Aalborg University Copenhagen
Medialogy MED4 2007
Sensors Technology
Basic electronic elements – transistor
Smilen Dimitrov
Basic electronic elements – transistor
1
Contents
1
2
3
4
5
6
7
8
9
Introduction ............................................................................................................... 3
Transistor ....................................................................................................................5
Bipolar junction transistor (BJT) .............................................................................. 6
3.1
Hydraulic analogy of a transistor ....................................................................... 13
Transistor (BJT) as an electronic element ...............................................................16
4.1
Measuring (testing) transistors.......................................................................... 25
FET transistors..........................................................................................................27
Transistor construction ........................................................................................... 28
Basic circuits ............................................................................................................ 29
7.1
Common emitter amplifier.................................................................................29
7.2
Current source and current mirror .................................................................... 36
7.3
Astable multivibrator .........................................................................................38
7.4
Differential amplifier.......................................................................................... 41
Sensing application.................................................................................................. 44
8.1
Phototransistor...................................................................................................44
PE Questions ............................................................................................................ 45
Basic electronic elements – transistor
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1 Introduction
We have previously introduced the model that conceptualizes the focus we have in ST:
In these parts of the lectures, we focus on the hardware – electronics part of our sensorbased interaction input system:
The understanding of this part of the process requires in essence two things:
understanding of the conversion from a given physical parameter into an electric
parameter – which is the sensing process: process of electric measurement that the
sensor performs; and understanding concepts in electrical circuits which are used to
perform signal conditioning. Both of these require understanding of electrical
properties of matter.
Basic electronic elements – transistor
3
So far, we have discussed circuit theory analysis of resistive circuits - and among them,
sensor resistive circuits as well. We have also looked at the capacitor as a non-ohmic
element, its influence in an electric circuit and the possibilities to use it in a sensor
context. We have also looked at the diode (PN junction) as a semiconductor element.
We now continue introducing basic electronic elements - this lecture is concerned with
discussing the transistor. The transistor too is a semiconductor element. Special types of
transistors also can be applied as sensor devices - these will be briefly introduced as
well.
Basic electronic elements – transistor
4
2 Transistor
Like the diode, the transistor is a semiconductor device as well - however, it commonly
has three terminals, so it is a bit more complex to understand. The main purpose of a
transistor is to control large current flow through two of those terminals, by smaller
current/voltage applied on the third terminal.
At this level, we should understand that there are two basic types of transistors, which
differ in construction and usage (but not in the main purpose) - these are bipolar
junction transistors (BJT) and field effect transistor (FET).
“Transistors are divided into two main categories: bipolar junction transistors
(BJTs) and field effect transistors (FETs). Transistors have three terminals
where, in simplified terms, the application of voltage to the input terminal increases
the conductivity between the other two terminals and hence controls current flow
through those terminals. The physics of this 'transistor action' are quite different
between the BJT and FET…
In analog circuits, transistors are used in amplifiers, audio amplifiers, radio frequency
amplifiers, regulated power supplies, and in computer PSUs, especially in switching
power supplies. Transistors are also used in digital circuits where they function
similarly to electrical switches. Digital circuits include logic gates, RAM (random
access memory) and microprocessors.[1]”
Note however, that the division between BJT and FET is not the only categorisation of
transistors - “Transistors are categorized by:
• Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide
• Type: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
• Polarity: NPN, PNP, N-channel, P-channel
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF),
microwave
• Application: switch, general purpose, audio, high voltage, super-beta, matched
pair
• Physical packaging: through hole metal, through hole plastic, surface mount, ball
grid array
Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN,
low power, high frequency switch. [1]”
In this lecture, we will mostly focus on describing the principle of work of a BJT
transistor, provide a simplified circuit theory model of it, and discuss application of an
Basic electronic elements – transistor
5
NPN BJT (as the calculations with it are easier) in several basic circuits. We will not go
in further details of transistor design and analysis, which in itself is a rather complex
area; we are only briefly going to mention FETs.
3 Bipolar junction transistor (BJT)
The following excerpt gives a good introduction to BJTs: “The bipolar junction
transistor (BJT) was the first type of transistor to be commercially mass-produced.
Bipolar transistors are so named because conduction channel uses both majority and
minority carriers for main electric current. The terminals are named emitter, base
and collector. Two p-n junctions exist inside the BJT, collector-base junction and
base-emitter junction. It is commonly described as a current operated device
because the collector current is controlled by the current flowing between base and
emitter terminals. [1]” Note that in a P semiconductor, majority carriers are holes, and
minority carriers are free electrons; in a N semiconductor, majority carriers are free
electrons, and minority carriers are holes.
As an introduction, let’s include the following excerpt: “A BJT consists of three
differently doped semiconductor regions, the emitter region, the base region and the
collector region. These regions are, respectively, p type, n type and p type in a PNP
transistor, and n type, p type and n type in a NPN transistor. Each semiconductor
region is connected to a terminal, appropriately labeled: emitter (E), base (B) and
collector (C).
Figure 1. Simplified cross-section of an npn BJT (left), closeup of a transistor (right) (Ref. [2])
The base is physically located between the emitter and the collector and is made from
lightly doped, high resistivity material. The collector surrounds the emitter region,
making it almost impossible for the electrons injected into the base region to escape
Basic electronic elements – transistor
6
being collected, thus making the resulting value of a very close to unity, and so, giving
the transistor a large ß.
The bipolar junction transistor, unlike other transistors, is not a symmetrical
device. This means that the interchange of the collector and the emitter makes the
transistor leave the forward active mode, starting to operate in the reverse mode.
Small changes in the voltage applied across the base-emitter terminals causes the
current that flows between the emitter and the collector to change significantly. This
effect can be used to amplify the input current. BJTs can be thought of as voltagecontrolled current sources, but are usually characterized as current amplifiers due to
the low impedance at the base. [2]”
As a discussion of the principle of operation of a BJT, the following long excerpt is
included: "
A Bipolar Transistor essentially consists of a pair
of PN Junction Diodes that are joined back-toback. This forms a sort of a sandwich where one
kind of semiconductor is placed in between two
others. There are therefore two kinds of Bipolar
sandwich, the NPN and PNP varieties. The three
layers of the sandwich are conventionally called
the Collector, Base, and Emitter. The reasons for
these names will become clear later once we see
how the transistor works.
