding of how the
circuit works.
The voltage regulator circuit consists of 5 different components; a 9V battery,
a resistor, a diode, a transistor, and a capacitor. You may wish to review the
description of the operation of these components in the Lab Guide.The circuit
you will be building is shown in Fig. 6.1. The pin out for the transistor is shown
in Fig. 6.2 - please be very careful, interchanging the base and collector will
result in immediate destruction of the transistor.
Fig. 6.1 Power Supply Regulator
If you are using the replacement transistor rather than the one that comes in
the robot kit, its leads come out in a different order (shown on the left in Fig. 6).
For more details on identifying the E, B, and C terminals of your transistors see
the Lab Guide. Another potential problem is that the capacitor is electrolytic which means that it can only stand to have voltages applied in one direction. If
voltages are applied contrary to the sign of the label on the capacitor it will be
destroyed. Note, this circuit is also shown in the Graymark Robot Assembly
Manual in Figure T2 - but it may be WRONG. The Zener diode may be
incorrectly connected with the arrow pointing toward ground. Follow the
schematic shown above in Fig.6.1 or in the schematic.
A brief description of the circuit operation is as follows. Before the 9V
battery is attached, all points on the circuit are at 0V (ground). Let us first
consider the operation of the circuit without the capacitor (C11). When the
switch is closed, a voltage is applied to the Zener diode through R14. The value
of R14 is chosen so that the Zener diode is in the reverse break-down region.
Consequently, the voltage across the diode is held constant at 5.6V. This 5.6V
also appears across the base-emitter junction of the transistor and the load
resistor in series. Since this voltage is much greater than 0.7V, the base-emitter
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Electronics Engineering Technician
diode is forward biased and current flows intothe base of the transistor. The
voltage across the resistor is therefore fixed at 5.6-0.7=4.9 V as long as the
Zener diode is in the reverse breakdown region and the base-emitter diode is
forward biased.
To see how this circuit is always able to hold the voltage across the load at
approximately 5 V, let us consider the current flows in the circuit. The current
flowing through R14 is split between the Zener diode and the base-emitter diode
of the transistor. If we denote the current flowing into the base as IB, a current
equal to (symbol)IB flows from the battery into the Collector. Since the transistor
is connected to operate in the “Forward Active” region, a current equal to
(symbol+1)IB flows out of the emitter and through the Zener diode changes so
that the base current and therefore the emitter current has the proper value to
give the required 5V across the load resistor.
Fig. 6.2 Transistor Lead Configurations
The capacitor is a circuit element that stores electrical change. It is used in
this circuit to help keep the voltage regulator’s output voltage constant over
time. The rate of change of the voltage across a capacitor is proportional to the
current flowing out of it divided by the capacitance. Therefore, the larger the
capacitor, the smaller the changes in voltage at the output of the regulator over
time for a fixed current drain.
1. First, build the circuit ,on your proto board. The transistor is TR3, to
determine how the voltage regulator circuit should be wired together. Connect
the voltage regulator output to a 4.7K resistor (which will give you a load
current of about 1mA) which acts as the load resistance, R load. For testing
purposes, we are replacing the battery with the +20V power supply. Connect
the Common terminal to the bottom of the diode and C11. Make sure you set
Paper - II Electronic Devices and Circuits
279
the +20V output of the power supply output voltage to 0V before you connect
the +20V terminal of the power supply to the transistor collector and R14.
Measure the output capacitor voltage, VOUT,for varying values of power supply
voltage, V1N, starting at 0 volts and increasing the supply by 1 volt steps until it
reaches 9 volts. As you increase the power supply volage keep an eye on the
ammeter on your power supply. If it moves noticeably, immediately turn the
voltage back down and check your circuit.
2. One important characterization of a voltage regulator is how well it
holds the output constant in the face of changing input voltage - this is called line
regulation. Line regulation is characterized by the change in output voltage divided
by the change in input voltage. That is,
Aline =  VOUT /  VIN
The line regulations error for the ideal voltage regulator is 0%. With a
single 4.7K resistor as a load for the voltage regulator output, and a power
supply voltage of 9V, measure the output voltage. Change in the power supply
voltage to 8V and then measure the output voltage. What is Aline for your voltage
regulator?
3. Next, we will explore what is called the “load regulation” of your voltage
regulator. Good load regulation means that the output voltage does not change
much with changing load resistance. To characterize the load regulation of your
regulator circuit, set the power supply voltage at 9V, and see how the output
voltage varies as you draw current from (load down) the voltage regulator output.
Measure the regulator output voltage with a 4.7K resistance (which means a
load current of about 1mA) and with a load resistance of wo 4.7K resistors in
parallel (which means a load current of approximately 2mA. We formulate the
voltage regulator’s load regulation in terms of its incremental output resistance change in load voltage divided by the change in load current.
Rsupply =  VOUT /  IOUT =  VOUT / 1mA
Note, an ideal voltage source would have
Rsupply = 0
What is Rsupply for your regulator?
4. After you have verified that the circuit functions properly, solder the
voltage regulator circuit onto the robot PC board. Note, do not out off the leads
of the components flush with the board. You should leave enough wire protruding
so that a clip lead can be attached for testing. Verify that the connections are
correct by examining the underside of the board. Set the +20V part of your
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Electronics Engineering Technician
power supply to +9V and attach it to the +9V pin of the robot PC board
between R14 and the power switch SW1. Attach the power supply common to
one of the GND pins of the robot PC board. Turn SW1 on the check to see that
your voltage regulator still works by measuring the voltage of the output pin with
respect to ground. If the output voltage is not 5V, identify the problem and fix it.
If you cannot rectify the problem ask your TA for help.
6.2 Shunt voltage Regulator
Trans
Fig. 6.3 Shunt Voltage Regulator Circuit
Regulator Circuit Description
In this case the Shunt Regulator shown is a DC to DC converter, with an
Unregulated input DC voltage and a Regulated output DC voltage.
The operational voltage of the shunt regulator will depend on the Zener
diode used [CR1] and the transistor [Q1].
Transistor Q1 is a NPN transistor which needs to be able to work with the
voltages used in the design.
With the addition of Resistor Rs the reference iput is not connected to the
reference of the output [the negative terminals].
6.3 IC Voltage Regulators
With the advet of micro-electronics, it is possible to incorporate the complete
circuit on a monolithic silicon chip. This gives low cost, high reliability, reduction
in size and excellent performance. Examples of monolithic regulators are 78
XX/79 XX series and 723 general purpose regulators.
78 XX series are three terminal, positive fixed voltage regulators. There
are seven output voltage options available such as 5,6,8,12,15,18 and 24 V. In
Paper - II Electronic Devices and Circuits
281
78 XX, the last two numbers (XX) indicate the output voltage. thus 7815
represents a 15V regulator. There are also available 79 XX series of fixed output,
negative voltage regulators which are complements to the 78 XX series devices.
There are two extra voltage options -2 V and -5.2 V available in 79 XX series.
These regulators are available in two types of packages.
Metal Package (TO - 3 type)
Plastic Package (To - 220 type)
Data Sheet Regulator:
7805 is a voltage regulator integrated circuit. It is a member of 78xx
series of fixed linear voltage regulator ICs. The voltage source in a circuit may
have fluctuations and would not give the fixed voltage output. The voltage
regulator IC maintains the output voltage at a constant value. the xx in 78xx
indicates the fixed output voltage it is designed to provide. 7805 provides +5V
regulated power supply. Capacitors of suitable values can be connected at input
and output, pind depending upon the respective voltage levels.
Fig. 6.4
Pin Description
1
Input voltage (5V-18v)
Input
2
Ground (0V)
Ground
3
Regulated output, 5V (4.8V-5.2V)
Output
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Electronics Engineering Technician
Figure 6.02 shows the standard repressentation of monolithic voltage
regulator. A capacitor Ci (0.33 F) is usually connected betwen input terminal
and ground to cancel the inductive effects due to long distribution leads. The
output capacitor C0 (1)F) improves the transient reponse
Unregulated
input Vin
Regulated
output Vo
Fig. 6.5 Standard representation of a three terminal positive monolithic regulator
National Semiconductor also produces three terminal voltage regulators in
LM317 series. There are three series available for different operating temperature
ranges;
LM 100
series -550C to
+1250C
LM 200
series -250C to
+850C
LM 300
series -00C
+700C
to
The popular series are LM 340 positive regulators and LM 320 negative
regulators with output ratings comparable to 78 XX/79 XX series.
Characteristics
There are four characteristics of three terminal IC regulators which must be
mentioned.
1. V0 : The regulated output voltage is fixed at a value as specified by the
manufacturer. There are a number of modesl available for different output voltages,
for example 78 XX series has output voltage at 5,6,8 etc.
2. |Vin | >= |V0| + 2 volts: The unregulated input voltage must be atleast 2
V more than the regulated outpt voltage. For example, if V0 = 5 V, then Vin = 7
V.
3. I0 (max) : The load current may vary from 0 to rated maximum output
current. The IC is usually provided with a heat sink, otherwise it may not provide
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the rated maximum output current.
4. Thermal shutdown: The IC has a temperature sensor (built-in) which turns
off the IC when it becomes too hot(usually 1250 C to 1500 C). the output current
will drop and remains there until the IC has cooled significantly.
The electrical characteristics 7805 voltage regulator and the connection
diagram of packages available. Some of the important performance parameters
listed in the data sheet are explained as follows:
Line/Input Regulation
It is defined as the percentage change in the output voltage for a change in
the input voltage. It is usually expressed in millivolts or as a percentage of the
output voltage. Typical value of line regulation from the data sheet of 7805 is 3
mV.
Absolute Maximum Ratings
Input Voltage (5 V through 18 V)
35 V
(24 V)
40 V
Internal Power Dissipation
Internally limited
Storage Temperature Range
-650C to +1500C
Operating Junction
Temperature Range
(symbol)A7800
(symbol)A7800C
-550C to +1500C
-00C to +1250C
Electrical Characteristics VIN = 10 V, IOUT = 500 mA, 00C <= Tj <= 1250C,
CIN = 0.33 (symbol)F, COUT = 0.1 f , unless otherwise specified.
The reference voltage is typically 7.15V. So the output voltage V0 is
Vo = 7.15 x R2 / R1 + R2
which will always be less than 7.15V. So in the circuit of Fig. 6.06 is used
as low voltage (<7V) 723 regulator.
If it is desired to produce regulated output voltage greater than 7V, then the
circuit of Fig. 6.07 can be used. The NI terminal is connected directly to Vref
through R2. So the voltage at the NI terminal is Vref. The error amplifier operates
as a non-inverting amplifier with a voltage gain of
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Av = 1 + ( R1 / R2)
So the output voltage for the circuit is
Vo = 7.15 x (1+ R2 / R1)
6.3.1 Advantages of IC Regulators
· Available source input voltages
· Desired supply output voltage magnitudes
· Ability to step-down or step-up output voltages, or both
· DC-DC converter efficiency (POUT / PIN)
· Output voltage ripple
· Output load transient response
· Solution complexity (one IC solution, # of passive components,
controller and external FETs)
· Switching frequency (for switch-mode regulators)
Data Sheets
The circuits of Fig.6.07 have no protection. If the load demands more
current e.g. under abort circuit condition, the IC tries to provide it at a constant
output voltage getting hotter all the time. This may ultimately burn the IC.
The IC is, therefore, provided with a current limit facility. Currenet limiting
refers in the ability of a regular to prevent the load current from increasing above
a present value. The characteristic curve of a current limited power supply. The
output voltage remains constant for load current below Ilimit. As current
approaches to the limit, the output voltage drops. The current limit Ilimit is set by
connecting an external resistor Rsc between the terminals CL and CS terminals.
The CL terminal is also connected to the output terminal V0 and CS terminal to
the load.
LM341,LM78M05,LM78M12,LM78M15
LM341/LM78MXX Series 3-Terminal Positive Voltage Regulators
Paper - II Electronic Devices and Circuits
6.4 Posotive Voltage I.C. Regulators78 XX Series
Fig. 6.8
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6.4.1 Negative Voltage Regulator Circuit Diagram using 79xx
Regulator IC DC - DC Converter, Power Supply, Using Regulator IC,
Voltage Regulator
This is a Negative Voltage Regulator Circuit Diagram. You want get best
performance via electronic circuits its need fixed DC power supply, especially
digital electronic circuits. This voltage regulator circuit designed using Fixed
Negative Voltage Regulator IC. You can use 79xx series regulator ICs (Ex:
7905, 7908, 7912) for this circuit and supply voltage is this circuit -8V to -30V.
Output voltage is indicating in last two numbers of IC. You can select regulator
IC using this table.
IC No
Voltage
IC No
Voltage
7905
-5V
7910
-10V
7912
-12V
7915
-15V
7918
-18V
7906
-6V
7908
-8V
7924
-24V
7909
-9V
You can select output current limits of regulator IC using this table.
IC No
Output Current (Amp) Package
79Lxx (Ex: 79L05)
100mA
TO 92
79Mxx (Ex: 79M05)
500mA
TO 220
79xx (Ex: 7905)
1A
TO 220
79Sxx (Ex: 79S05)
2A
TO 220
79Txx (Ex: 79T05)
3A
TO 220
79Hxx (Ex: 79H05)
5A
TO 3
If you use regulator IC maximum output current is over 100mA, don’
forget to installed proper heatsink with IC.
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6.5 Adjustable Voltage Regulator By Using LM 317
description/ordering information (Continued)
In addition to having higher performance than fixed regulators, this device
includes on-chip current limiting, thermal overload protection, and safe-operatingarea protection. All overload protection remains fully functional, even if the
ADJUST terminal is disconnected.
The LM317 is versatile in its applications, including uses in programmable
output regulation and local on-card regulation. Or, by connecting a fixed resistor
between the ADJUST and OUTPUT terminals, the LM317 can function as a
precision current regulator. An optional output capacitor can be added to improve
transient response. The ADJUST terminal can be bypassed to achieve very high
ripple-rejection ratios, which are difficult to achieve with standard three-terminal
regulators.
Fig. 6.9 Adjustable voltage Regulator IC 317
Description/ordering information
The LM317 is an adjustable three-terminal positive-voltage regulator capable
of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It
is exceptionally easy to use and requires only two external resistors to set the
output voltage. Furthermore, both line and load regulation are better than standard
fixed regulators.
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291
Since the feedback current is proportional to the output voltage, this circuit
is voltage shunt feedback amplifier.
The transfer trans resistance
Rmf = Vs / Is
Since Is = If (because IB isvery very small)
Therefore Rmf = Vs / If = 1/ B = Rf
6.6 The Differential Amplifier
A differential amplifier serves to amplify the difference between two signals.
A differential amplifiers forms the basic stage of an integrated op-amp with
differential inputs.
The circuit diagram of the emitter cuopled differential amplifier is shown in
Fig. 6.12. Ituses two identical npn transistors.The transistors are connected in
CE mode. The emitter bias is used here. The two inputs v1 and v2 and the
output is v0. If the inputs are similar, the output of the amplifier is zero. The
amplifier output is proportional to the difference of the two inputs v1 and v2.
Therefore it is called as differential amplifier. The emitter coupled differential
amplifier of Fig.6.12 posses the following properties.
Fig. 6.12 Emitter coupled differential amplifier
1. Low drift.
2. Very high input resistance.
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3. Cancels the effects of supply voltages.