Figure 2.Illustration of BJT transistor as two PN junctions (Ref.
[6])
Some of the basic properties exhibited by a Bipolar Transistor are immediately
recognisable as being diode-like. However, when the 'filling' of the sandwich is fairly
thin some interesting effects become possible that allow us to use the Transistor as an
amplifier or a switch.
Basic electronic elements – transistor
7
To see how the Bipolar Transistor works we can concentrate on the NPN variety.
Figure 3 shows the energy levels in an NPN transistor when we aren't externally
applying any voltages. We can see that the arrangement looks like a back-to-back pair of
PN Diode junctions with a thin P-type filling between two N-type slices of 'bread'. In
each of the N-type layers conduction can take place by the free movement of electrons in
the conduction band. In the P-type (filling) layer conduction can take place by the
movement of the free holes in the valence band.
However, in the absence of any
expernally applied electric field, we find
that depletion zones form at both PNJunctions, so no charge wants to move
from one layer to another.
Figure 3. NPN bipolar transistor (Ref. [6])
Consider now what happens when we apply a moderate voltage between the Collector
and Base parts of the transistor. The polarity of the applied voltage is chosen to increase
the force pulling the N-type electrons and P-type holes apart. (i.e. we make the Collector
positive with respect to the Base.)
This widens the depletion zone between the Collector and base and so no current will
flow. In effect we have reverse-biassed the Base-Collector diode junction.
The precise value of the Base-Collector voltage
we choose doesn't really matter to what
happens provided we don't make it too big and
blow up the transistor! So for the sake of
example we can imagine applying a 10 Volt
Base-Collector voltage.
Figure 4. Application of collector-base voltage (Ref. [6])
Basic electronic elements – transistor
8
Now consider what happens when we apply a relatively small Emitter-Base voltage
whose polarity is designed to forward-bias the Emitter-Base junction. This 'pushes'
electrons from the Emitter into the Base region and sets up a current flow across the
Emitter-Base boundary. Once the electrons have managed to get into the Base region
they can respond to the attractive force from the positively-biassed Collector region. As
a result the electrons which get into the Base move swiftly towards the Collector and
cross into the Collector region. Hence we see a Emitter-Collector current whose
magnitude is set by the chosen Emitter-Base voltage we have applied.
To maintain the flow through the
transistor we have to keep on putting
'fresh' electrons into the emitter and
removing the new arrivals from the
Collector. Hence we see an external
current flowing in the circuit.
Figure 5. Application of emitter-base voltage (to a
previously reverse-biased collector-base) (Ref. [6])
The precise value of the chosen Emitter-Base voltage isn't important to our argument
here, but it does determine the amount of current we'll see. For the sake of example
we've chosen a half a volt. Since the Emitter-Base junction is a PN diode we can expect
to see a current when we apply forward voltages of this sort of size. In practice with a
Bipolar transistor made using Silicon we can expect to have to use an Emitter-Base
voltage in the range from around a half volt up to almost one volt. Higher voltages tend
to produce so much current that they can destroy the transistor!
It is worth noting that the magnitude of the current we see isn't really affected by the
chosen Base-Collector voltage. This is because the current is mainly set by how easy it is
for electrons to get from the Emitter into the Base region.
Basic electronic elements – transistor
9
Most (but not all!) the electrons that get into the Base move straight on into the
Collector provided the Collector voltage is positive enough to draw them out of the Base
region.
That said, some of the electrons get 'lost' on
the way across the Base. This process is
illustrated in Figure 6.
Figure 6. Some electrons fall into a hole (Ref. [6])
Some of the free electrons crossing the Base encounter a hole and 'drop into it'. As a
result, the Base region loses one of its positive charges (holes) each time this happens. If
we didn't do anything about this we'd find that the Base potential would become more
negative (i.e. 'less positive' becuase of the removal of the holes) until it was negative
enough to repel any more electrons from crossing the Emitter-Base junction. The
current flow would then stop.
To prevent this happening we use the applied Emitter-Base voltage to remove the
captured electrons from the Base and maintain the number of holes it contains. This
have the overall effect that we see some of the electrons which enter the transistor via
the Emitter emerging again from the Base rather than the Collector. For most practical
Bipolar Transistors only about 1% of the free electrons which try to cross Base region get
caught in this way. Hence we see a Base Current, IB, which is typically around one
hundred times smaller than the Emitter Current, IE. [6]"
Note that the discussion up to this point is concerned with the real (or electron)
direction of current; in circuit analysis - and on the transistor symbol themselves - the
technical direction of current is used (opposite of the real direction of current).
In this kind of a setup of NPN transistor, we say that the base
is directly polarised (forward biased - P base potential is more
positive than N emitter potential), and the collector is
inversly polarised (reverse biased - N collector potential is
more positive than P base potential)
Figure 7. Example of direct polarisation of base, and inverse polarisation of
collector junction in NPN transitor (Ref. [2])
Basic electronic elements – transistor
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Another representation of the conduction process in an NPN transistor is given below,
this time through the technical direction of current:
Figure 8. Process of transistor conduction, illustarted with conventional (technical) direction of current. (Ref. [5] )
In an NPN transistor, electrons flow into the emitter, which is the real direction of
current; so the technical (conventional) direction of emitter current will be out of the
emitter.
"Bipolar transistors, having 2 junctions, are 3 terminal semiconductor devices. The
three terminals are emitter, collector, and
base. A transistor can be either NPN or PNP.
See the schematic representations below:
Figure 9. Schematic symbols of PNP and NPN BJT
transistors (Ref. [6])
Note that the direction of the emitter arrow defines the type of transistor. Biasing and
power supply polarity are positive for NPN and negative for PNP transistors. The
transistor is primarily used as an current amplifier. When a small current signal is
applied to the base terminal, it is amplified in the collector circuit. This current
amplification is referred to as hFE or beta (β) and equals Ic/Ib.
As with all semiconductors, breakdown voltage is a design limitation. There are
breakdown voltages that must be taken into account for each combination of terminals.
i.e. Vce, Vbe, and Vcb. However, Vce(collector-emitter voltage) with open base,
designated as Vceo, is usually of most concern and defines the maximum circuit voltage.
Basic electronic elements – transistor
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Also as with all semiconductors there are undesireable leakage currents, notably Icbo,
collector junction leakage; and Iebo, emitter junction leakage. [6]"
Obviously, we can see that the transistor is a complex element, and can be used in a
variety of ways. We will mostly discuss the configuration known as a 'common emitter',
which was described above: base-emitter PN junction is directly polarised, and
collector-base PN junction is inversly polarised. For this mode of operation, it is
important we remember the following points in analysis (of an NPN transistor):
- The base-emitter junction behaves like a diode - that is, it has a turn-on voltage (as
in a diode, approx 0.7V), after which the base-emitter junction leads, and base current IB
flows through it; or in other words we control whether the transistor is turned on
through applying voltage to base-emitter junction
- When the base-emitter junction is turned on, and base current IB flows, the collector
current IC is related to IB, and amplified by a factor of hFE or beta (β) - that is, IC =
hFE* IB. (hFE is as paramater which is usually 20 or 30, and in high gain transistor this
factor can be all the way up to 100 or 200).
Figure 10.Visualing construction and symbols of PNP and NPN BJT
transistors ( Ref. [5])
Basic electronic elements – transistor
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3.1 Hydraulic analogy of a transistor
Obviously, the transistor is an element more complex to understand than a diode.
However, here too we can make use of a hydraulic analogy, to simplify the working
principle and try to understand how it works in a circuit.
Again, there is an applet on the ST state dealing visualising the hydraulic analogy of a
transistor. In this case, it is an analogy to an NPN transistors - as the direction of the
flow of 'water' in the applet, represents the technical direction of current in an NPN
transistor. In addition, it represents the 'common emitter' configuration we are focusing
on.
The two PN junctions (base-emitter and collector-base) are modelled as two valves, as
hydraulic models of diode (as in the diode applet). In addition, the fact that the collector
current must be related to the base current, is modelled by a (unrealistic) system of a
reel, connecting the base-emitter (BE) valve and the collector-base (CB) valve, in a such
a way, that whenever the BE valve is opened, the CB valve correspondingly opens as
well.
At first, the applet shows a hydraulic pipetransistor, where there is no water applied
- which corresponds to a ransistor that is
not polarised, and hence turned off.
Figure 11. ST applet showing a hydraulic model of a
transistor which is not polarised (off)
By directly polarising the base, we actually
start 'pushing' water into the base pipe,
which necessarily opens the BE check
valve. This water continues flowing
through
the
emitter
pipe.
Correspongingly, the CB check valve
opens too, but as the collector is not
polarised, no amplification is seen, as
there is no collector current.
Figure 12. ST applet showing a hydraulic model of a transistor, with unpolarised collector-base, and polarised
base-emitter
Basic electronic elements – transistor
13
If we only polarise the base-collector
junction (reverse), and do not polarise
the base-emitter (direct),
in the
hydraulic water it corresponds to pushing
water 'into' the collector pipe only;
however, because of the orientation of the
CB valve, the water cannot continue
flowing in the emitter.
Figure 13. ST applet showing a hydraulic model of a
transistor, with unpolarised collector-base, and
polarised base-emitter
That is why we need to polarise the base - so through flow of base current, we can
control the flow of collector current.
So, once both the base (BE) and collector (CB) are polarised, that means we 'push' water
in both the base and collector. Then, by changing the flow of water through the base
(here simulated by dragging the BE valve with the mouse) , we can see that the flow of
collector
current
is
changed
correspondingly.
Figure 14. ST applet showing a hydraulic model of a
transistor, with both base-emitter and collector
base polarised
Thus, we have modelled that for small values of base current (IB) we have corresponding
larger values of collector current (IC = hFE* IB). We have also modelled, that whatever
flows in from the base and the collector, must leave through the emitter - the Kirchoff
current law for the transistor (IE = IB + IC).
Basic electronic elements – transistor
14
This type of a water analogy can be also found on other resources on the Internet: "The
base is actuated by closing the switch. The collector emitter line becomes conductive, the
lamp lights up. This can be shown clearly with the help of the water analogy. When the
small flow of water reaches the base, this flow of water opens the slide and makes the
way free for the large water flow (collector emitter line), the water can flow. [7]":
Figure 15. Another hydraulic analogy of a transistor (Ref. [7])
"[Transistor has a hydraulic component equivalent of] A device similar to an EGR valve,
where a diaphragm controlled by a low-current signal (either
constant current — BJT, or constant pressure — FET) moves a
plunger which allows a larger current to flow through another
section of pipe, like a globe valve. [8]"
Figure 16. Yet another hydraulic analogy of a transistor (Ref. [9])
See also: [15]
Basic electronic elements – transistor
15
4 Transistor (BJT) as an electronic element
Transistors are common electronic elements, that can be found in a variety of shapes,
sizes and functions. They also have own schematic symbol for use in circuit theory
analysis; occasionally, additional information is added to the symbol, to specify the
transistor function.
Figure 17. Image of different transistors (left, Ref. [1] );symbols of BJT and FET transistors (center, Ref. [1]);
Q2N2222A Bipolar Junction Transistor with symbol (right, Ref. [10] )
At this point, what we need is an approximative transistor model, which we can apply in
circuit theory analysis: " Transistors are complicated devices. In order to ensure the
reliable operation of circuits employing transistors, it is necessary to model the physical
phenomena observed in their operation analytically using transistor models. There
exists a variety of different models, that range in complexity.
Transistor models are used for almost all modern electronic design work. Modern
designs are … difficult to predict without accurate models of the devices used.