4. High CMRR.
5. Very high stability.
6.7 Operational AmplifierWorking
The operational amplifier is a basc analog building block common to a
number of electronic functions performed in instrumentation, computation and
control. Op-amp is basically a differential amplifier whose function is to amplify
the difference between two input signals. Op-amp is available in IC form.
Basic Concepts and Characteristics of Operational Amplifier
The ideal operational amplifier is shown as Fig.6.13, its equivalent circuit
6.13. A signal appearing at the negative terminal v1 is inverted at the output, a
signal at the positive terminal v2 appears at the output with no change in sigh.
Hence the negative terminal is called the ‘inverting terminal’ and the positive
terminal the ‘non inverting terminal.’ In general the output voltage is directly
propertional to the difference of the input voltage. The constant of proportionality,
- A; is the voltage gain of the amplifier.
Fig. 6.13 Equivalent Circuit
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6.8 Ideal OP-AMP Characteristics
The ideal op-amp has the following characteristics.
1. Infinite gain A = 
2. I1 = I2 = 0 of infinite input impedance Zi = .
3. Zero output impedance Z0 = 0.
4. Zero output voltage for vd = 0 i.e., zero offset
5. Infinite bandwidth Bw = 
6. Infinite common mode rejection ratio CMRR = 
Operational Amplifier
Earlier we have used an ideal op-amp, and assumed that the op-amp
responds equally well to both ac and dc input voltages. However, a practical
op-amp does not behave this way. A practical op-amp has some dc voltage at
the output even with both the inputs grounded. The factors responsible for this
and the suitabe compensating techniques are discussed. Also, under ac conditions
the characteristics of an op-amp are frequency dependent. The limitations of an
op-amp under ac conditions and methods of compensation are discussed.
DC Characteristics
An ideal op-amp draws no current from the source and its response is
also independent of temperature. However, a real op-amp does not work this
way. Current is taken from the source into the op-amp inputs. Also the two
inputs respond differently to current and voltage due to mismatch in transistors.
A real op-amp also shifts its operation with temperature. These non-ideal dc
characteristics that add error components to the dc output voltage are:
Input bias current
Input offset current
Input offset current
Thermal drift.
Input Bias Current
The op-amp’s input is a differential amplifier, which may be made of BJT
or FET. In either case, the input transistors must be biased into their linear region
by supplying currents into the bases by the external circuits. In an ideal op-amp,
we assumed that no current is drawn from the input terminals. However,
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practically, input terminals do conduct a small value of dc current to bias the
input transistors. The base currents entering into the inverting an non-inverting
terminals are shown as I-B and I+B respectively. Even though both the transistors
are identical, I-B and I+B are not exactly equal due to internal imbalances between
the two inputs. Manufacturers specify input bias current IB as the average value
of the base currents entering into the terminals of an op-amp.
The various electrical parameters supplied in the data sheet as follows:
Input offset voltage: It is the voltage that must be applied between the input
terminals of an op-amp to nollify the output. Since this voltage could be positive
or negative its absolute value is listed on the data sheet. For 741C, the maximum
value is 6 mV.
Input offset current: The algebraic difference between the currents into the () input and (+) input is referred to as input offset current. It is 200 nA maximum
for 741C.
Input bias current: The average of the currents entering into the (-) input and
(+) input terminals of an op-amp is called input bias current. Its value is 500 nA
for 741C.
Input resistance: This is the differential input resistance as seen at either of the
input terminals with the other terminal connected to ground. For the 741C,the
input resistance is 2 M.
Input capacitance : It is the equivalent capacitance that can be measure at
either of the input terminal with other terminal connected to ground. Atypical
value of Ci is 1.4 pF.
Offset voltage adjustment range: A special feature of the 741 family opamp is the provision of offset voltage null capability. For 741C offset voltage
adjustment range is +- 15 mV.
Input voltage range : This is the common-mode voltage that can be applied to
both input terminals without disturbing the performance of an op-amp. For the
741 C, the range of the input common-mode voltage is +-13 V. Commonmode configuration is used only for test purpose to determine the degree of
matching between the inverting and non-inverting terminals.
Common-mode rejection ratio: For 741C, CMRR is typically 90 dB. CMRR
is usually measured under the test condition that the input source resistance R8
<= 10 k. The higher the value of CMRR, better is the matching between the
two input terminals and smaller the output common-mode voltage.
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Supply voltage rejection ratio: The change in an op-amp’s input offset voltage
due to variations in supply voltage is called the supply voltage rejection ration
(SVRR). Some manufacturers use terms like power supply rejection ratio
(PSRR) or power supply sensitivity (PSS). These terms are expressed in
microvolts per volt or in decibels. For 741C, SVRR = 150 V/V. Obviously,
lower the value of SVRR, better the op-amp.
Large Signal Voltage Gain : An op-amp amplifies the difference voltage
between the two input terminals and, therefore, its voltage gain is defined as
Voltage gain = output voltage / differential input voltage
Sincethe amplitude of the output signal is much larger than the input signal,
the voltage gain is commonly referred as large signal voltage gain. For 741C,
typical value is 2,00,000 under test conditions, RL >+ 2 k and V0 = +- 10 V.
Output Voltage Swing: The output voltage swing indicates the value of positive
and negative saturation voltages of an op-amp, and never exceeds the supply
voltage V+ and V-. For 741C, the output voltage swing is guaranteed to be
between +13 V and -13Vfor RL > = 2 k.
Output Resistance: Output resistance R0 is the resistance measured between
the output terminal of the op-amp and the ground. It is 75  for the 741Copamp.
Output Short Circuit Current: This is the current that may flow if an op-amp
gets shorted accidentally and is generally high. The op-amp must be provided
with short circuit protection. The short circuit current Isc for 741C is 25 mA This
means that the built-in short circuit protection is guaranteed to withstand 25 mA
of current.
6.9 OP - AMP Specification
The manufacturers supply data sheets for the IC’s they produce. These data
sheets provide information regarding pin diagram, absolute maximum ratings,
electrical characteristics, equivalent circuit of the devices etc.In this section,
significance of the electrical parameters supplied in a typical op-amp data sheet
is discussed.
The data sheet for a Fairchild A741 op-amp, 741 series are available in
models 741, 741A,741C and 741E. The schematic diagram and electrical
parameters for all these models are the same with only the values of the parameters
differing from one model to another. We will consider specifications for 741C
op-amp.
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From the data sheet it can be seen that:
1. 741 is internally frequency compensated op-amp.
2. 741 is a monolithic IC fabricated using planar epitaxial process.
3. It is useful for integrator, summer, voltage follower and other feed
back applications.
4. Absolute maximum ratings are specified for supply voltage, internal
power dissipation, differential input voltage, input voltage, storage and
operating temperature ranges, soldering pin temperature and output short
circuit duration.
5. 741 is available in all three packages viz 8-pin metal can, 10-pin flat
pack and 8 or 14 pin DIP. The pin diagrams for all these packages are
shown in the data sheet.
6. For 741C, two sets of electrical specifications are available, one set is
applicable at room temperature (250C) and other set applies to the commercial
temperature range (00 to + 700C). As we are interested only in showing the
significance of the parameters listed, we limit the discussion to only one model,
that is 741C at 250C.
6.10 Operational Amplifier Pin Diagram
Supply current: Supply current I8 is the current drawn by the op-amp from the
power supply. It is 2.8 mA for 741C.
Power consumption: This gives the amount of quiescent power
(Vi =
0V) that must be consumed by the op-amp so as to operate properly. It is 85
mW for 741C.
Transient response: The rise time and overshoot are the two characteristics
of the transient response of any circuit. These parameters are of importance
whenever selecting an op-amp for ac applications. The transient response test
circuit is included in the data sheet. For 741C, rise time is 0.3 (symbol)s and
overshoot is 5%.
Slew rate: This is another parameter of importance whenever selecting an opamp for high frequency applications. Op-amp 741C has a low slew rate (0.5 V/
s) and therefore cannot be used for high frequency applications.
CMRR:
Input impedence:
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6.11 Scale Changer/Inverter
In the basic inverting amplifier of 6.15, if the ratio Rf/R1 = K, where K is a
real constant, then the closed loop gain ACL = -K. The circuit thus could be used
to multiply by a constant factor if Rf and R1 , ACL = -1 and the circuit is called an
inverter, i.e., the outpu is 1800 out of phase with respect to input through the
magnitudes are same.
Fig. 6.15 Scale Changer
Summing Amplifier
Op-amp may be used to design a circuit whose output is the sum of several
input signals. Such a circuit is called a summing amplifier or a summer. An inverting
summer or a non-inverting summer may be obtained as disccussed now.
Invertor Summing Amplifier
A typical summing amplifier with three input voltages V1, V2 and V3, three
input resistors R1,R2 ,R4 and a feedback resistor Rf is shown in Fig.6.16. The
following analysis is carried out assuming that the op-amp is an ideal one, that is,
AOL = (symbol) and Ri = (symbol). Since the input bias current is assumed to be
zero, there is no voltage drop across the resistor Rcomp and hence the noninverting input terminal is at ground potential.
The voltage at node ‘a’ is zero as the non-inverting input terminal is grounded.
The nodal equation by KCL at node ‘a’ is
(V1 / R1 ) + (V2 / R2) + (V3 / R3) + (Vo / Rf ) = 0
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Fig 6.16 Inverting Summing amplifier
or Vo = -
Rt
Rt
Rt
--- V1 + --- V2 + --- V3
R2
R1
R3
Thus the output is an inverted, weighted sum of the inputs. In the special
case, when R1 = R2 = R3 = Rfwe have
V0 = -(V1 + V2 + V3)
in which case the output V0 is the inverted sum of the input signals. we may
also set
R1 = R2 = R3 = 3Rf
in which case
Vo= -(V1+V2+V3)
Thus the output is the average of the input signals (inverted). In a practical
circuit, input bias current compensating resistor Rcomp should be provided. to
find Rcomp make all inputs V1 = V2 = V3 = 0. So the effective input resistance R1
= R1 || R2 || R3. Therefore, Rcomp = R1|| Rf = R1 || R2 || R3 || Rf.
AC Voltage Follower
The circuit of a practical ac voltage follower is shown in Fig.6.17. The
circuit is used as a buffer to connect a high impedance signal source to a low
impedance load which may even be capacitive. The capacitor C1 and C2are
chosen high so that they are short circuit at all frequencies of operation. Resistors
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R1 and R2 provide a path for dc input current into the non-inverting terminal. C2
acts as a bootstrapping capacitor and connects the resistance R1 to the output
terminal for ac operation. Hence the input resistance that the source sees is
approximately R1/(1 - ACL) [from Miller’s theorem] where ACLis the gain of the
voltage follower which is close to unity (0.9997). Thus very high input impedance
can be obtained.
Fig. 6.17 AC Voltage follower
1. Voltage to Current Converter (Transconductance Amplifier)
2. Current to Voltage Converter
In many applications, one may have to convert a voltage signal to a
proportional output current. For this, there are two types of circuits possible.
V-I Converter with floating load
V-I Converter with grounded load
Figure 6.18 shows a voltage to current converter in which load ZL is floating.
Since voltage at node ‘a’ is vi, therefore,
vi = iL R1
(I-B = 0)
or iL=Vi / R1
That is the input voltage vi is converted into an output current of vi /R1. It
may be see that the same current flows through the signal source and load and,
therefore, signal source should be capable of providing this load current.
A voltage-to-current converter with grounded loud is shown in Fig.6.18.
Let vi be the voltage at node ‘a’. Writing KVL, we get
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i1 + i2 = iL
or (vi - v1 / R) + (vo - v1 / R) = iL
or vi - v1 - 2 v1 = iLR
Therefore v1 = v1 + v0 - iLR / 2
Fig. 6.18 Voltage to current converter with (a) floating load, (b) Grounded load
Since the op-amp is used in non-inverting mode, the gain of the circuit is 1
+ R/R = 2. The output voltage is,
v0 = 2 v1 = v1 + v0 - iLR
that is,
vi = iLR
or,
iL = vi /R
As the input impedance of a non-inverting amplifier is very high, this circuit
has the advantage of drawing very little current time from the source. A voltage
to current converter is used for low votage dc and ac voltmeter, LED and zener
diode tester.
Current to Voltage Converter (Transresistance Amplifier)
Photocell, photodiode and photovoltaic cell give an output current that is
proportional to an incident radiant energy or light. The current through these
devices can be converted to voltage by using a current-to-voltage converter
and thereby the amount of light or radiant energy incident on the photo-device
can be measured.
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Fig. 6.19 Current to Voltage converter
Figure. 6.19. shows an op-amp used as I to V converter. Since the (-)
input terminal is at virtual ground, no current flows through R8 and current i8
flows through the feedback resistor Rf. Thus the output voltage v0 = i8Rf. It may
be pointed out that the lowest current that this circuit can measure will depend
upon the bias current IB of the op-amp. This means that A741(IB = 3 nA) can
be used to detect lower currents. The resistor Rf is sometimes shunted with a
capacitor Cf to reduce high frequency noise and the possibility of oscillations.
Finding Square Roots
A divider circuit can be used to find square roots by connecting both the
inputs of the multiplier tothe output of an op-amp as shown in Fig.6.20.
VA = V2o / Vref
and VA = -Vin
So Vo2 = -Vin Vref
Fig. 6.20 Finding Square roots
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303
Thus, output V0 is proportional to square root of magnitude of Vin. Note
the Vinmust be negative orelse op-amp will saturate. The rangeof Vin lies between
-1 and -10 V.
Differentiator
One of the simplest of the op-amp circuits that contain capacitor is the
differetiating amplifier, or differentiator. As the same suggests, the circuit performs
the mathematical operation of differentiation, that is, the output waveform is the
derivative of input waveform. A differentiator circuit is shown in Fig.6.21.
Analysis
The node N is at virtual ground potential i.e., vN = 0. The current iC through
the capacitor is,
iC = C1 d/dt (vi-vN)=C1 dvi./dt
The current if through the feedback amplifier is v0 / Rf and there is no
current into the op-amp. Therefore, the nodal equation at node N is,
C1 dvi / dt + vo/RF = 0
from which we have
vo = RFC1 dvi / dt
Fig. 6.21 Op-amp differentiator
For a square wave input, say 1V peak and 1 KHz, the output waveform
will consist of positive and negative spikes of mangitude Vsat which is
approximately 13V for +- 15V op-amp power supply.
During the time periods for which input is constant at +- 1V, the differentiated
output will be zero. However, when input transits between +- 1V levels, gets
clipped to about +- 13 V for a +- 15V op-amp power supply as shown in Fig.
6.22
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Fig. 6.23 (a) Sine-wave input and cosine output (b) Square wave input and spike output
Integrator
If we interchange the resistor and capacitor of the differentiator, we have
the circuit of Fig. 6.23 which as we will see, is an integrator. The nodal equation
at node N is,
(vi / R1)+ Cf (dvo / dt) = 0
or, dvo / dt = -(1 / R1Cf ) vi
Integrating on both sides,
Fig. 6.23 (a) Op-amp integrator
where v0 (0) is the initial output voltage.
The circuit, thus provides an output voltage which is proportional to the
time integral of the input and R1CF is the time constant of the integrator. It may
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305
be noted that there is a negative sign in the output voltage, and therefore, this
integrator is also known as an inverting integrator. A resistance Rcomp = R1 is
usually connected to the (+) input terminal to minimize the effect of input bias
current.