Comprehensive models include the primary terminal current-voltage characteristics,
capacitances between terminals, and parasitic capacitance, resistance, and inductance,
time delays, and temperature effects. [11]"
Note that there are several types of transistor models: "Non-linear, or large signal
transistor models fall into three main types:
• Physical models - These are models based upon device physics, describing in detail
the specific physical phenomena within a transistor….
• Empirical models - This type of model is entirely based upon curve fitting
techniques, using whatever equations most accurately fits the measured data to specify
the operation of the transistor in simulation…
• Table models - The third type is model is a form of look-up table containing a large
number of values for common device parameters …
Basic electronic elements – transistor
16
Non-linear models are used with a computer simulation program, such as SPICE. The
use of non-linear models, which describe the entire operating area of a transistor, is
required for digital designs and large signal circuits such as power amplifiers. [11]"
First of all, remember that so far, when discussing eletronic elements, what we needed
as a model, is a mathematical relationship between voltage and current for a given
element. So far we have discussed only two-terminal elements, which means that we
could have perceived them as a single branch (connecting two nodes) - that is, they have
a single voltage (across the branch) and single current (through the branch) associated
with them; so a model for them is a single equation relating voltage across and current
through the element, or in other words, a single IU characteristic.
Now, with a transistor, we have three terminals (collector, ba se, emitter), and
correspondingly we have three possible voltages (VBE, VCB, VCE - but only two are
independent due to KVL) and three currents (IB, IC, IE - but only
two are independent due to KCL) that can exist for this element.
Vcb
A mathematical model would thus have to relate all of these
variables; correspondingly a transistor would be described with a
system of equations (or a family of IU curves).
Figure 18. Currents and voltages of a transistor (from Ref. [12])
Among the large-signal transistor models we find:
• Gummel–Poon model
• Ebers–Moll model
• H-Parameter model
At this point, we will only briefly include some equation from the Ebers-Moll model. The
reason we do this, is because we have already introduced the Shockley expression, as an
exponential dependency between voltage and current in a diode; similarly, these
expressions are exponential, and hence can help recognize the relationship of a
transistor to a single PN junction. "The DC emitter and collector currents in normal
operation are well modeled by the Ebers–Moll model:
⎛ VBE
⎞
I E = I ES ⎜ e VT − 1⎟
⎜
⎟
⎝
⎠
⎛ VBE
⎞
I C = α T ⋅ I ES ⎜ e VT − 1⎟
⎜
⎟
⎝
⎠
Eq 4-1
where
•
•
•
IE is the emitter current
IC is the collector current
α F is the common base forward short circuit current gain (0.98 to 0.998)
Basic electronic elements – transistor
17
• IES is the reverse saturation current of the base-emitter diode (on the order of 10-15
to 10-12 amperes)
• VT is the thermal voltage kT/q (approximately 26 mV at room temperature ˜ 300 K).
• VBE is the base-emitter voltage
• β F (aka hFE) is current gain - the ratio of the allowed collector-emitter current to the
base-emitter current.
The collector current is slightly less than the emitter current, since the value of αT is
very close to 1.0. In the BJT a small amount of base–emitter current causes a larger
amount of collector–emitter current. The ratio of the allowed collector–emitter current
to the base–emitter current is called current gain, β or hFE. A β value of 100 is typical
for small bipolar transistors. In a typical configuration, a very small signal current flows
through the base–emitter junction to control the emitter–collector current. β is related
to α through the following relations:
I C = αT ⋅ I E
Eq 4-2
IC = βF ⋅ I B
Eq 4-3
βF =
αT
1 − αT
Eq 4-4
[2]"
Note that a typical transistor
characteristic is given as a family of
curves, which show certain voltagecurrent characteristics for a pair of
terminals, obtained when a parameter
of a third terminal is changed. A
typical collector characteristic curve dependence of Ic on Uce, when Ib is
changed - is shown here:
Figure 19. Typical collector characteristic curve
for a transistor (Ref. [6])
Basic electronic elements – transistor
18
However, we will need a model far more simpler than this - a model which could be
considered a small signal model: "A transistor’s parameters represent its electrical
properties. Engineers employ transistor parameters in production-line testing and
circuit design. A group of a transistor’s parameters sufficient to predict circuit gain,
input impedance, and output impedance is also referred to as its small-signal model.
Small signal models were the first type of models used. A small signal model is
generated by taking derivatives of the current-voltage curves about a bias point. As long
as the signal is small relative to the bias voltage the derivatives do not vary significantly.
Small signal models are still used to design circuits such as radio receivers and audio
pre-amplifiers. [11]"
The simplest small signal model that we can
adopt is shown to the right:
Figure 20. Small signal linear model for the common emitter
transistor (Ref. [13])
"Here we have replaced the diode with a linear element (a resistor, called rπ ) and we
have changed the notation for the currents from IB and IC to ib and ic respectively, to
remind us that we are now talking about small signal ac quantities, not large signal ones.
The bias currents IB and IC are still flowing through the device … but they do not appear
in the small signal model. This model is only used to figure out how the transistor
behaves for the ac signal going through it, not have it responds to large DC values. Now
rπ the equivalent small signal resistance of the base-emitter diode is given simply by the
inverse of the conductance of the equivalent diode. [13]"
However, remember that for the purposes in this course, we took that in the model of a
diode, its resistance is zero - we only model it as an open switch, or as a small source
with the diode turn-on voltage VDON. So the only part of the small signal model of a BJT
transistor that we are going to use, is that there is a linear dependency between base and
collector current, that is IC = hFE* IB (Eq 4-3). In the schematic of the model on Figure
20, that current relationship is expressed by the symbol of a dependent current source meaning it will force current in the collector branch with a value β*iB.
However, in order to use the model in analysis, we need to make it slightly more
accurate.
Basic electronic elements – transistor
19
To make out transistor model slightly more accurate, we will also model the transistor as
a switch. This is closely related to the output characteristic of a transistor: "
When a bipolar junction transistor (BJT) is used in any circuit, its function is
determined by the devices characteristic curves.