A simple low pass RC circuit can also work as an integrator when time
contant is very large. This requires very large values of R and C. The components
R and C cannot be made infinitely large because of practical limitations. However,
in the op-amp integrator of Fig.6.24, by Miller’s theorem, the effective input
capacitance becomes CF (1 - Av) where Av is the gain of the op-Amp.The gain
Av is infinite for an ideal op-Amp, so the effective time constant of the op-Amp
integrator becomes very large which results in perfect integration.
The operation of the integrator can also be studied in the frequency domain.
In phase notation, Equation can be written as
Vo(8) = - 1 / sR1CF) Vi(s)
In steady state, put s = jw and we get
Vo(jw) = - 1 /jwR1CF) Vi(jw)
So, the magnitude of the gain or integrator transfer function is
The frequency response (or Bode Plot) of this basic integrator is shown n
fig.6.23. The Bode Plot is a straight line of slope -6B/octave (or equavalently 20 dB/decade). The frequency fb in Fig 6.24 is the frequency at which the gain
of the integrator is 0 dB and is given by
fb = 1 / 2 R1CF
Fig. 6.24 Frequency response of a basic and lossy integrator
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It can further be seen from that at w = 0, the magnitude of the integrator
transfer function is infinite. At dc, the capacitor CF behaves as an open circuit
and there is no negative feedback. The op-amp thus operates in open loop,
resulting in an infinite gain. In practice, of course, output never becomes infinite,
rather the output of the amplifier saturates at a voltage close to the op-Amp
positive or negative power supply depending on the polarity of the input dc
signal.
As the gain of the integrator decreases with increasing frequency, the
integrator circuit does not have any frequency problem as faced in a differentiator.
However, at low frequencies suhc as at dc (=0), the gain becomes infinite (or
saturates). The solution to the problem is discussed in the following.
OP - AMP Comparitor
The challenge sounds simple enough - take a 60 Hz (or 50 Hz) sinewave
from the AC power line and convert it to a square wave. This signal will serve as
a clock to drive counters for a 24 hour time clock. So you hook up an op-Amp
as a comparator to do the job. But your surprised to see the clock running too
fast! With oscilloscope in hand you discover the AC line is noisy! And to your
horror, you see glitches (additional edges) at the comparator’s output, causing
the counters to advance too quickly. What you need is a better comparator,
immune to the noise swinging above and below the comparator’s threshold.
Fig. OP - Amp Comparitor
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307
6.12 Block Diagram of IC 555
In industrial applications, the word timer isused more commonly. The timer
is basically a small subsystem as the control element of a much larger electrical
power system.
Energy
Source
Timer
Controlled
Device
Fig. 6.27 Block diagram of Timer System
Fig.6.27 illustrates the block diagram of a timer controlling the operation of
a device. A timer is basically as small unit of a control element of a system. To
perform particular operation or to operate a machine for a particular time,
manually it is difficult. The quality and rate of product gets affected if operated
manually. Therefore the introduction of timer became must and overcome all
these problems. Usually times is a small in size and often permanently connected
to the machine.
Classification of Timers
Timers may be classified in the following ways.
1. According to the function performed by it. They are
(a) Delay timers
(b) Interval timers
(c) Repeat cycle timer
(d) Reset timer
2. According to the techique used to achieve the industrial timing
(a) Thermal timers
(b) Electromechnical timers
(c) Electrochemical timers
(d) Mechanical timers
(e) Pneumatic timers
(f) Electronic timers
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Applications of Timers
1. Control systems
2. Industrial use
3. Computers
4. Communication system
5. Measuring systems
6. Clock circuits
7. Motor starters etc.
IC 555 TIMER (ELECTRONIC TIMER)
Fig 6.28 gives the functional block diagram of 555 IC timer. The three
equal resistors R1, R2 and R3 serves as internal voltage divider for the source
voltage. Thus one third of source voltage VCC appears across each resistor. The
voltages at point P1 and P2 serves as reference voltages for the comparators.
The reference voltage for comparator 2 is + 1/3 VCC. If a trigger pulse is applied
at the negative input of the comparator drops below + 1/3 VCC it causes a
change in state. Comparator 1 is reference at voltage + 2/3 VCC . The output of
each comparator is fed to the input termints of the flip-flop.
Fig. 6.28 Block diagram of IC 555 Timer
The flip-flop changes states according to the the voltage values of its input.
Thus if the voltage at the threshold terminal rises above +2/3 VCC it causes
comparator 1 to cause flip-flop to change its state. On the other hand, if the
trigger voltage falls below +1/3 VCC it causes comparator 2 to change its state
and hance cause flip-flop to change its states. Thus the output of the flip-flop is
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controlled by the voltages of the two comparators. A change in state occurs
when he threshold voltage rises above +2/3 VCC or when the trigger voltage
drops below +1/3 VCC .
The output of the flip-flop is used to drive the discharge transistor and the
output voltage. A high flip-flop output turns ON both the discharge transistor
and the output stage.The discharge transistor becomes conductive and behaves
as a low resistance short circuit to ground. The output stage behaves similarity.
When the flip-flop output assumes the low state, reverse action takes place.
Thus the output stage is high. Thus the output and discharge transistor state
depends on the voltage applied to the threshold and the trigger input terminals.
IC 555 as Monostable Multivibrator
Fig.6.29 shown 555 IC connected as a monostable multivibrator which
uses only one resistors R and one capacitor C.
Fig. 6.29 IC 555 as Monostable Multivibrator
In monostable multivibrator, a simple output pulse is generated in response
to one input trigger pulse. When a negative input going place is applied at the
trigger input (pin 2) this result in outut (at pin 3) to go high to VCC . The trigger
pulse cause the comparator 2 to drop below its reference voltage +1/3 VCC and
this in turn causes the flip-flops to go its low state. A negative voltage to the
discharge transistor causes its resistance to become infinite. This in turn removes
the shunt to ground for capacitor C. Hence the voltage across C begins to rise
in accordance witht he time constant RC when this voltage across C exceeds
+ 2/3 VCC . It causes the comparator to change states and result in discharge
transistor again becoming conductive. Capacitor then discharges very quickly
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to ground through pin 7. The output voltage drop to its low or ground state.
Thus the output stage follows the change in the trigger level. The time duration T
of the output is given by T = 1.1 RC.
6.13 IC 555 Astable Multivibrator:
The astable multivibrator using IC 555 is shown in Fig.6.30. During the
charging up period transistor T1 is held open by the flip-flop and the capacitor
charges through the series connected resistors RA and RB. Whe the voltage
across the capacitor reaches the reference level of the upper comparator 2 VCC
/ 3, the comparator changes the state of the flip-flop and this terms the transistor
T1 ON. The capacitor discharges through resistor RB until its voltage reaches
the reference level of the lower comparator VCC /3. This comparator changes
the state of flip-flop again, which in turn makes the transistor T1 OFF and the
cycle repeats
The charging time of the capacitor is determined by
T1 = C (RA + RB) loge (VCC - VCC /3) / VCC - 2 VCC/3
The above equation immediately follows from the fact that the charging of
capacitor starts from VCC /3 instead of zero. Further the charging continues upto
2 VCC / 3, after which the upper comparator changes state. The equation ()
simplifies to
T1 = C (RA + RB) loge 2
(or) T1 = 0.7 (RA + RB) C
Fig. 6.30 IC 555 Astable Multivibrator
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6.14 Phase locked Loop (PLL)
A phase locked loop (PLL) is basically a closed loop feedback system.
The action of PLL is to lock or synchronise the frequency of a controlled oscillator
to that of an incoming signal. Basically, PLL consists of three functional blocks.
1. Phase detector
2. Voltage controlled oscillator (VCO)
3. Low pass filter
The basic loop may also contain an amplifier
Block diagram of basic PLL is shown in Fig.6.31. The phase detector
exhibits a multiplier characteristics. With no input signal applied to the PLL, the
output from the phase detector is zero. The error voltage applied as the control
signal to V.C.O is also zero and VCO operates at its free running frequency f0.
Fig. 6.31 Block diagram of Basic PLL
This frequency is referred to as the centre frequency. If an input signal is
applied to the loop the phase detector produces an output signal which contains
components as the sum and differences frequencies i.e., fs + f0 and fs - f0. If f2 is
significantly different from f0 then both components do not fall into the pass band
of low pass filter, and hence, are attenuated. Under such condition, the frequency
of VCO is not changed and the loop does not acquire a lock.
If the input signal frequency fs has values such that the frequency component
fs - f0 lies within the passband of the low pass filter, then this component is
amplified and applied as a control signal to the VCO. This causes the VCO
frequency to vary in a direction which reduces the frequency difference between
fs and f0 . If fs is sufficiently close to fo the feedback action of the loop causes the
VCO frequency is identical to that of the input signal and the component fs - fo is
a direct voltage of magnitude proportional to cos (symbol) where (symbol) is
the phase difference between the input signal and the VCO signal. The action of
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the loop is to cause (symbol) to take on just that value which is required to
generate the d.c. control voltage necessary to change the frequency of the VCO
from free running value to frequency of the input signal. This action allows the
PLL to ‘track’ any frequency changes of the input signal once lock has been
acquired.
6.15 Functional Block Description Of PLL Type LM -565
Functional block diagram of LM - 565 PLL is shown in Fig. 6.32 and pin
diagram is shown in Fig.6.32. It is self-contained, adaptable filter and
demodulator for the frequency range from 0.001 Hz to 500 kHz. The circuit
consists of a VCO, a phase comparator, an amplifier and a low pass filter.
R2, C2 form a low pass filter. The capacitor C2 is connected externally while
R2 is an internal resistor of value 3.6 k(symbol). Here the free running frequency
of the VCO is determined by the values of a external resistor R, connected
between pin 8 and the positive supply line and an external capacitor C, connected
between pin 9 and the negative supply line. A capacitor ofvalue typically 0.001mF
is normally connected between pins 7 and 8 to eliminate possible oscillation in
the VCO voltage controlled current source.
The square wave output signal of VCO is available at pin 4 and in order to
close the loop, pin 4 must be connected externally to the phase comparator
input Pin 5. The amplified loop error voltage which is applied as the control
signal tothe VCO is available at pin 7. This signal is referenced to the positive
supply line. A reference voltage which is normally equal to the voltage at Pin 7 in
available at Pin 6 and this allows differential stages to be both biased and driven
by connecting them to Pin 6 and 7. The signal inputs to the phase comparator
are differential at Pin 2 and Pin 3, and the d.c. level at these two Pins must be
made the same. If dual power supplies are used it is simplest to bias 2 and 3 at
the potential of the common power supply line. With single supply operation
they should be biased to a level in the lower half of the total power supply
voltage by means of a suitable potential divider.
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Fig. 6.32 LM 565 PLL
6.16 Operation of VCO LM566.
LM566C Voltage Controlled Oscillator
General Description
The LM566CN is a general purpose voltage controlled oscillatorwhich
may be used to generate square and triangular waves, the frequency of which is
a very linear function of a control voltage. The frequency is also a function of an
external resistor and capacitor.
The LM566CN is specified for operation over the 0oC to a 70oC
temperature range.
Features
• Wide supply voltage range: 10V to 24V
• Very linear modulation characteristics
• High temperature stability
• Excellent supply voltage rejection
• 10 to 1 frequency range with fixed capacitor
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6.17 PLL Lock Range
The range of frequencies over which a PLL can maintain lock with a input
signal is called the ‘lock range’ of the system. This is always longer than the
band of frequencies over which the PLL can acquire lock with an incoming
signal. The lock range is decreases as higher order odd harmonics are used to
achieve lock.
6.18
PLL Capture RangeWorking
The range of frequencies over which a PLL can acquire lock is known as
the ‘capture range.’ The greatest capture range possible is equal to the lock
range but in general the capture range is less than the lock range. The capture of
an input signal does not takes place as soon as the signal is applied, but takes
finite time called the ‘Pull-In’ time to establish lock.
6.19 Application of PLL
1. For FM demodulation
2. For AM demodulation
3. For frequency multiplication
4. For frequency shift keying
5. In modems
6. For frequency division
7. In Telemetry receivers.
6.20 Frequency Multiplier Using PLL
Fig 6.33 shows a practical circuit for frequency multiplication using PLL.
Frequency multiplication can be achieved in two different ways.
1. Locking to harmonic of the input signal (or)
2. Including a counter in the loop between the VCO and the phase
comparator.
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Fig. 6.33 Frequency multiplier using PLL
Fig. 6.33 shows the second method which can provide large multiplication
of frequency. To set up the circuit, the frequency limits of the input signal are
determined. The f0 is adjusted by means of R1 and C1 which ensures the output
frequency of the divider midway between the input frequency limits. The value
of C2 is selected large enough to eliminate variations in the demodulated output
voltage at Pin 7 so that the VCO frequency is established. The output can now
be taken as the VCO square wave output, and its fundamental will be the desired
multiple of the input frequency as long as the loop is in lock.
Summary
1. An op-amp can be used to perform mathematical operations such as
scale changer, addition and subtraction.
2. An instrumentation amplifier is useful for amplifying low level signals
which are obtained by sensing with a transducer in the measurement of physical
quantities like temperature, water flow etc.
3. Op-Amps can be used for amplifying both ac and dc inputs. A capacitively
coupled amplifier is used for amplifying ac signals only.
4. The V - to - I converters are useful in low voltage dc and ac voltmeters,
LED and zener diode testers.
5. The I-to -V converters are used for testing photo devices.
6. A diode in the feedback loop of an op-amp behaves as a precission
diode as its cut-in voltage gets divided by the open-loop gain of op-amp.
7. A precision diode may be used for half-wave rectification, full wave
rectification, peak-value detector, clipper and clamper.
8. A sample and hold circuit samples an input signal and holds on to its last
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sampled value until the input is sampled again. Thie circuit is used in analog to
digital interfacing and pulse modulating system.
9. Op-Amp may be used to perform functions such as In, log, antilog,
multiply or divide signals.
10. The op-Amp integrator and differentiator are useful for signal wave
shaping.
11. Integrators are preferred over differentiators for analog computers as
the gain of integrator decreases with increasing frequency and hence signal to
noise ratio of integrator is higher than that of differentiator.
12. Monolithic audio power amplifiers with built in heat sink are available.
13. The operational transconductance amplifier (OTA) outputs a current
proportional to its input voltage. OTAs are used to build programmable gain
voltage amplifiers, voltage controlled resistances, neural networks etc.
Short Answer Type Questions
1. Mention the types voltage regulators.
2. Mention the types of IC regulators.
3. What are the applications of Differential amplifiers. ?
4. What are the IC numbers of positive/negative regulators?
5. What is adjustable voltage regulator ?
6. Define Op-Amp input impedence,open loop gain.
7. Define Op-Amps slew rate,CMRR.
8. Define Op-Amps input offset voltage,input offset current.
9. Mention the number of pins in IC741.
10. Write applications of Op-Amp.
11. What are the applications of IC555 ?.
12. What is PLL.?
13. What are applications of LM565 ?
14. What is lock range of PLL?.
15.What is capture range of PLL ?
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16.What are the applications of PLL ?
Long Answer Type Questions
1. Explain the operation of transister series voltage regulator.
2. Explain the operation of shunt voltage regulator.
3. Explain the operation of positive/negative voltage regulator using IC
78XX.
4. Explain the operation of adjustable voltage regulator by using LM317.
5. Draw and explian working of an operational amplifier.
6. What are the specifications of ideal Op-Amp?
7. Draw and explain working of IC 741.Write applications.
8. Draw and explain working of IC555.
9. Explain working of astable multivibrator using 555ic.
10. Draw and explain working of PLL-LM565.
11. Explain the operation of LM566.
12. Explain the frequency multifier and FM demodulater using PLL.
Practical/OJT Questions
1. Study the operation of series/shunt voltage regulators.
2. Study the operation of IC regulators.
3. Study the operation of Op-Amp and its applications
4. Study the operation of differential amplifier and its applications.
5. Study the operation of 555IC.
6. Study the operation of LM 565,LM566 and its applications.
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UNIT
7
Power Electronic Devices
Learning Objectives
• Study of power electronics devices SCR,DIAC,TRIAC,GTOs,PUT,
QUADRACS, SIDACS, SLRs, UJT, RCT, MCT, IGBT, etc,.symbols.
• Study of construction and working of SCR.
• Study the construction and working of DIAC and TRIAC.
• Study the triggering of SCR by using UJT.
• Study the operation and working of Thtrister.
7.0 Introduction
Thyristers are a family of semiconductor devices that exibits bi-stable currentvoltage characteristics and can be switched between a high impedence, low
current of state and a low impedance, high current on state.
7.1 The Thyrister Family Devices are
(1) DIACS
(2) Gate Turn - off (GTO) Thyristers
(3) Programmable Unijunction Transisters
(4) Quadracs
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(5) Sidacs
(6) Silicon Controlled Rectifies SLR
(7) Thyrister surge suppressors
(8) Thyristors
(9) TRIACS
(10) Unijunction Transistors
The diode acts as a switch during forward bias condition. The characteristic
curve of the PNPN diode is shown in the figure.
7.2 Thyristor Family Devices:
The impartent multilayer devices of the Thyristor family as follows,
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7.3 SCR ISI Specification (Silicon Controlled Rectifier)
The basic structure and circuit symbol of SCR is shown in fig. It is a four
layer three terminal device in which the end P-layer acts as anode, the end Nlayer acts as cathode and P-layer nearer to cathode acts as gate. As leakage
current in silicon is very small compared to Germanium, SCRs are made of
Silicon and not Germanium.
Fig. 7.2 Basic Structure and Circuit Symbol of SCR
Constructional Details of SCR
The internal equivalent circuit of SCR is two transistors PNP,NPN are
arranged back to back,the feedback occures because collector of Q1 drives the
base of Q2 and vice-versa.Assuming normal transistor common emitter phas
reversal and producing from Q2 to Q1 we can observe that positive going voltage
at the base of Q2 is inverted as its collector.This is common with Q1 s
base.Transistor Q1 also inverts ,and signal at its collector is then in phase with
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the original input voltage.Thae overall current gain of the two transistor
arrangement is equal to 1*2.The total anode to cathode current is given by
Ia=Ic1+Ic2
consider the gate voltage,if Vgk is zero or negative the NPN trasistor Q2 is
biased off and therefore Q1 is never turned on.Thus Ia equals the sum of the
leackage currents of Q1 and Q2.Tresistance from anode to cathode is very high
and the anod to cathode voltage drop is high.Therefore the switch is open.If Vgk
is sufficiently positive,Q2 is forward biased.Therfore Ic2 increases and Q1 is tured
ON. Thus the becomes regenerative.
Both transistors saturate.This reduses the forward resistance and voltage
drop drastically,so the switch is closed .Once the SCR is on ,there is no need
for gate current.This is because Ic1 is sufficient to maintain Ib2 and keep Q2
ON.
The only way to turn off the SCR is reduce Ia below some minimum holding
current.This sometimes disadvantage with SCR.
Due to this ON-OFF action SCR can be triggered from the open or
blocking state to the closed or low resistance high conducting stage.
7.4 Volt-Ampere, Charactrtistics of SCR
This is a graph between Ia anode current versus V ak voltage for different
gate currents.When the anode voltage is negative the SCR works as two revrese
biased PN junctions.A small leackage current flows .When VAK exceeds the
reverse break down voltage
Fig.7.3 SCR VI Characteristics
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The characteristics of SCR are shown in Fig. SCR acts as a switch when it
is forward biased. When the gate is kept open, i.e. gate current IG = 0, Operation
of SCR is similar to PNPN diode. When IG < 0, the amount of reverse bais
applied to J2 is increased. So the breakover voltage VBO is increased. When IG
>0, the amount of reverse bias applied to J2 is decreased thereby decreasing the
breakover voltage. With very learge positive gate current breakover may occur
at a very low voltage such that the characteristics of SCR is similar to that of
ordinary PN diode. As the voltage at which SCR is switched ‘ON’ can be
controlled by varying the gate current IG . It is commonly called as controlled
switch. Once SCR is turned ON, the gate loses control i.e. the gate cannot be
used to switch the device OFF. One way to turn the device OFF is by lowering
the anode current below the holding current IH by reducing the supply voltage
below holding voltage VH, keeping the gate open.
SCR is used in relay control, motor control, phase cotrol,heater control,
battery chargers inverters, regulated power supplies and as static switches.
Two transitor version of SCR The operation of SCR can be explained in a
very simple way by considering it in terms of two transistors, called as the two
transistor version of SCR. As shown in fig 7.4., an SCR can be split in to two
parts and displayed machanically from one another but connected electrically.
Thus the device may be considered to be constituted by two transistors. T1
(PNP) and T2 (NPN) connected back to back.
Ib1=IA = Ie1 = IA - 1IA =(I - 1) IA
(7.01)
Also, from the Fig 7.03, it is clear that
Ib1=Ic2
(7.02)
and Ic2=2Ik
(7.03)
Substituting the values given in Eqs (7.02) and (7.03) in Eq. (7.01) we get
(1-1) IA = 2IK
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Fig. 7.4 Two Transistor Version of SCR
We know that IK - IA + 1g.
(7.03)
Substituting Eq. (7.03) in Eq (7.02) we obtain
(7.04)
Equation(7.04) indicates that (1+ 2) = I then IA = infinity, i.e. the anode
current IA suddenly reaches a very high value approaching infinity. Therefore,
the device suddenly triggers into ON state from the original OFF state. This
characteristics of the device is known as its regenerative action.
The value of (1+ 2) can be made almost equal to unity by giving a
proper value of positive current Ig for a short duration. This signal Ig applied at
the gate which is the base of T2 will cause a flow of collector current IC2 by
transferring T2 to its ON state. As IC2 = Ib1, the transistor T1 will also be switched
ON. Now the action is regenerative since each of the transistors would supply
base current to the other. At this point even if the gate signal is removed, the
device keeps on conducting, till the current level is maintained to a minimum
value of holding current.
7.5 SCR Ratings
The following are the ratings of SCR
1. Current at break over point
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2. Off state fotward current
3. Holding current
4. Gate Current
5. Latching current
6. Reverse Current
7. On State current
8. On state forward average current
9. On state forward RMS current
10. Repetitive peak forward current
11. Maximum surge forward current
12. Power Dissipation
13. Gate power dissipation
14. Blocking resistance
15. Dynamic forward resistance
16. Thermal resistance
17. Delay time
18. Junction recovery time
19. Turn on time
20. Turn off time
21. Raised Time
22. Recovery time
23. Forward blocking RMS voltage
24. Forward brake over voltage
25. Reverse breakover voltage
26. Repetative Peak off straight forward voltage
27. Non repitative off stage forward voltage
28. Gate voltage
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29. Gate voltage
30. Repitative peak reverse voltage
31. On State forward voltage drop.
7.6 Constructional Details of DIAC and TRIAC.
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Fig. 7.6
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7.7 DIAC, TRIAC Volt-Ampere Characteristics Forward
and Reverse BIAS
Construction – Working – Characteristics – Diac as bi-directional switch.
(for learning activity )DIAC symbol:
Fig. 7.5(a) Diac Symbol
Fig. 7.5(b) Diac VI Characteristics
Construction:
The DIAC is basically a two terminal parellel-inverse combination of
semiconductor layers that permits triggering in either direction. The basic
arrangement of the semiconductor layers of the diac is shown in the figure, along
with its graphical symbol. Nore that either terminal is referred as the cathode.
Instead, there is an anode 1 and an anode 2. When the anode 1 is positive with
respect to anode 2, the semiconductor
Operation:
DIAC circuits use the fact that a DIAC only conducts current only after a
certain breakdown voltage has been exceeded. The actual breakdown voltage
will depend upon the specification for the particular component type.
When the diac break down voltage occurs, the resistance of the component
decreases abruptly and this leads to a sharp decrease in the voltage drop across
the diac, and a corresponding increase in current. The DIAC will remain in its
conducing state until the current flow through it drops below a particular value
known as the holding current. When the current falls below the holding current,
the DIAC switches back to its high resistance, or non-conducting state.
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DIAC are widely used in AC applications and it is found that the device is
“reset” to its non-conducting state, each time the voltage on the cycle falls so
that the current falls below the holding current. As the behaviour of the device is
approximately equal in both directions, it can provide a method of providing
equal switching for both halves of an AC cycle, e.g for triacs.
Most DIAC s have a breakdown voltage of around 30 volts, although the
exact specifications will depend upon the particular type of device.. Interestingly
their behaviour is somewhat similar to that of a neon lamp, although they offer a
far more precise switch on voltage and thereby provide a far better degree of
switching equalisation.
TRIAC Symbol
Fig. 7.6 TRIAC Symbol
The structure of a TRIAC may be considered as a p-n-p-n structure and
the triac may be considered to consist of two conventional SCRs fabricated in
an inverse parallel configuration.
In operation, when terminal A2 is positive with respect to A1, then a positive
gate voltage will give rise to a current that will trigger the part of the triac consisting
of P1 N1 P2 N2 and it will have an identical characteristic to an SCR. When
terminal A2 is negative with respect to A1 a negative current will trigger the part
of the triac consisting of P2 N1 P1 N3. In this way conduction on the TRIAC
occurs over both halves an alternating cycle.
TRIAC structure
Triacs do not fire symmetrically as a result of slight differences between the
two halves of the device. This results in harmonics being generated, and the less
symmetrical the triac fires, the greater the level of harmonics produced. It is
generally undesirable to have high levels of harmonics in a power system and as
a result TRIACS are not favoured for high power systems. Instead two thyristors
may be used as it is easier to control their firing.
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To help in overcoming this problem, a device known as a diac (diode AC
switch) is often placed in series with the gate. This device helps make the switching
more even for both halves of the cycle.
Fig. 7.7 Structural Symbol of TRIAC
This results from the fact that the diac switching characteristic is far more
even than that of the TRIAC. Since the diac prevents any gate current flowing
until the trigger voltage has reached a certain voltage in either direction, this
makes the firing point of the TRIAC more even in both directions.
Fig. 7.8 V.I Characterstics of TRIAC
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7.8 Different Modes of TRIAC Triggering
TRIAC Triggering modes are as follows
a. First quadrant operation.
b. Second quadrant operation
c. Third quadrant operation
d. Fourth quadrant operation
7.9 SCR Circuit Triggered by UJT
One common application of the Uni Juntion Transistor is the triggering of
the other devices such as the SCR, TRIAC etc. The basic elements of such a
triggering circuit are shown in figure. The resistor RE is chosen so that the load
line determined by RE passes through the device characteristic in the negative
resistance region, that is, to the right of the peak point but to the left of the valley
point, as shown in figure. If the load line does not pass to the right of the peak
point P, the device cannot turn on.
For ensuring turn-on of UJT
RE < VBB – VP­ / IP
V BB
Motor lamp
heater or some
other device
UJT Triggering of an SCR
Peak Point P
R1 (Load Line)
Negative Resistance
Region
Valley Point
Fig. 7.9 How to trigger TRIAC by using UJT
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This can be established as below
Consider the peak point at which IRE = Ip and VE = VP
(the equality IRE = IP is valid because the charging current of capacitor, at
this instant is zero, that is, the capacitor, at this particular instant, is changing
from a charging state to
a discharging state). Then VE = VBB – IRE RE
So, RE(MAX) = VBB – VE­ / IRE = VBB – Vp­ / IP at the peak point.
At the valley point, V
IE = IV and VE = VV so that
VE = VBB – IRE RE
So RE(MIN) = VBB – VE / IRE = VBB – VV / IV or for ensuring turn-off.
RE > = VBB – VV / IV
So, the range of resistor RE is given as
VBB – VP / IP >RE > VBB – VV / IV
The resistor R is chosen small enough so as to ensure that SCR is not
turned on by voltage VR when emitter terminal E is open or IE = 0
The voltage VR = RVBB/R + RBB for open-emitter terminal.
The capacitor C determines the time interval between triggering pulses and
the time duration of each pulse. By varying RE, we can change the time constant
RE C and alter the point at which the UJT fires. This allows us to control the
conduction angle of the SCR, which means the control of load current.
7.10 Power Control Circuits of DIAC, TRIAC and SCR
Power electronic converters can be found wherever there is a need to
modify a form of electrical energy (i.e. change its voltage, current or frequency).
The power range of these converters is from some milli Watts (as in a mobile
phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With
“classical” electronics, electrical currents and voltage are used to carry information,
whereas with power electronics, they carry power. Thus, the main metric of
power electronics becomes the efficiency.
The first very high power electronic devices were mercury-arc valves. In
modern systems the conversion is performed with semiconductor switching
devices such as diodes, thyristors and transistors, as pioneered by R. D.
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Middlebrook and others beginning in the 1950s. In contrast to electronic systems
concerned with transmission and processing of signals and data, in power
electronics substantial amounts of electrical energy are processed. An AC/DC
converter (rectifier) is the most typical power electronics device found in many
consumer electronic devices, e.g. television sets, personal computers, battery
chargers, etc. The power range is typically from tens of watts to several hundred
watts. In industry the most common application is the Variable Speed Drive
(VSD) that is used to control an induction motor. The power range of VSDs
start from a few hundred Watts and end at tens of mega Watts.
The power conversion systems can be classified according to the type of
the input and output power
· AC to DC (rectifier)
· DC to AC (inverter)
· DC to DC (DC-to-DC converter)
· AC to AC (AC-to-AC converter)
Switching
As efficiency is at a premium in a power electronic converter, the losses
that a power electronic device generates should be as low as possible. The
instantaneous dissipated power of a device is equal to the product of the voltage
across the device and the current through it P= V x I. The losses of a power
device are at a minimum when the voltage across it is zero (the device is on) or
when no current flows through it (off). Power electronic converters are built
around one (or more) device operating in switching mode.
Practical devices have non-zero voltage drop and dissipate power when
on, and take some time to pass through an active region until they reach the “on”
or “off” state. These losses are a significant part of the total lost power in a
converter.
Devices
The capabilities and economy of power electronics system are determined
by the active devices that are available. Their characteristics and limitations are
a key element in the design of power electronics systems. Formerly, the mercury
arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered
devices such as the thyratron and ignitron were widely used in power electronics.
As the ratings of solid-state devices improved in both voltage and current-
Paper - II Electronic Devices and Circuits
339
handling capacity, vacuum devices have been nearly entirely replaced by solidstate equivalents, or by solid state devices that have no thermionic equivalent.