For a BJT there are input, output and transfer characteristic curves, the most useful
being the output characteristic curve. The output curves dictate the range of collectoremitter voltage Vce for variations in collector current, Ic. For use as an amplifier, biasing
is arranged so that the linear part of
the output curves (the almost
horizontal sections) are used. If the
circuit uses the transistor as a switch,
then biasing is arranged to operate
on regions of the output curves
known as saturation and cut-off. See
the diagram below:
Figure 21. Output characteristic of a BJT
transistor, with indicated cut-off and saturation
regions (Ref. [14] )
The yellow shaded area represents the 'cut-off' region. Here the operating conditions of
the transistor are:
zero input base current, zero output collector current and
maximum (supply rail) collector voltage.
In 'saturation', as depicted by the red shaded area, the BJT will be biased so that
the maximum amount of base current is applied, resulting in maximum collector
current flow and
minimum collector emitter voltage.
In both cut-off and saturation, minimum power is dissipated in the transistor. [14]"
We are going to take these effects into consideration into the model of the transistor we
will use.
Basic electronic elements – transistor
20
Finally, we can introduce the small signal model of transistor we are going to use note that "the following models are valid for moderate or small operating currents and
voltages, i.e., breakdown is not considered. [16]"
OFF
ON
SAT
Figure 22. Table of NPN transistor models for OFF, ON, and Saturation state (Ref. [16] )
In short - when we do not have enough voltage applied to the base-emitter junction to
turn it on as a diode (that is, we applied less than 0.6 or 0.7 V), neither the base-emitter
nor the collector-base lead, so all currents are zero - and we model both junctions as
open switches (OFF state). When the base-emitter junction is opened, at first there is
the linear relationship between collector and base current Eq 4-3, modelled by a current
generator - this is the ON state; the base-emitter junction we model the same as a
leading diode - that is, with a small voltage source with a value of the diode turn-on
voltage (around 0.6 or 0.7 V). The Vce voltage in this case is determined by external
elements.
When Vbe grows, the base current grows, and the collector current does as well, and
eventually the transistor reaches saturation (SAT state) - here we model both baseemitter and colelctor-base PN junctions as voltage sources, since the collector and base
currents do not have a linear relationship anymore, and they will be determined by
external components in the circuit. Here are several reformulations of the saturation
state in a transistor: "When a transistor is saturated the collector-emitter voltage VCE
is reduced to almost 0V. When a transistor is saturated the collector current Ic is
Basic electronic elements – transistor
21
determined by the supply voltage and the external resistance in the collector circuit, not
by the transistor's current gain. [18]". Also "generally if you apply enough base current
that the maximum current possible will always flow, then the transistor is operating like
a switch (rather than operating like an amplifier). In this mode, the transistor is said to
be saturated. … A BJT transistor will cause a drop in voltage between the collector and
the emitter even when it is saturated (it isn't a perfect conductor). This voltage drop is
called out in the data sheet as Vce(sat). Typically, this will be between .2 and 1 volts for
silicon transistors. [19]"
"For active operation, the collector-base junction must be reversed biased (positive
Vcb). Once Vcb enters the -.5V ballpark it ceases to be reversed biased, and the collector
current now depends on
Vcb. Practically speaking
this provides an output
voltage minimum for Vce
[which is VceSAT] [27]"
Figure 23.Common saturation
voltage range of an NPN transistor
(Ref. [27])
This is also included in the following excerpt: "The Bipolar Junction Transistor (BJT) is
an active device. In simple terms, it is a current controlled valve. The base current (IB)
controls the collector current (IC).
Regions of BJT operation:
Cut-off region: The transistor is off. There is no conduction between the collector and
the emitter. (IB = 0 therefore IC = 0)
Active region: The transistor is on. The collector current is proportional to and
controlled by the base current (IC = βIB) and relatively insensitive to VCE. In this region
the transistor can be an amplifier.
Saturation region: The transistor is on. The collector current varies very little with a
change in the base current in the saturation region. The VCE is small, a few tenths of
volt. The collector current is strongly dependent on VCE unlike in the active region. It is
desirable to operate transistor switches will be in or near the saturation region when in
their on state.
Rules for Bipolar Junction Transistors (BJTs):
•
For an npn transistor, the voltage at the collector VC must be greater than the
voltage at the emitter VE by at least a few tenths of a volt; otherwise, current will
not flow through the collector-emitter junction, no matter what the applied
voltage at the base.
Basic electronic elements – transistor
22
•
For the npn transistor, there is a voltage drop from the base to the emitter of 0.6
V. In terms of operation, this means that the base voltage VB of an npn transistor
must be at least 0.6 V greater that the emitter voltage VE; otherwise, the
transistor will not pass emitter-to-collector current. [17]"
So, for this kind of a model, all we need are the basic equations of the BJT, below.
Figure 24. Basic equations of the BJT in our simplified model (Ref. [17] )
Let's also include the following figure as a reminder of the basic principles of an NPN
BJT transistor:
Figure 25. NPN transistor in active state (Ref. [5])
Before we conclude, let us just repeat that the discussion so far was related to an NPN
BJT transistor, for the most used configuration (common emitter). For different
configurations, a slightly different analysis should be applied. For PNP transistors, the
conclusions are the same, except the polarisation voltages and the directions of the
Basic electronic elements – transistor
23
currents are going to be reversed, in respect to NPN transistors. (Note that most of the
resources included in this document discuss both NPN and PNP transistors).
Basic electronic elements – transistor
24
4.1 Measuring (testing) transistors
The reason for measuring transistor is to find its proper pins - and to also make sure it is
working properly.
For most transistors, one can identify the pin function by the form of the packaging here are some notes about transistor packages: "There are many transistor case designs.