Power electronic devices may be used as switches, or as amplifiers. [1] An
ideal switch is either open or closed and so dissipates no power; it withstands
an applied voltage and passes no current, or passes any amount of current with
no voltage drop. Semiconductor devices used as switches can approximate this
ideal property and so most power electronic applications rely on switching devices
on and off, which makes systems very efficient as no power is wasted in the
switching devices. By contrast, in the case of the amplifier, the current through
the device varies continuously according to a controlled input. The voltage and
current at the device terminals follow a load line, and the power dissipation
inside the device is large compared with the power delivered to the load.
Several attributes dictate how devices are used. Devices such as diodes
conduct when a forward voltage is applied and have no external control of the
start of conduction. Power devices such as silicon controlled rectifiers and
thyristors (as well as the former mercury valve and thyratron) allow control of
the start of conduction, but rely on periodice reversal of current flow to turn
them off. Devices such as gate turn-off Thyristors, bipolar junction transistors.
(BJT), and MOSFET transistors provide full switching control and can be turned
on or off without regard to the current flow through them. Transistor devices
also allow proportional amplificaton, but this is rarely used for systems rated
more than a few hundred Watts. The control input characteristics of a device
also greatly affect design; sometimes the control input is at a very high voltage
with respect to ground and must be driven by an isolated source.
Devices vary in switching speed. Some diodes and Thyristors are suited
for relatively slow speed and are useful for power freqauency switching and
control; certain thyristors are useful at a few KHz. Devices such as MOSFETS
and BJTs can switch at tens of KHz up to a few megahertz in power applications,
but with decreasing power levels. Very high power (hundreds of KW) at very
high frequency (hundreds or thousands of megahertz) is still the area where
vacuum tube devices predominate. The use of faster switching devices minimizes
energy lost in the transitions from on to off and back, but may create problems
with radiated electtromagnetic interference. Gate drive (or equivalent) circuits
must be designed to supply sufficient drive current to achieve the full switching
speed possible with a device. A device that doesn’t get sufficient drive to switch
rapidly, may be destroyed by excess heating.
Power handling and dissipation of devices is also a critical factor in design.
Power electtronic devices may have to dissipate tens or hundreds of watts of
waste heat, even switching as efficiently as possible between conducting and
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non-conducting states. In the switching mode, the power controlled is much
larger than the power dissipated in the switch. The forward voltage drop in the
conducting state translates into heat that must be dissipated. High power
semiconductors require speicalized heat sinks or active cooling systems to keep
their junction temperature from rising too high; exotic semicoductors such as
Silicon carbide have an advantage over straight silicon in this respect, and
Germanium, once the main-stay of solid-state electronics is now little used due
to its unfavorable properties at high temperature.
Semiconductor devices exist with ratings up to a few KV in a single device.
Where very high voltage must be controlled, multiple devices must be used in
series, with networks to equalize voltage across all devices. Again, switching
speed is a critical factor since the slowest-switchind device will have to withstand
a disproportionate share of the overall voltage. The former mercury valves were
available with ratings to 100 KV in a single unit, simplifying their application in
HVDC systems.
The current rating of a semiconductor device is limited by the heat generated
within the dies and the heat developed in the resistance of the interconnecting
leads. Semiconductor devices must be designed so that current is evenly distributed
within the device across its internal junctions (or channels); once a “hot spot”
develops, breakdown effects can rapidly destroy the device. Certain SCRs are
available with current ratings to 3000 Amperes in a single unit.
Applications
Power electronic systems are found in virtually every electronic device.
For example:
DC/DC converters are used in most mobile devices (mobile phones, PDA
etc.) to maintain the voltage at a fixed value whatever the voltage level of the
battery is. These converters are also used for electronic isolation and power
factor correction.
AC/DC converters (rectifiers) are used every time an electronic device is
connected to the mains (computer, television etc.). These may simply change
AC to DC or can also change the voltage level as part of their operation.
AC/AC converters are used to change either the voltage level or the
frequency (international power adapters, light dimmer). In power distribution
networks AC/AC converters may be used to exchange power between utility
frequency 50 Hz and 60 Hz power grids.
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341
DC/AC converters (inverters) are used primarily in UPS or renewable
energy systems or emergency lighting systems. When mains power is available,
it will charge the DC battery. If the mains fails, an inverter will be used to produce
AC electricity at mains voltage from the DC battery.
7.11 Working of Reverse Conducting Thyristor
Reverse conducting Thyristor
A reverse conducting thyristor (RCT) has an integrated reverse diode, so
is not capable of reverse blocking. These devices are advantageous where a
reverse or freewheel diode must be used. Because the SCR and diode never
conduct at the same time they do not produce heat simultaneously and can
easily be integrated and cooled together. Reverse conducting Thyristors are
often used in frequency changers and inverters.
7.11(a) Thristor (RCT)
The thyristor is a four-layered, three terminal semiconducting device, with
each layer consisting of alternately N-type or P-type material, for example PN-P-N. The main terminals, labelled anode and cathode, are across the full four
layers, and the control terminal, called the gate, is attached to P-type material
near to the cathode. (A variant called an SCS - Silicon Controlled Switch brings all four layers out to terminals.) The operation of a thyristor can be
understood in terms of a pair of tightly coupled bipolar junction transistors,
arranged to cause the self-latching action:
Structure on the physical and electronic level, and the Thyristor symbol.
Thyristors have three states:
1. Reverse blocking mode - Voltage is applied in the direction that would
be blocked by a diode
2. Forward blocking mode - Voltage is applied in the direction that would
cause a diode to conduct, but the Thyristor has not yet been triggered into
conduction
3. Forward conducting mode - The Thyristor has been triggered into
conduction and will remain conducting until the forward current drops below a
threshold value known as the “holding current”
7.11(b)Function of the gate terminal
The thyristor has three P-N junctions (serially named J1, J2, J3 from the
anode).
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When the anode is at a positive potential VAK with respect to the cathode
with no voltage applied at the gate, junctions J1 and J3 are forward biased, while
junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place
(Off state). Now if VAK is increased beyond the breakdown voltage VBO of the
thyristor, avalanche breakdown of J2 takes place and the thyristor starts
conducting (On state).
I f a positive potential VG is applied at the gate terminal with respect to the
cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By
selecting an appropriate value of VG, the thyristor can be switched into the on
state suddenly.
Once avalanche breakdown has occurred, the thyristor continues to
conduct, irrespective of the gate voltage, until: (a) the potential VAK is removed
or (b) the current through the device (anode”cathode) is less than the holding
current specified by the manufacturer. Hence VG can be a voltage pulse, such as
the voltage output from a UJT relaxation oscillator.
These gate pulses are characterized in terms of gate trigger voltage (VGT)
and gate trigger current (IGT). Gate trigger current varies inversely with gate
pulse width in such a way that it is evident that there is a minimum gate charge
required to trigger the thyristor.
Switching characteristics
V - I Characteristics.
In a conventional thyristor, once it has been switched on by the gate terminal,
the device remains latched in the on-state (i.e. does not need a continuous supply
of gate current to conduct), providing the anode current has exceeded the latching
current (IL). As long as the anode remains positively biased, it cannot be switched
off until the anode current falls below the holding current (IH).
A thyristor can be switched off if the external circuit causes the anode to
become negatively biased. In some applications this is done by switching a second
thyristor to discharge a capacitor into the cathode of the first thyristor. This
method is called forced commutation.
After a thyristor has been switched off by forced commutation, a finite time
delay must have elapsed before the anode can again be positively biased and
retain the thyristor in the off-state. This minimum delay is called the circuit
commutated turn off time (tQ). Attempting to positively bias the anode within this
time causes the Thyristor to be self-triggered by the remaining charge carriers
(holes and electrons) that have not yet recombined.
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343
For applications with frequencies higher than the domestic AC mains supply
(e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast
thyristors are made by diffusing into the silicon heavy metals ions such as gold or
platinum which act as charge combination centres. Alternatively, fast thyristors
may be made by neutron irradiation of the silicon.
7.11 (c) Insulated Gate BipoalarTransistor (IGBT)
Widely used in any accession where need amplify and drive at grids gate,
realize the safety electrical isolation between the power semiconductor device
and control circuit by using opto-coupler. The switching frequency high up to
20K Hz, with short protection and output fault, output soft off when over-current,
timing and reset function, etc. This series include QP series ( with isolated power)
and hot sale QC series.
• QP series is built with isolated power, with high reliability and layout and
very easy to use.
• QC series can cross lots of competitors’ item.
• QA series is the assistant driving power of QC series
Application: Inverter, uninterrupted power supply (UPS,), servo drive,
welding machine and other occasion with high power IGBT.
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Electronics Engineering Technician
7.11 (d) Bipolar Junction Transistor (BJT)
BJT has three terminals:
Fig. 7.10 Signal Level Transistor Structure
Power transistor of npn types are easy to manufacture and cheaper.
Used in high-voltage and high current application.
Working
The base-emitter diode (forward) acts as a switch
Base
Collector Base
Terminal Terminal Current Emitter
Terminal
Fig. 7.11
When v1>0.7 it lets the electrons flow toward collector, so we can control
our output current (Ic) with the input current (Ib) by using transistors.
Vertical Cross Section
Paper - II Electronic Devices and Circuits
347
Fig. 7.12
Steady State Characteristics of Signal Level BJT
IB versus VBE input characteristics
Fig. 7.13
IC versus VCE output characteristics
Fig. 7.14
Steady State Characteristics of Power Transistor
In transistor, base current is effectively the input current and collector current
is output current.
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Fig. 7.15
Output Characteristics
Curve I ? IB=0
Curve II? IB?0
Initial part of the curve II, characterized by VCE ? called saturation region.
In this region transistor acts like switch.
Flat part of the curve ?with increasing VCE, almost IC is constant ? called
active region. In this region transistor acts as amplifier.
Almost vertically rising curve is the breakdown region, which must be
avoided at all cost.
The load line IC=(VCC-VCE)/RC. The line joining A and B.
When transistor is ON, VCE=0, the IC=VCC/RC. This collector current is
shown by point A
When transistor is OFF, or in cut-off, VCC appears across collector-emitter
and there is no collector current. This value is indicated by point B.
Transfer Characteristics
Forward current gain a=IC/IE
The ratio of collector current (O/P) IC and base current IB (I/P) called
current gain.
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Fig. 7.16
Working
Transistors work in 3 regions
Active: Always on —IC=BIb
Saturation :Ic=Isaturation On as a switch
Off :Ic=0 Off as a switch
Transistors have three terminals:
Fig. 7.17
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Transistors can be used as switches.
Transistors can either conduct or not conduct current.
Fig. 7.18
Transistor Switching Example
When VBE is less than 0.7V the transistor is off and the lamp does not light.
When VBE is greater than 0.7V the transistor is on and the lamp lights.
Transistor operation as switch means that transistor operates either in
saturation region or in cut-off region and nowhere else on the load line.
As an ideal switch operate at A. At point B in cut-off state as an open
switch.
Fig. 7.19
Large base current will cause the transistor work in saturation region at
point A’ with small saturation voltage VCES.
When the control or base is reduce to 0, the transistor is turn-off and its
operation is shift to B’ in the cut-off region. A small leakage current ICEO flow in
the collector circuit when the transistor is off.
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351
When the control or base is reduce to 0, the transistor is turn-off and its
operation is shift to B’ in the cut-off region. A small leakage current ICEO flow in
the collector circuit when the transistor is off.
If VCE(S) is the collector –emitter saturation voltage, then the collector
current ICS is:The ratio of ICS to IB is called forced current gain and less than ß.
the time during which collector current rises from 0.1 Ics to 0.9Ics. This
shows the total turn on time ton=td+tr.
Safe-Operating Area
The safe operating area of a power transistor specifies the safe-operating
limits of collector current versus collector emitter voltage.
For reliable operation of the power transistor, the collector current and
voltage must always lie within this area.
Two types of safe-operating areas are specified by manufacturer:
FBSOA (Forward-biased safe-operating area)
RBSOA (Reverse-biased safe-operating area)
FBSOA ? belongs to the transistor operation when base-emitter junction is
forward biased to turn-on the transistor.
Forward Biased Safe Operating Area
(FBSOA) DC AS WELL AS SINGLE PULSE OPERATION
FBSOA INCREASES ?PULSE-WIDTH DECRESASES
Reverse biased safe operating area (RBSOA)
Reverse biased safe operating area (RBSOA)
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Fig. 7.20
During turn-off, a transistor is subjected to high-current and high voltage
with BJT reverse biased.
Safe-operating area of transistor during turn-off is specified as RBSOA.
RBSOA specifies the limit of transistor operation at turn-off when the base
current is zero or when the base emitter junction is reversed biased (with –ve
base current). With increasing reverse bias, area RBSOA decreases in size.
Advantages of BJTs
• Have high switching frequencies.
• Turn-on losses are small.
• Controlled turn-on & turn-off characteristics.
• No commutation circuit required.
Disadvantages of BJTs
• Drive circuit is complex.
• Has the problem of charge storage.
• Has the problem of second breakdown.
• Cannot be used in parallel
• Problems of negative temperature coefficients.
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7.12 Manufacturer data Sheet of Power Electronic Devices
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357
7.13 Applications of all Power Electronic Devices
Introduction
Power electronic converters can be found wherever there is a need to
modify a form of electrical energy (i.e. change its voltage, current or frequency).
The power range of these converters is from some milliwatts (as in a mobile
phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With
“classical” electronics, electrical currents and voltage are used to carry information,
whereas with power electronics, they carry power. Thus, the main metric of
power electronics becomes the efficiency.
The first very high power electronic devices were mercury-arc valves. In
modern systems the conversion is performed with semiconductor switching
devices such as diodes, thyristors and transistors, as pioneered by R. D.
Middlebrook and others beginning in the 1950s. In contrast to electronic systems
concerned with transmission and processing of signals and data, in power
electronics substantial amounts of electrical energy are processed. An AC/DC
converter (rectifier) is the most typical power electronics device found in many
consumer electronic devices, e.g. television sets, personal computers, battery
chargers, etc. The power range is typically from tens of watts to several hundred
watts. In industry the most common application is the variable speed drive (VSD)
that is used to control an induction motor. The power range of VSDs start from
a few hundred watts and end at tens of megawatts.
The power conversion systems can be classified according to the type of
the input and output power
· AC to DC (rectifier)
· DC to AC (inverter)
· DC to DC (DC-to-DC converter)
· AC to AC (AC-to-AC converter)
Switching
As efficiency is at a premium in a power electronic converter, the losses
that a power electronic device generates should be as low as possible. The
instantaneous dissipated power of a device is equal to the product of the voltage
across the device and the current through it. The losses of a power device are at
a minimum when the voltage across it is zero (the device is on) or when no
current flows through it (off). Power electronic converters are built around one
(or more) device operating in switching mode.
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Electronics Engineering Technician
Practical devices have non-zero voltage drop and dissipate power when
on, and take some time to pass through an active region until they reach the “on”
or “off” state. These losses are a significant part of the total lost power in a
converter.
Devices
The capabilities and economy of power electronics system are determined
by the active devices that are available. Their characteristics and limitations are
a key element in the design of power electronics systems. Formerly, the mercury
arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered
devices such as the thyratron and ignitron were widely used in power electronics.
As the ratings of solid-state devices improved in both voltage and currenthandling capacity, vacuum devices have been nearly entirely replaced by solidstate equivalents, or by solid state devices that have no thermionic equivalent.
Power electronic devices may be used as switches, or as amplifiers. [1] An
ideal switch is either open or closed and so dissipates no power; it withstands
an applied voltage and passes no current, or passes any amount of current with
no voltage drop. Semiconductor devices used as switches can approximate this
ideal property and so most power electronic applications rely on switching devices
on and off, which makes systems very efficient as no power is wasted in the
switching devices. By contrast, in the case of the amplifier, the current through
the device varies continuously according to a controlled input. The voltage and
current at the device terminals follow a load line, and the power dissipation
inside the device is large compared with the power delivered to the load.
Several attributes dictate how devices are used. Devices such as diodes
conduct when a forward voltage is applied and have no external control of the
start of conduction. Power devices such as silicon controlled rectifiers and
thyristors (as well as the former mercury valve and thyratron) allow control of
the start of conduction, but rely on periodice reversal of current flow to turn
them off. Devices such as gate turn-off thyristors, bipolar junction transistors
(BJT), and MOSFET transistors provide full switching control and can be turned
on or off without regard to the current flow through them. Transistor devices
also allow proportional amplificaton, but this is rarely used for systems rated
more than a few hundred watts. The control input characteristics of a device
also greatly affect design; sometimes the control input is at a very high voltage
with respect to ground and must be driven by an isolated source.
Devices vary in switching speed. Some diodes and thyristors are suited for
relatively slow speed and are useful for power freqauency switching and control;
certain thyristors are useful at a few KHz. Devices such as MOSFETS and
Paper - II Electronic Devices and Circuits
359
BJTs can switch at tens of KHz up to a few MHz in power applications, but
with decreasing power levels. Very high power (hundreds of kilowatts) at very
high frequency (hundreds or thousands of MHz) is still the area where vacuum
tube devices predominate. The use of faster switching devices minimizes energy
lost in the transitions from on to off and back, but may create problems with
radiated electtromagnetic interference. Gate drive (or equivalent) circuits must
be designed to supply sufficient drive current to achieve the full switching speed
possible with a device. A device that doesn’t get sufficient drive to switch rapidly,
may be destroyed by excess heating.
Power handling and dissipation of devices is also a critical factor in design.
Power electtronic devices may have to dissipate tens or hundreds of watts of
waste heat, even switching as efficiently as possible between conducting and
non-conducting states. In the switching mode, the power controlled is much
larger than the power dissipated in the switch. The forward voltage drop in the
conducting state translates into heat that must be dissipated. High power
semiconductors require speicalized heat sinks or active cooling systems to keep
their junction temperature from rising too high; exotic semicoductors such as
silicon carbide have an advantage over straight silicon in this respect, and
Germanium, once the main-stay of solid-state electronics is now little used due
to its unfavorable properties at high temperature.
Semiconductor devices exist with ratings up to a few kilovolts in a single
device. Where very high voltage must be controlled, multiple devices must be
used in series, with networks to equalize voltage across all devices. Again,
switching speed is a critical factor since the slowest-switchind device will have
to withstand a disproportionate share of the overall voltage. The former mercury
valves were available with ratings to 100 KV in a single unit, simplifying their
application in HVDC systems.
The current rating of a semiconductor device is limited by the heat generated
within the dies and the heat developed in the resistance of the inter connecting
leads. Semiconductor devices must be designed so that current is evenly distributed
within the device across its internal junctions (or channels); once a “hot spot”
develops, breakdown effects can rapidly destroy the device. Certain SCRs are
available with current ratings to 3000 Amperes in a single unit.
Applications
Power electronic systems are found in virtually every electronic device.
For example:
· DC/DC converters are used in most mobile devices (mobile phones,
PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of
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Electronics Engineering Technician
the battery is. These converters are also used for electronic isolation and power
factor correction.
· AC/DC converters (rectifiers) are used every time an electronic device
is connected to the mains (computer, television etc.). These may simply change
AC to DC or can also change the voltage level as part of their operation.
· AC/AC converters are used to change either the voltage level or the
frequency (international power adapters, light dimmer). In power distribution
networks AC/AC converters may be used to exchange power between utility
frequency 50 Hz and 60 Hz power grids.
· DC/AC converters (inverters) are used primarily in UPS or renewable
energy systems or emergency lighting systems. When mains power is available,
it will charge the DC battery. If the mains fails, an inverter will be used to produce
AC electricity at mains voltage from the DC battery.
7.14 Power Control Schematic
Power supplies and control schematics
· +9V *and* -9V from one battery
· 0-14 volt, 0-2 amp current limited variable power supply regulator
· 12 Vdc - 120 Vac Inverter Schematic
· 12 volt battery monitor
· 12 Volt Gel Cell Charger
· 12 volt power supply
· 12 Volt Switching Power Supply circuit diagram and PCB layout
· 12V 30A power supply
· 12V Lead-Acid Battery Monitor using LM3914
· 12V to 120V Inverter
· 12V, 4-AA Cell Differential Temperature Charger
· 13.8V 30-40A Power Supply (PDF)
· 1A Variable Regulated Power Supply
· 200 Watt Modified PC Power Supply 13.5 Volt 14 Amp
· 3.3V / 5V Regulated Power Supply Circuit
Paper - II Electronic Devices and Circuits
361
· 3rd harmonic distortion meter for measuring the quality of AC supply
· 5 volt power supply
· 500W low cost 12V to 220V inverter
· 6V to 12V Converter
· 6V to 12V Converter
· AC Power Meter
· Active Power Zener
· Adjustable power supply using LM317
· Adjustable Voltage Regulator using a 7805 or other fixed linear voltage
regulator instead on LM317
· Advanced High Voltage PSU circuit
· Alkaline battery charger
· Alternative power source for Magellan GPS receivers
· Amplified zener regulator
· Assorted power source and control circuits
· Automatic 12V Lead Acid Battery Charger
· Automatic 9V NiCad battery charger
· Back And Forth - Bidirectional Bipolar Stepper Motor Driver
· Basic 78xx series regulator mains power supply circuit diagram
· Basic Power Supply
· Basic Solid State Relays
· Basic UPS Power Supply
· Battery Characterizer
· Battery Charger Ideas
· Battery Charger, Current and Voltage Regulated for Sealed Lead Acid
batteries
· Battery Low Voltage Beeper
· Battery Low Voltage Beeper
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Electronics Engineering Technician
· Battery voltage monitor
· Bench power supply that allows a number of varying output voltages to
be preset. Includes PCB layout
· Breadboard supply - very low dropout adjustable power supply
· Build A 10 Amp 13.8 Volt Power Supply
· Build a breadboard power module for integrated circuits
· Build A High Performance Voltage Regulator From Discrete
Components
· Build A Simple Rechargeable CMOS Battery
· Car Ignition Coil Driver from 110V AC
· Car Ignition Coil Driver from 12V DC - Can be used as an electric
fence
· Charge Monitor for 12V lead acid battery
· Charger for gel lead acid batteries
· Cockcroft-Walton voltage multipliers (PDF)
· Compressor-mate power protection for refrigerators, freezers and air
conditioners
· Controller for hybrid (photovoltaic- wind turbine and diesel engine)
power plant
· Current booster for 78nn series voltage regulators
· DC to AC inverter using a 555 timer
· DC Voltage and Current Source
· Digital bench power supply based on a PIC16F870
· Dual (postive and negative) 12V power supply
· Dual Polarity Power Supply
· Dual Polarity Power Supply
· Dual Polarity Unregulated PSU For High-End Audio Amps
· Dual power supply
· Dual regulated power supply
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363
· Dynamo Current and Voltage Regulator
· Efficient unipolar stepper motor driver (only uses power when it makes
a step)
· Emergency power system
· Expanded Scale Battery Volt Meter
· Expanded Scale Battery Volt Meter
· Fast NiMH / NiCd Battery charger
· Filtering PC bus POWER
· Fixed Voltage Power Supply
· Fixed Voltage Power Supply
· Flyback transformer driver
· Fuse blown indicator
· Fuse monitor / alarm
· General purpose portable DC power supply using rechargeable C cells
· Generating -5VDC from +5VDC
· Gyrator circuit
· High Current Power Supply
· High Current Power Supply
· High current regulated power supply
· High Side Current Monitors (LM358, Zetex - ZXCT-1009)
· High Voltage Converter: 90V From 1.5V
· High voltage DC generator
· High Voltage High Current Power Supply
· High Voltage High Current Power Supply
· High-Voltage Pulse Generator
· HV supply: 12VDC in, 12KV out
· Inverter, 12 volt unit, MOSFET design
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Electronics Engineering Technician
· Inverter, A 12 volt unit, Very Basic type
· Lead acid battery charger with float
· Lead/acid battery charger
· Lead-Acid Battery Monitor
· LED battery voltage monitor. A fuel gage for your gel cell battery.
· Lithium Battery Rejuvinator
· Lithium Ion Battery Charger based on a PIC micro including circuit
diagram and source code
· LM311 Thermostat circuit diagrams
· LM317 Regulator Circuit
· LM3914 battery monitor
· Low Battery Voltage Cutout Circuits
· Low Power LED Voltmeter
· Low Power LED Voltmeter
· Low Voltage Alarm for batteries and other volatile DC power sources
· Low-dropout 12V regulator (LM324)
· Machine power loss beepter (PDF)
· Multiple voltage power supply
· N.O. Magnetic Reed Switch ON /OFF Circuit (SCR equivalent)
· Nagative voltage generation using 555 timer
· Negative Supply from single positive Supply using 555 timer
· Negative voltage generator
· Nicad battery charger
· NiCad Discharger for Tx & Rx Packs
· NiCd Cell Charger
· Nine Volt Battery Eliminator
· One 9V battery gives +18, +25, +33V
Paper - II Electronic Devices and Circuits
365
· Op-Amp Current Source with Floating Load including SPICE simulation
· Power reminder beeper (PDF)
· Power supply metering circuits for measuring both voltage and current
· Power supply provides +5VDC regulated, +10VDC unregulated and
7.5VAC
· Preselect Twin Coil Switch Machine Circuit
· Pulse Charger for reviving tired Lead Acid batteries
· PWM DC Motor Speed Control
· PWM Motor Speed Controller / DC Light Dimmer
· PWM Motor/Light Controller
· PWM Motor/Light Controller
· PWM Motor/Light Controller variants
· Regulated 12V supply
· Regulated Power Supply Circuits
· Simple +5V power supply circuit
· Simple Capacitance Multiplier Power Supply For Class-A Amplifiers
· Simple constant current source
· Simple DC Adapter Power Supply
· Simple NiCad battery charger using LM317
· Simple switching power supply
· Simple switching power supply (mains operated)
· Simple switching regulator (experimental)
· Simple voltage booster based on Linear Technologies LT1372, includes
PCB design
· Single to 3-phase power conversion
· Small battery-powered USB charger including circuit diagram and PCB
layout
· Snowmobile GPS power adapter
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· Solid state relay circuit
· Solid State Tesla Coil/High Voltage Generator
· Student DC power supply
· Team digital - SCR16 - Twin Coil Switch Machine Adapter
· Temperature Controlled Nicad Charger
· Temperature Controlled NICD Charger
· Tesla coil / HV generator
· Transformer Secondary Voltage Reduction
· Transformerless Power Supply
· Transformerless Power Supply
· TTL power supply with crowbar protection
· Unplugged power cord alarm
· Unregulated power supply
· USB charger
· Using Pass Transistors Beef Up Voltage Regulator current output
· Variable Dual Lab Power Supply
· Variable power supply
· Voltage and current regulated power supply
· Voltage doubler
· Voltage Inverter
· Voltage Inverter
· Voltage inverter
· Voltage Inverter using 555 Timer
· Voltage Inverter using LM380 audio amplifier IC
· Voltage Monitor using UA741 operational amplifier
· Voltage monitor with LED indicator
· Windmill DIY Analog MPPT (maximum power point tracker) Circuit
Paper - II Electronic Devices and Circuits
Short Answer Type Questions
1.Mention the names of thyristor family devices.
2.Write ISI symbols of power devices.
3.Draw the symbols of DIAC,TRIAC,SCR.
4.Write applications of SCR .
5Write applications of DIAC.
6.Write applications of TRIAC.
7.Draw the symbol of UJT.
8.Write applications of UJT.
Long Answer Type Questions
1. Explain construction working V I characteristics of SCR.
2.Explain construction and working of DIAC and TRIAC.
3.Explainconstruction and working of UJT.
4.Write applications of power electronic devices.
Practical/OJT Questions .
• Study the V I characteristics of SCR.
• Study the V I characteristics of DIAC and TRIAC.
• Study the V I characteristics of UJT.
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UNIT
8
Opto Electronic Devices
Learning Objectives
• Study of Opto electronic devices.
• Study of classification of opto eletronic devices.
• Study of operation of LDR.
• Study of operation and working of LED applications.
• Study of construction and working of LCD applications.
• Study of consrtuction and working of Photo diode.
• Study of construction and working of Photo transister.
• Study of construction and working of Opto coupler.
• Study of working of Photo conductive cells.
8.0 Introduction of OPTO Electronic Devices
In semiconductor material, the process of conduction is achieved by using
different techniques of liberating the valence elections. The liberation of these
electrons can be achieved by impartly some external energy viz heat, light, photon,
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369
bombardment etc. Out of this light is one of the common sources used for imparting
external energy. The device especially made to change their properties with light
are called as Optoelectronic devices; the optoelectronic devices are product of
a technology that combines optics with electronics.
A phase locked loop (PLL) is basically a closed loop feedback system.
The action of PLL is to lock or synchronise the frequency of a controlled oscillator
to that of an incoming signal. The implementation of PLL with discrete
components involves circuits of considerable cost and complexity. For the reason,
the use of PLL in the past has been limited to specialized measurements. The
development of integrated circuits PP now makes it highly economical as well as
reliable. In this chapter we will study the application of 555IC timer and description
of face locked loop.
In this chapter we will also study the different opto electronic devices.
8.1 Classification of OPTO Electronic Devices
Opto electronic devices are basically classified into
1. Sensors
2. Emitters
3. Couples of insulators
Opto electronic devices are of three categories
1. Photo conductive device
Example of Photo conductive device are
(a) Photo bodies
(b) Photo transistor
(c) Light Dependent Resistor.
2. Photo Emissive Devices
Example of photo emissive devices are
(a) Light Emitting Device or (LED)
(b) Liquid Crystal Display (LCD)
(c) Photo Tube
(d) Photo multiplier
(e) Light Activated Simulating Emitter Radiation(LASER)
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3. Photo voltaic devices
Example of photo voltaic devices are
(a) Solar cells
(b) Light detectors
Opto Electronics Devices
Sensors
Proconductive
Emitters
Solar Cells
Photo diode
Photo Emissive
Photo voltaic
Photo Transistor
Light detector
LCD LED Laser PhotoTube
Light Dependent
Resistor (LDR)
Fig. 8.1
8.2 Light Dependent Resistor (LDR)
A Light dependent resistor (LDR) or photo resistor is made from
semiconductor. Materials whose resistance various with the amount of light energy
imparted to it. The LDR made with cadmium sulphide (CdS) cadmium solenoid
(CdSe) and lead simple (PbS). Resistance of LDR is very high when kept in
total darkness and is very low and kept in well illuminated area. The resistance
(R) is indirectly proportional to the amount of light energy (E) falling in its surface.
Working Principle of LDR
When energy (E) is imparted to the materials like CdS, the liberation of
valence electronic generate electronic whole pairs within the material. Those
pairs act as charge which initiate the conduction process. The resistance R of
the materials is reduced according as AE –a
Where A =constant and a = lies between 0.7and0.9 for CdS .the greater
the amount of light falling on the surface, lower will be the value of resistance of
the material and vice-versa.