Some conform to JEDEC Standards and are defined by Transistor Outline (TO)
designations … Solid-state devices other than transistors are also housed in these same
packages. In general, the larger the unit, the greater the current or power rating of the
device. [22]"
Figure 26. Different transistor packages and pinouts (Ref. [22] )
Figure 27. Different transistor packages and pinouts (Ref. [23])
Occasionally, however, it will not be possible to read the polarity of the terminals
directly from the terminals of the transistor- in that case, one has the option of finding
which terminal is base, emitter or collector, by using a test procedure with a multimeter,
set to measure resistance - as in the case of diodes. Again, we use the fact that when
measuring resistance, each multimeter 'becomes' a small voltage source. This would be
the ohmmeters view of the BJT: "Clearly a transistor cannot be made on the bench by
combining two diodes. … Most ohmmeters not only measure resistance, but also
Basic electronic elements – transistor
25
measure the forward voltage drop across a diode. From this perspective you can identify
the base and the type of transistor based on the [a diode] equivalent circuit. [17]"
The procedure for identifying the terminals of a
transistor is given below:
Figure 28. Using an ohmmeter to identify terminals of a transistor
(Ref. [26] )
"Set a digital multimeter to diode test and an analogue multimeter to a low resistance
range such as × 10, as described above for testing a diode.
Test each pair of leads both ways (six tests in total):
•
The base-emitter (BE) junction should behave like a diode and conduct one
way only.
•
The base-collector (BC) junction should behave like a diode and conduct one
way only.
•
The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an
NPN transistor. The diodes are reversed in a PNP
transistor but the same test procedure can be used.
[21]"
Figure 29. Behavior of junctions in an NPN transistor, upon
measurement (Ref. [21] )
The expected values in measuring are shown in the table.
Figure 30.Table of expected measured resistance values
(ohmmeter readings) for transistor tests (Ref. [4])
For a more detailed account of the measuring procedure, see also [21].
Basic electronic elements – transistor
26
5 FET transistors
Let's just include a few excerpts on FET - field effect transistors: "The FET is a three
terminal device like the BJT, but operates by a different principle. The three terminals
are called the source, drain, and gate. The voltage applied to the gate controls the
current flowing in the source-drain channel. No current flows through the gate
electrode, thus the gate is essentially insulated from the source-drain channel. Because
no current flows through the gate, the input impedance of the FET is extremely large (in
the range of 1010–1015 Ω). The large input impedance of the FET makes them an
excellent choice for amplifier inputs.
The two common families of FETs, the junction FET (JFET) and the metal oxide
semiconductor FET (MOSFET) differ in the way the gate contact is made on the sourcedrain channel.
In the JFET the gate-channel contact is a reverse biased pn junction. The gate-channel
junction of the JFET must always be reverse biased otherwise it may behave as a diode.
All JFETs are depletion mode devices—they are on when the gate bias is zero (VGS = 0).
In the MOSFET the gate-channel contact is a metal electrode separated from the
channel by a thin layer of insulating oxide. MOSFETs have very good isolation between
the gate and the channel, but the thin oxide is easily damaged (punctured!) by static
discharge through careless handling. MOSFETs are made in both depletion mode (on
with zero biased gate, VGS = 0) and in enhancement mode (off with zero biased gate).
Two versions of the symbols are in common use. The symbols in the top row depict the
source and drain as being symmetric. This is not generally true. Slight asymmetries are
built into the channel during manufacturing which optimize the performance of the
FET. Thus it is necessary to distinguish the
source from the drain. [] We will use the
asymmetric symbols found on the bottom row,
which depict the gate nearly opposite the source.
The designation n-channel means that the
channel is n doped and the gate is p doped. The
p-channel is complement of n-channel. [17]"
Figure 31.Two versions of common JFET symbols (Ref. [17] )
Basic electronic elements – transistor
27
6 Transistor construction
Transistors, like diodes, are produced using a variety of chemical processes. All of these
elements are made by different combinations of P and N semiconductor - a visual
overview of the differences in construction are provided below:
Figure 32.Differences in construction of semiconductor elements (Ref. [25])
A very good illustration of the transistor - both BJT and FET - fabrication process is
given in [24].
Figure 33. Cross-section of a semiconductor FET and BJT transistor (Ref. [24] )
Basic electronic elements – transistor
28
7 Basic circuits
In this section, we analyze several basic circuits that are based on the functionality of a
BJT NPN transistor.
7.1 Common emitter amplifier
The common emitter amplifier is the simplest transistor circuit. Before we start
discussing the common emitter amplifier, let us look at an even simpler example.
We have previously mentioned that in order for a transistor to work, it needs its baseemitter junction directly polarised, and collector base junction inversely polarised; this
resulted with illustration like Figure 7. We can find similar illustrations elsewhere.
However, we should keep in mind that this is NOT a minimal common emitter
amplifier! Actually, as our model of a diode assumes that a diode has resistance near
zero when it's turned on, notice that in the
image, the base-emitter loop (loop A) has no
B
resistance whatsoever - this means that in
reality, this would represent a short circuit, and
most likely burn the base-emitter junction!
A
Figure 34.Illustration of NPN transistor biasing in a common
emitter configuration - as there are no resistances in neither
loop A nor loop B, the circuit is in fact a short circuit (Ref. [28])
So remember that this kind of a diagram is only an ilustration of the biasing, not a
schematic of a 'legal' circuit! A next step might be to add a single resistor in the emitter
branch - this ensures that both loops see some sort of a resistance.
Thus, we obtained a 'legal' circuit, but note that
this is not an amplifier; if we write KVL for loop A:
B
Vi - Vbe = Vo
we can see that the output is simply always going
to be the input minus 0.7 V (VbeON).
A
Figure 35.Common emitter NPN with a single resistor
Basic electronic elements – transistor
29
Finally, a minimal common emitter configuration is represented below - and it contains
a base resistor and a collector resistor - with no resistor in the emitter branch. The input
voltage, which should be amplified, is Vi, and is applied to the base resistor - whereas
the output voltage is taken from the collector (in reference to ground, which in this case
is the emitter). Where Vi is the input voltage, which eventually directly polarizes the
base-emitter junction; Vcc is the collector power supply, which ought to polarise the
collector-base junction inversely.