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Working and VI Characteristic:
(a) Construction
(b) Circuit Symbol
Fig. 8.2 Light Dependent Resistor
Fig 8.2 shows Photo Conducting Cell and its symbol. The photo resistor is
deposited on top ceramic substrate. After fabrication it has translucent to and
hermetically sealed. Ratings and performance of the devices are characterized
by the value of current flowing through the device at a given voltage and the
amount flux. In the presence of light, it poses a little reactance in the circuit giving
ON stage. In darken it poses a very high resistance which causes almost no
flow of current thus resulting in OFF state of the switch. Hence the device acts
like an automatic switch whose ON and OFF states are dependent on total
illumination and dark condition respectively. The characteristic of CdS LD is
shown figure 8.3.
Fig. 8.3 Ilumination Characteristics of LDR
Advantages and disadvantages of LDR
Advantages:
1. Low cost
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2. Easy Operation
3. High photo sensitivity
Disadvantages:
1. Effect of light intensity
2. Poor temperature stability
3. Narrow spectral response
Application of LDR
1. Automatic street lighting
2. Burglar alarm
3. Relay circuits
4. Light meters
8.3 Light Emitting Diode
The LED is basically a device which convert input electrical energy into
output optical radiation in the visible or infrared portion of spectrum depending
on the semi conductor material used. LEDs have replaced incandescent lamps
in much application because of low voltage long life and fast ON-OFF switching.
The material used in manufacturing the LEDs are
1.Arsenide phosphide (GaAsP): it provides either red light or yellow light
2.Galliun phosphide (GaP) :it provide red or green light
3.Galliun Arsenide (GaAss):it provide infrared radiation
Principle :
When a PN junction is forward baised, charge carrier recombination take
place at a junction as electrons cross then-side and combine with holes on the
P-side. Free electrons are in the conduction band of energy levels while holes
are in the valence band. Therefore electrons are at higher energy level then
holes. When recombination take place, some of its energy given up in the form
of heat and light. If the semiconductor material is translucent, the light will be
emitted and junction becomes light source, which is called as Light Emitting
Diode (LED)
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Construction and Working :
Diffused P-type
Epitaxial a-type
(b) Circuit Diagram
Gold Film Cathode Connection
(a) Cross Section view of LED
Fig. 8.4 LED
Construction and Working of LED
The cross sectional view of a typical LED is shown in Fig 8.4 (a) the
semiconductor material employed is Gallium Arsenate or Gallium Arsenatic
Phosphate or Gallium phosphate. An N type bar is grown up on a substract
and P – region is created by diffusion since the charge carrier recombination
occur in the p – region it must be kept upper most . The P – region therefore
becomes the surface of the device and the metal film anode connection must be
patterned to a allows most of the light to be emitted. A gold film is applied to the
bottom of substrate to reflect as much as possible of the light toward the surface
of the device and to provide cathode connection
FIG 8.4(b) shows the circuit Symbol of LED. The outward arrow indicates
emission of light .Fig 8.5 shows the V-I characteristic of LDR. From V-I
characteristic one can expect a typical operating current of 10mA operating
voltage range of LED is from 1.7 V to 3.0 V.
Opto Electronic Devices, Timers & Phase Locked Loops
Advantages and Disadvantages of LED
Advantages :
1. Low working Voltage and current
2. Less Power Consumption
3. No warm up time
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4. Very fast action
5. Emission of monochromatic light
6. Small size less in weight
7. No effect of mechanical vibrations
8. Less fragile than glass
9. Extremely long life
Fig. 8.5 Characteristics of LED
Disadvantages :
1. Sensitive to damage by over voltage or over current
2. Wide optical band width compared to LASERS
3. Temperature dependent of radiant output power and wave length4
4. Theoretical overall efficiency is not achieved except in special cases or
pulsed conditions.
Applications of LED
1.Calculators
2. Multi meters
3. Picture phones
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4. Burgl alaram system
5. Digital meters
6. Microprocessors
7. Digital computers
8. Electronic telephone exchange
9. Intercomes
10. Digital watches
11. Electronic panels
12. Sold state video display
13. Optical communication system.
8.4 Generation of Different Colour LED’S
As part of the National Instruments Introduction to NI ELVIS II, NI
Multisim, and NI LabVIEW courseware, the labs introduces students to the
capabilities of the NI ELVIS II educational design and prototyping platform.
Students can explore how NI ELVIS II allows for an easy transition from design,
simulation to prototype as it interfaces with both NI Multisim and LabVIEW
software.
The courseware includes 11 lab experiments starting with the an introduction
to the NI ELVIS environment and steps you through AC circuits to
communications. The labs are designed as a starting point for your own curriculum
design, demonstrations in the classroom, and method to inspire students to be
imaginative and creative in their design projects.
View all of the labs for the Introduction to NI ELVIS II, NI Multisim and
NI LabVIEW courseware.
Goal
This lab focuses on using NI ELVIS II to illuminate diode properties, diode
test methods, bit patterns for a two-way stoplight intersection, and the use of NI
ELVIS II APIs in a LabVIEW program to run the stoplights automatically. A
Multisim challenge encourages the reader to design a two-way stoplight
intersection using discrete transistor-transistor logic (TTL) ICs.
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Exercise 8.1: Testing Diodes and Determining Their Polarity
A semiconductor junction diode is a polar device with a band on one end
which indicates the cathode. The other end is called the anode . While there are
many ways to indicate this polarity in the packaging of a diode, one thing is
always the same – a positive voltage applied to the anode results in the diode
being forward-biased so that current can flow. You can use NI ELVIS II to
determine the diode polarity.
Complete the following steps to set up NI ELVIS II for diode and polarity
tests:
1. Launch the NI ELVIS II Instrument Launcher strip and select DMM.
2. Click on the diode test button .
3. Connect one of the LEDs to the workstation banana sockets DMM
(VÙ ) and (COM).
When you apply a positive voltage to the cathode, the diode blocks the
current. The display, which reads the same value as it does when no diode is
connected (open circuit), shows the word OPEN (see Figure 7.1).
Fig 8.6. Reverse-biased Diode Reading
When you apply the positive voltage to the anode, the diode allows current to
flow. The display reads a low voltage level.
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377
Fig. 8 .7 Forward-biased Diode Reading
For example, a silicon rectifying diode in the forward-bias direction displays
a voltage ~0.6 V. In the reverse-bias direction, the display shows the word
OPEN.
NOTE: You can use this simple test to determine the polarity of a colored
LED. Connect a red LED to your test leads. In one direction, you see light
(forward-biased) and, in the other direction, no light (reverse-biased). The DMM
display does not change, but there is enough current to produce some light.
Check closely “ the LED is dimly lit and may be difficult to see with bright lights
in the room. When the LED is lit, the red lead connection is the anode.
The way this works is that the display shows the voltage required to generate
a small current flow of about 1 mA. In the forward-bias region, this voltage level
is usually smaller than the open circuit voltage. In the reverse-bias direction, no
current flows and the tester displays the word “open”. For LEDs, the voltage
threshold is often larger than the open circuit voltage. The 1 mA test is not
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sufficient to discern the forward-bias test (GOOD), but it is enough to generate
a low light intensity.
Exercise 8.2 Characteristic Curve of a Diode
The characteristic curve of a diode, that is, a plot of the current flowing
through the device as a function of the voltage across the diode, best displays
the diode’s electronic properties.
Complete the following steps to display the characteristic curve of a diode:
1. Place a silicon diode across the DMM/Impedance Analyzer pin sockets
DUT+ and DUT-. The anode diode pin goes to the + input. For reference, the
flat side of the LED is the cathode.
2. Launch the NI ELVIS II Instrument Launcher strip and select the TwoWire Current-Voltage Analyzer (2-Wire). A new SFP opens so you can display
the characteristic (I-V) curve for the device under test. This SFP applies a test
voltage to the diode from a starting voltage level to an ending level in incremental
voltage steps, all of which you can select.
3. For a silicon diode, set the following parameters:
Start: -2 V
Stop: +2.0 V
Increment: 0.05 V
4. Set the maximum current in either direction to ensure the diode does not
operate in a current region where damage may occur. Check the diode
specifications.
In the reverse-bias direction, the current should be very small (mA) and
negative. In the forward-bias direction, you should see that above a threshold
voltage, the current rises exponentially to the maximum current limit.
6. Change the Display buttons [Linear/Log] to see the curve plotted on a
different scale.
7. Try the Cursor operation. It gives the (I,V) coordinate values as you
move the cursor along the trace.
The threshold voltage is related to the semiconductor material of the diode.
For silicon diodes, the threshold voltage is about 0.6 V, and for germanium
diodes, it is about 0.3 V. One way to estimate the threshold voltage is to fit a
Paper - II Electronic Devices and Circuits
379
tangent line in the forward-bias region near the maximum current (refer to Figure
7.8). The point where the tangent intersects the voltage axis defines the threshold
voltage. Observe the (I,V) characteristic curve for a light emitting diode. For
this LED, the threshold voltage given by the intersection of the tangent with the
voltage axis is about 1.56 V.
Fig. 8 .8 Current-Voltage Characteristic Curve of a Silicon Diode
8. Using the Two-Wire Current-Voltage Analyzer, determine the
threshold voltage for a red, yellow, and green LED, and complete the chart
below.
Red LED ____________ V
Yellow LED ____________ V
Green LED ____________ V
Exercise 8.3: Manual Testing and Control of a Two-Way Stoplight
Intersection
Complete the following steps to build and manually test and control a twoway stoplight intersection.
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1. Install two each of red, yellow, and green LEDs on the NI ELVIS II
protoboard, positioned as a two-way stoplight intersection.
Fig. 8.9 LED layout of a Two-way Stoplight Intersection
2. Connect the pin socket DIO <0> to the anode of the red LED in the
North-South (Up-Down) direction.
3. Connect the other end of the LED through a 220 W resistor to digital
ground (not pictured).
NOTE: The resistor is used to limit the current through the LED.
4. Connect the remaining colored LEDs in a similar fashion.
Here is the complete mapping scheme.
DIO <0> Red
N-S direction
DIO <4> Red
E-W direction
DIO <1> Yellow
N-S direction DIO <5> Yellow
E-W direction
DIO <2> Green
N-S direction DIO <6> Green
E-W direction
5. From the NI ELVIS II Instrument Launcher strip, select Digital Writer
(DigOut).
6. Using the vertical slide switches, select any 8-bit pattern and output that
pattern to the NI ELVIS II digital lines. Recall that Bit 0 is connected to the pin
socket on the protoboard labeled DIO <0>.
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381
7. Set the Generation Mode to (Run Continuous) and Pattern to
(Manual),
8. To activate the port, click on the Run button.
Fig. 8.10 Digital Writer for Testing LEDs
When all switches (Bits 0-2 and 4-6) are HI, all the LEDs should be lit.
When all these switches are LO, all the LEDs should be off.
You can now use these switches to find out which 8-bit codes are necessary
to control the various cycles of a stoplight intersection.
Here are some clues for an intersection. The basic operation of a stoplight
is based on a 60-second time interval with 30 seconds for red, followed by 25
seconds for green, followed by 5 seconds for yellow. For example, in a twoway intersection, the yellow light in the North-South direction is on while the red
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light in the East-West direction is on. This modifies the 30-second red timing
interval to two timing intervals: a 25-second cycle followed by a 5-second cycle.
There are four timing periods (T1, T2, T3, and T4) for two-way stoplight
intersection operation.
9. Study the following chart to see how a two-way stoplight intersection
works.
Direction N-S E-W
Lights RYG RYG 8-Bit Code Decimal Value
Bit #
012 456
T1 25 s 0 0 1 1 0 0 00001100 12
T2 5 s 0 1 0 1 0 0 ________ ___________
T3 25 s 1 0 0 0 0 1 ________ ___________
T4 5 s 1 0 0 0 1 0 ________ ___________
10. Use the Digital Writer to determine which 8-bit codes need to be written
to the digital port to control the stoplights in each of the four timing intervals.
For example, timing period 1 requires the code 00101000. Computers
read the bits in the reverse order (least significant bit on the right). This code
then becomes 00010100. In the white box above the Manual Pattern Line
switches display, you can read the radix of the switch pattern in binary
{00010100}, decimal {20}, or hexadecimal {14}.
11. Click on the black ^ to left of the white display box to change the radix.
You can use this feature to determine the numeric codes for the other timing
intervals T2, T3, and T4. If you output the 8-bit code for each of the timing
intervals in sequence, you can manually operate the stoplights.
NOTE: You can also change the radix in the Line States display by clicking
on the white x beside the Numeric Value display.
Repeating this four-cycle sequence automates your intersection.
Exercise 8.4: Automatic Operation of the Two-Way Stop light
Intersection
Complete the following steps to automate the timing cycle on the stop light
circuit.
1. Close NI ELVIS II SFPs and launch LabVIEW.
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2. Open the program StopLightsMx.vi. There is only one control on the
front panel “ a Boolean switch used to stop the operation of the stoplights.
NOTE: This LabVIEW program is configured to connect to”Dev1" for
your NI ELVIS workstation. If your device is configured to another device
name, you need to rename your NI ELVIS workstation to “Dev1,” in
Measurement & Automation Explorer (MAX) or modify the LabVIEW programs
to your current device name.
3. Switch to the block diagram (Window»Show Block Diagram).
4. Observe the four-cycle sequence generated by the for loop.
The NI-ELVISmx Digital Writer API is the structure that outputs the light
code to the stoplights. This API expects the input code to be an 8-bit Boolean
array. For example, the first timing interval T1 requires the code 12 (twelve
decimal). Its value is placed in the first element of an integer array labeledLights
Pattern. You must transfer the other integer codes from the table in Exercise
7.3 into the three blank elements of the Lights Pattern array.
Figure 8.11. Block Diagram for Automated Operation of a Two-way Stoplight
Intersection
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Electronics Engineering Technician
In operation, one of the elements of the Lights Pattern array is selected on
the boundary of the for loop (inner loop) and converted into an 8-bit Boolean
array. In a similar way, the appropriate time delay is selected at the for loop
boundary and passed to the Wait function.
The timing intervals are stored in the four elements of the Time Delay
array. To speed up operation, the 25-second time interval is reduced to 5 seconds
and the 5-second time interval is reduced to 1 second.
What’s Cool!
LEDs are amazing devices. If you multiply the threshold voltage, VT, times
the electronic charge, e, the product is energy that is close to the band gap
energy of the semiconductor material used to manufacture the semiconductor
diode. Further, in the forward-biased region, the light from the LED has an
energy of hc/ë, where h is Planck’s constant, c is the speed of light, and ë is the
wavelength of the center of the energy distribution. Conservation of energy yields
the equation:
From the LED specifications, you can determine the wavelength or the
LED color. For example, red LEDs have a wavelength of about 560 nm. From
the I-V characteristic curve of the LED , you can measure the threshold voltage
VT. If you plot VT versus 1/ë for the three different colored LEDs, you find a
straight line with a slope approximately equal to (hc/e), a mixture of three
fundamental constants of nature.