Figure 36. Different representations of the common emitter amplifier
So, actually, it is the power given by Vcc that we modulate by Vi, and that is what we
perceive as amplification.
So let us write the KVL expressions for this circuit:
Ic
+
B
Ib +
Vi − VRB − Vbe = 0
Vo + V RC − VCC = 0
(loop A)
(loop B)
And KCL for the transtor as a node
A
Ib + Ic = Ie
Ie
Notice that in this circuit, Vo = Vce. In addition, we can write the branch equations Ohms law - for the two resistors:
V RB = RB I b
;
Basic electronic elements – transistor
V RC = RC I c
30
From loop A we can express the base-emitter voltage:
Vbe = Vi − VRB = Vi − R B I b
If this voltage is less than the turn-on voltage of base-emitter junction (assumed 0.6V),
then the transistor is OFF. If it is greater then 0.6V, then the transistor turns on. We can
express this differently - if we express the current Ib (from KVL for loop A):
Ib =
Vi − Vbe
RB
From here, as Vbe keeps a value of around 0.6V when the transistor is ON, the input
voltage Vi must be at least greater than 0.6V, so we have positive base current (flowing
into the base) so to turn the transistor ON.
Once the transistor is turned on, while in the active state, we additionally have the linear
relationship between collector and base current:
I c = hFE ⋅ I b
In this state, we can directly express the relationship between input and output voltage from KVL for loop B:
V o = VCC − V RC = VCC − RC I c = VCC − RC h FE I b
V o = VCC −
h FE RC
(Vi − Vbe ) ≈ VCC − hFE RC (Vi − 0.6 )
RB
RB
From here, we can see that the output voltage Vo (in this case Vce as well) is maximum
when the input voltage is <= 0.6; before the transistor turns on, the output voltage Vo =
Vcc.
As Vi grows, Vo (which is also Vce) drops (gets smaller); and this is the case as long as
the transistor stays in the active state.
However, as Vi grows, the base current grows as well, so does the collector current, and
the transistor eventually goes in saturation (SAT). For saturation, the linear dependancy
between base and collector current is not valid anymore, and we know that Vce reaches
the value of VceSAT for that transistor - typically some 0.2 V. Thus, in saturation, the
collector current will be fixed on:
I c max =
VCC − Vo VCC − VceSAT VCC − 0.2
=
≈
RC
RC
RC
Basic electronic elements – transistor
31
The corresponding maximum base current, at the moment of transition between ON
and SAT will be:
I b max =
I c max VCC − 0.2
≈
hFE
hFE RC
And so the input voltage when the transistor crosses into saturation is
Vi max = VRB + Vbe ≈ RB I b max + 0.6 = RB
VCC − 0.2
+ 0 .6
hFE RC
Of course, as here the output voltage is the collector-emitter voltage, so the output
voltage in saturation will be Vo = VceSAT = 0.2V.
So, now we have the ranges of our analysis:
Vi < 0.6V
-> T is OFF -> Vo = Vcc
0.6V < Vi < R B
-> T is ON
-> Vo = VCC −
-> T is SAT
-> Vo = 0.2V
VCC − 0.2
+ 0 .6
hFE RC
V − 0 .2
Vi > R B CC
+ 0 .6
hFE RC
hFE RC
(Vi − 0.6)
RB
Now, if we have Vcc = 5V, hFE = 100, and RC = RB, we obtain:
Vi < 0.6V
0.6V < Vi < 0.648
Vi > 0.648
-> T is OFF -> Vo = 5V
-> T is ON -> Vo = 65 - 100*Vi
(5 > Vo > 0.2)
-> T is SAT -> Vo = 0.2 V
Notice for a voltage change from 0.6 to 0.648 V (total change = 0.048V) we got a swing
from 5 to 0.2 V (total change = - 4.8 V). So the obtained change of output voltage is
about 100 times greater than the change of the input voltage - hence we obtained
amplification.
We can express change through the delta operator, and apply it to the equation relating
input and output voltage:
h R
V o ≈ VCC − FE C (Vi − 0 .6 ) / delta differentiate
RB
h R
∆ V o ≈ − FE C ∆ Vi
RB
Basic electronic elements – transistor
32
So, an amplification factor can be calculated as the relationship between output and
input change of voltage:
A=
h R
∆Vo
≈ − FE C
RB
∆Vi
And it only depends on the transistor current gain parameter hFE (or β), and the values
of the resistors. If Rc=Rb, then it solely depends on hFE, and knowing that a typical hFE is
between 50 and 200 - that means with a typical transistor in this configuration we'd get
between 50 and 200 factor of voltage amplification.
------------------------------Notice that in this simple case, one cannot simply amplify an AC signal (which typically
goes from negative to positive voltages, centered around the zero). That is because we
need at least 0.6V to have the transistor turn on. In this case, to analyse our circuit we
can only use an AC signal changing from 0.6V to 0.648V; this can be modelled with a
DC generator, and a zero centered AC signal:
The amplitude of the AC signal is ( 0.648 - 0.6 ) /2 = 0.024 V
The DC generator should have the central value ( 0.648 + 0.6 ) /2 = 0.624 V
So, our signal can be modelled with
Vi = 0.624 + 0.024 ⋅ sin (ωt )
This is what we are going to use, when simulating this circuit with the Falstad circuit
applet. It is also advisable to use Falstads circuit simulator applet, to visualise the flow of
current in this circuit.
Figure 37. Visualising the flow of current in a common emitter amp using Falstads applet (left), visualising voltage
(green) input (top) and output (bottom) of a common emitter amp (right) (Ref. [29])
Basic electronic elements – transistor
33
Note that in the Falstad simulator, the parameters of the transistor cannot be changed.
However, it is obvious by comparing the input and output signal, that the input signal as
been amplified - but also, the input is in counter-phase with the output voltage (when
the input voltage grows, the output drops and vice versa).
Notice that the applet has the finetuned voltage described above, so a clean sinusoid is
obtained on the output - this means the transistor works all the time in the active state
(ON). Try changing the DC and the AC component of the voltage component, to see how
soon the transistor will start going into either OFF or SAT, and thereby distort the
signal.