Multisim Challenge: Design a Control Circuit for a Two-Way Stoplight
Intersection
Modern-day stoplights use a cluster of red, yellow, or green LEDs to
produce the stoplight signals. In this lab, you have learned about the electrical
and optical characteristics of visible LEDs. You have used colored LEDs to
form a simple two-way stoplight intersection and a LabVIEW program to control
the light sequences. With Multisim, you can design a stoplight controller using
discreet logic ICs. A stoplight program requires a shift register and variable
delays. Recall that the red light is on for (25 + 5) seconds, the green light for 25
seconds, and the yellow light for 5 seconds. Load the Multisim program called
Stop Light Timing. Study the operation carefully.
This program uses two 7474 Dual D edge-triggered flip-flop ICs to form a
4-bit shift register. It uses a special clock circuit to generate the timing sequence
25, 5, 25, 5 seconds. This program controls only one set of red, yellow, and
green stoplights. Your challenge is to modify the program so that it can control
two sets of stoplights in a two-way stoplight intersection.
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8.5 Liquid Crystal Display (LCD):
LCD are passive display devices characterized by very low power
consumption and good contrast ratio the liquid crystal display does not emit
light or generate light. It require an external or internal light source. Although
LED give off light but LCDs are not light sources but controls light.LCD a
reflect part of surrounding light, while the other parts of the display absorb light.
Principle:
The molecules in ordinary liquids normally have random orientation. In liquid
crystals, the molecules are oriented in a definite crystal pattern. when an electric
field is applied to the crystal, the molecules, which are approximately cigar shaped,
tend to align themselves perpendicular to the field charge carrier flowing through
the liquid disrupt the molecular alignment and cause a turbulence within the liquid.
This is illustrated in Fig 8.12.
(a) Molecules in Liquid crystal when no
current is flowing
(b) Change carrier flow through
liquid crystal disturbs molecular
alignment and causes turbance
Fig. 8.12 Molecules in Liquid Crystals
When not activated, the liquid crystal is transparent. When activated, the
molecular turbulence causes the light to be scattered in all directions, so that the
activated areas appear bright. The phenomenon is known as dynamic scattering
and is shown in fig 8.13.
The actual liquid crystal material may be one of several organic compounds
which exhibit the optical properties of a solid while retaining the fluidity of a
liquid.
Example of such compounds are
1.Cholestaryl nonanoate
2. p- azoxy anisole
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Constructing and working
The constructional features consists of a layer of liquid crystal material
sandwiched between the glass sheets with transparent electrons deposited in
the inside surface and is shown in Fig. 8.13
Liquid Crystal
Transparent Electrodes
Spacer
Fig. 8.13 Construction of LCD
With both glass sheets transparent the cell is known as “transmitive type
cell”. When only one glass sheet is transparent and other had a reflective coating
the cell is termed as reflective type the liquid crystal cells do not generate light
but transmit or reflect light from external sources thus only energy required by
the cell is to produce :dynamic scattering effect”.
LCDs are usually seven segment type or dot matrix type displays. In these
displays LCDs are activated by applying voltage between the segments and
common electrode. Segments on the LCD are driven by low frequency a.c
typical driving voltage of 5 V rms. When segments is not activated, the transmitive
type cell will simply transmit rear or edge lighting through the segment in straight
line. In this condition the corresponding segment will not appear bright. In reflective
type the light is reflected in usual way from mirror surface and corresponding
segment will not appear bright. When the segment is activated the incident light
is diffusely scattered forward and the corresponding segment appears bright
Advantages and Disadvantages of LCD
Advantages
1. Low power consumption
2. Small voltage requirement
3. They are economical
4. Good contrast is display
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Disadvantages
1.Some time the output of the LCD is not visible clearly
2.They are not very reliable
3.Slow operation
4.Occupies large area
5.They are very sensitive to damage by over voltage or over current
6.They are limited temperature range.
7.In LCDs a.c Square wave drive of frequency less then 50Hz is employed.
Application of LCD
1. Display unit in calculators
2. Display unit in higher end CRO
3. Display unit in watches
4. Display unit in computer
5. They are used in televisions
6. Used as the slide in a projection system to obtain on enlarged image.
7. Used in all portable instrument display.
8.6 Comparision Between LED and LCD
Displays
The displays, used in electronic instruments, equipments etc are
1. Seven segment display
2. Dot matrix display
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S.No.
LED
1. Consumes more power 10250 mw power per digit.
2. Because of high power
requirement it requires
external interface circuitry
when driven from ICs.
LCD
Essentially act as a capacitor and
consume very less power requires
10-200 mirco W power per digit.
Can be driven directly from IC
chips
3. Good brigtness level
Moderate brightness level
4. Operating temperature
range - 40oC to 85oC.
Temperature range limited to 20oC to 60oC
5. LED Life is around
1,00,000 hours
Lifetime is limited to 50,000 hours
due to chemical degradation.
6. Emits light in red, orange,
yellow, greeen and white
Invisible in darkness - requires
external illumintion..
7. Operating voltage range is
1.5 to 5V dc
Operating voltage range is 3 to 20
V dc.
8. Response time is 50 to 500
ns
Has a slow decay time - response
time is 50 to 200 ns.
9. Viewing angle 150o
Viewing angle is 100o
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8.6.1 Seven Segment Display
For conventional Seven segment display (including the decimal point i:e the
segment) the wiring pattern is simplified by making the terminal of LED or LCD
common to all other segments. In LRDs the terminals can be common anode
(CA)form or common cathode (CC) form. Fig 8.15(a)shows a Seven segment
display. in this circuit the anodes of all the diodes are connected together to the
positive terminals of the d.c voltage source. The cathodes are connected to the
external resistor. By grounding the external resistor we can form any decimal
digit from 0 to 9.
(a) 7-Segments
(b) Schematic Digit
(c) Digit 2 in seven segment display
Fig. 8.15 Seven Segment display
For example by grounding a,b,g,e and d we can form the digit “2”.In similar
manner by grounding f,g,b,and c can form the digit “4” and so on. A Seven
segment display can also display the capital letters A,C,E, and F besides this, it
can also display the lower case letter b and d.
The Seven segment display are used in digital clocks calculators, Stereo
tuners, microwave ovens, digital multi meters etc.
8.6.2 DOT Matrix Display
The Seven segment display is not commonly used technology and is
also the easiest to implement electronically. However it is limited to displaying
numeric and a small range of alphabetic information. the dot matrix can display
a wide range of numeric, alphabetic information and other characters. The dot
matrix display is a method of generating characters with a matrix of dots.
The commonly used dot matrix for the display of prominent characters are
5 x 7, 5 x 8 and 7 x 9. The 5 x 7 dot matrix display is shown in Fig 8.16 The two
wiring patterns of dot matrix display are as follows.
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(a) Dot Matrix
(b) Bit 4 in Dot Matrix
Fig. 8.16 Dot Matrix Display
1.Common anode or common cathode connection
2. X_Y array connection (Economical and can be extended vertically or
horizontally using a minimum number of wires).
8.7 Photo Diode Construction and Working:
Photo Diode is a opto device which is designed to respond to photo
absorption. Under illumination, the carriers conduction is directly proportional
to the injected carrier generation. This device when operated with reverse voltage
applied, functions as a photo conductive cell and when operated without reverse
voltage, functions as a photo voltaic cell
Principle:
When a pn junction is reversed baised a reverse saturation current I flows
due to thermally generated holes and electrons crosses the junction as a minority
carriers. Increasing the junction temperature more electron hole pairs are
generated and so the minority carrier current will be increased.
Light Waves
Fig. 8.17 Principle of Working of Photodiode
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The same effect occurs if the junction is illuminated. Hole and electron
pairs are also generated by the incident light energy and minority charge carriers
cross the junction. the diode specially made to follow this phenomenon is called
Photo Diode.
Construction and Working:
The Photo Diode is made of semi conductor or P – N junction kept in
sealed plastic or glass casing. The cover is so designed the light rays are allowed
to fall on one surface across the junction the remaining sides of the casing are
painted to restrict the penetiation of light rays. A lens permits light to fall on
junction
When light falls on the reverse baised PN Photo Diode junction hole –
electrons pairs are created. The movement of these electron hole pairs in a
property connected circuit results
Fig. 8.18 Photo Diode
In current flow. The magnitude of photo current depends on the number of
charge carriers generated and hence, on the illumination on the diode element.
This current is also affected by the frequency of the light falling on the junction of
the Photo Diode.
Fig, 8.18 Shows the circuit symbol. The inward arrows in circuit symbol
represent the incoming light, illustrate the V- I characteristics of Photo Diode
for different intensity levels. When there is no light or the applied illumination is
zero, the current flowing through the device is known as dark current. As the
light becomes brighter or illumination increases, the magnitude if the reverse
current also increases. It may observe that the current will become zero only
with positive applied bias.
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Advantages and Disadvantages of Photo Diode
Advantages
1. Low noise
2. Very good spectral response
3. Fastest photo detector
Disadvantages
1. Sensitive device
2. Current increased with temperature
3. Could not exceed the working temperature limit specially by the
manufacturers
Application of Photo Diode
1. Light decetor
2. Demodulators
3. Encoders
4. Optical communication spectrum
5. High speed counting and switching circuits
6. Computers card punching and tapes
7. Light operated Switches
8. Sound tracks Films
9. Electrionic control circuits
8.8 OPTO Transistor (Photo Duo Diode):
A transistor is similar to an ordinary bipolar junction transistor except that
no base is provided. Instead of base current the input to the transistor is provided
in the of light.
Principle:
Contain ordinary transistor with its base terminals open circuited. the
collector base age current ICBO will act base current.
In transistor ICBO is increased when the collector base junction is illuminated.
where is increased the collector current is also increased therefore for a given
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amount illumination on a very small area of the photo transistor provides much
largest output current. Therefore the photo transistor is a light detector which
combines a photo diode and a transistor amplifier.
Construction and Working:
The Construction of a photo transistor is just like a conventional NPN
transistor with a little hole made on the surface near to collector base junction. A
small lens is fixed on the hole for allowing a focused light beam to concentrate
on the collector – base junction. In the modern methods of fabrication highly
light effective materials are used instead of making a hole and fixing a lens on it.
From fig. 8.19 is clear that emitter base junction JE is forward biased,
here as the collector base junction JC is reversed baised. when the transistor is
kept in darkness there will be very few minority charge carriers (Thermally
generated which will cause the flow of reverse saturation collector current .This
current for obvious reasons, will be negligible small. On light being focused at
the collector base junction additional photo generated minority charge carriers
will be available which will add to the reverse saturation current thus as soon as
the light source is applied the transistor starts conducting and amplified current
starts flowing through the reverse biased junction. thus owing to the transistor
amplifier action, the current caused by the luminous flux will increase a lost.
Fig 8.19 (b) shows circuit symbol of photo transistor.
Fig 8.19 shows the characteristics of photo transistor drawn for Ic verses
Vce as a function of illumination H. The current is a photo transistor is dependent
mainly on the intensity of light entering the lens and is less affected by the voltage
applied to the external circuit.
Housing
Light
Lens
(b) Symbol
(a) Construction
Fig. 8.19 : Photo Transistor
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Fig. 8.20 Characteristics of Phototransistor
Application of Photo Transistor
1. High speed reading of computer punched cards and tapes.
2. Light detection system
3. Light operated switches.
4. Realin of film sound track
5. Production line counting of objects which interrupt a light beam.
8.9 Applications of Photo Transistor
1. High speed reading of computer punched cards and tapes.
2. Light detection system.
3. Light operated switches.
4. Realin of film sound tracks.
5. Production line counting of objects which interrupt a light beem.
8.10 OPTO Coupler (Opto isolator):
Opto coupler is a solid state component in which light emitter, the light path
and light detector are all enclosed within the component and cannot be altered
externally. Usually infrared emitting Diode (IRED) can be used as a light emitter.
photo transistor can be used as a light detector.
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Principle:
There is one way of transfer of electrical signal from LED(IRED) to photo
transistor without any electrical connection between the input and output circuitry.
The degree of isolation between input and output depends on the kind of materials
used in the light path and on the distance between the light emitter and light
detector. If the distance between emitter and detector is greater than & the
isolation is better but current transfer ratio i:e,ratio of detector to emitter current
is lower.
LED
Photo
Transister
Fig. 8.21 Opto Coupler
Working :
Fig 8.21, shows the symbol of opto coupler. The coupler may be operated
as a switch in which cause both the LED and photo transistor are normally
OFF.A pulse of current through the LED (IRED) causes the transistor to be
switched ON for the duration of the pulse. Operation as a linear coupler is also
possibly by setting usp a bias current in the LED. The signal is then capacitively
coupled to the LED and causes its brightness to increase or decrease thus the
photo transistor receives a light signal which increases and decreases linearly
around the constant bias level.
Advantages of Opto Coupler
1. The electrical isolation can be superior to that of a transformer isolation.
the charge less photons are not influenced by electrostatic or electromagnetic
fields.
2. The conditions of load changing will not affect the input as the signal
transfer is unilateral.
3. These are faster than isolation transformer
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4. In switching or chopping application, the inherent mechanical problems
are all eliminated by contactless operation of these isolators.
Application of Opto coupler
1. Opto couplers are used where the electronic circuit isolations required.
2. To eliminate common ground connection.
3. To reduce common mode noise.
4. In fibre optic communication
5. In switching applications.
8.11 Photo Conductive Cell
The Photo Conductive Cell is a two terminal semiconductor device whose
terminals resistance will vary linearly with the intensify of the incident light. For
obvious reason, it is frequently called a photo resistive device.
Construction and Working:
Light Sensitive
material
Resistance (k
100
Top View
10
1
Side View
(b) Circuit Symbol
0.1
(a) Construction
10
100
1000 10,000
(c) Illunination Characteristics
Fig. 8.15 Photo Conductive Cell
The photo conductive material most frequently used include Cadmium
Sulphide (CdS) and Cadmium Selenide (CdSe). Both materials respond rather
slowly to changes in light intensity. The essential element of a photo conductive
materials, metallic electrodes to connect the device in to a moisture resistance
enclosure. The circuit symbol and construction of a typical photo conductive
cell are shown in Fig. 8.22
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Light sensitive material is arranged in the form of a long strip zigzagged
across a disc shape base with protective sides for added protection, a glass or
plastic cover may be included. The two ends of the strip are brought out to
connecting pins below the base.
The illumination characteristic of a typical photo conductive cell are shown
in Fig4.15 (c) when cell is not illuminated its resistance may be more than 1K
this resistance is called the dark current. when cell is illuminated the resistance
may fall to few hundred ohms. Note that the plot is drawn in log scale.
Applications of Photo Conductive Cell
1.ON – OFF
2.Relay Circuits
3.Light Meters.
Short Answer Type Questions
1.Mention the names optoelectronic devices.
2.Write applications of LDR.
3.Write working principle of LED.
4.Write applications of LED.
5.Write any specifications of LED.
6.Write working principle of LCD.
7.Write applications of LCD.
8.Write working principle of photo diode.
9.Write applications of photo diode.
10.Write working principle of photo transistor.
11.Write applications of photo transistor.
12.Write applications of opto couplers.
Long Answer Type Questions
1.Write construction and working of LDR.
2.Draw and explain construction working of LED.
3.Explain construction and working of LCD with neat diagram.
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4.Write specifications of LCD and LED.
5.Write construction and working of photo diode.
6.Write construction and working of photo transistor.
7.Explain working of opto coupler with neat diagram.
8.Explain working of photo conductive cells.
Practical/OJT Questions
• Study the working of LDR.
• Study the working of LED and LCD.
• Study the working of photo diode/transistor.
• Study the opto coupler/photo conductive cell.