----------------Note finally, that for practical applications, this simplest form of the common emitter
amplifier is rarely used. A more common, practical form of the common emitter
amplifier is shown below.
Figure 38.A more practical version of the common emitter amplifier (Ref. [30] ); applet simulation ([32])
The analysis is slightly more complicated, and will not be included here. However, note
that R1 and R2 form a voltage divider, which keeps the base polarised at all times; so they
have the purpose of the DC generator above, supposed to shift the DC level. The AC
source, now centered around zero, is coupled to the divider through a capacitor C1 which ensures that the input changes are added to the voltage set by the divider;
however, it will be somewhat diminished because of the RC filtering effects. C2 on the
output ensures that the 'lifted' amplified AC signal on the output, will again be centered
around zero on the output; RL symbolises the load (or rather, the circuit consuming the
amplified signal).
Note that the common emitter amplifier also has equivalent input resistance (looking
into the base) and output resistance (looking into the collector): "The resistance of
Basic electronic elements – transistor
34
the input loop is the base resistance in series with the resistance in the emitter-side of
the circuit, referred to the base by the β transform.
rin (CE ) = R B + (1 + β ) ⋅ (rE + RE )
At the output node, the BJT transistor model shows a current source (infinite
resistance) in parallel with load resistance RL. The output resistance is therefore RL.
The CE amplifier has relatively high input resistance due to the β -transform effect at the
base. It is better as a voltage-input port. Its output resistance is relatively low if the load
resistor is not made too large. [35]" Note that in the previous simple case, RE was zero,
so this effect of increase of input resistance due to hFE could not be seen.
For more on the common-emitter amplifier, see [30], [31], [32], [33], [34] …
---------------------------------------------Note that when talking about powering transistors, some special notation is used for the
supplies: "VCC (note: lower case is often used instead of subscript, e.g. 'Vcc') is an
electronics designation that refers to voltage from a power supply connected to the
'collector' terminal of a bipolar transistor. In an NPN bipolar junction transistor, it
would be +VCC, while in a PNP transistor, it would be −VCC. There is debate over the
origins of the double letter subscript naming convention. One proposal is that it
originated as an abbreviation of the supply voltage for a common collector amplifier
with the other power supply names mimicking this fashion. Double letters may also have
been used to clearly indicate that a power supply voltage is being referred to. [37]"
Basic electronic elements – transistor
35
7.2 Current source and current mirror
So far we have mostly discussed voltage sources as power supplies (emf generators) in a
circuit - their job is to keep constant potential difference on their ends; which means
they adjust the current that flows through them depending on the total resistance of the
circuit. We haven't so far discussed the other types of sources - current sources; which
try to maintain constant current, whereas the voltage is set by the resistance of the rest
of the circuit.
A very simple way of implementing a current source, is
simply biasing a common emitter transistor
permanently; as the base current is fixed, so will be the
collector current, flowing through the RL, as long as we
are in the transistor active state: "Once the bias is set,
this circuit will supply a constant current to the load
resistor RL so long as operation is withing the linear
range of the transistor. [3]"
Figure 39. Schematic of a simple comm0n-emitter current source (Ref.
[3])
Another circuit, related to current sources, is the so-called current mirror: "A current
mirror is a circuit designed to copy a current through one active device by controlling
the current in another active device of a circuit, keeping the output current constant
regardless of loading. The current being 'copied' can
be, and sometimes is, a varying signal current.
Conceptually, an ideal current mirror is simply an ideal
current amplifier with unity current gain.
Transistor Q2 is connected such that it behaves as a
forward-biased diode. The constant current through it
(due to Rbias and Vcc) is determined mainly by the
series resistance Rbias as long as Vcc is significantly
larger than 0.7V, the typical forward VBE voltage for a
silicon BJT.
Figure 40. Schematic of a current mirror
Basic electronic elements – transistor
36
It is important to have Q2 in the circuit instead of a regular diode because, assuming the
two transistors are closely matched, the base current for each transistor should be nearly
identical since VBE for each transistor is identical. With nearly identical base currents,
the matched transistors should then have nearly identical collector currents as long as
VCE2 is not significantly larger than VBE. [36]" Notice that the analysis is based mostly on
the Ebers-Moll equations.
Of course, one needs to complete the circuit extending above Q1 in order to have current
Ic1 flowing. Note also that the figure also shows a -Vcc supply; however this potential
can be well set to ground without a problem - as long as the conditions described above
are satisfied.
It is also advisable to use Falstads circuit simulator applet, to visualise the flow of
current in this circuit (there is a built example).
Figure 41. Visualising the flow of current in a PNP current mirror using Falstads applet (Ref. [29])
Basic electronic elements – transistor
37
7.3 Astable multivibrator
Another interesting basic circuit is the astable multivibrator. For one, it is an oscillator
circuit, that generates a pair of complementary square-wave voltages. As such, it also be
seen as a digital clock - or more generally, switching, or logic circuit.
The starting point in the analysis of this
circuit is being aware that it oscillates
between two stable semi-stable states;
only one transistor can lead in each
state.
Figure 42. A schematic of an astable multivibrator
We include a very nice analysis of the principle of operation from [38]: "
Astable Multivibrator is a two stage switching circuit in which the output of the first
stage is fed to the input of the second stage and vice versa. The outputs of both the
stages are complementary. This free running multivibrator generates square wave
without any external triggering pulse. The circuit has two stable states and switches back
and forth from one state to another, remaining in each state for a time depending upon
the discharging of the capacitive circuit.
The multivibrator is one form of relaxation oscillator, the frequency of which may be
controlled by external synchronizing pulses.
In our experiment we are using transistor, as the amplifying device and also it is a
collector coupled multivibrator. In the basic symmetrical astable multivibrator, []
components in one half of a cycle of the circuit are identical to their counterpart in the
other half. Square wave output can be obtained from the collector point of [either] Q1 or
Q2.
Operation
When supply voltage, VCC is applied, one transistor will conduct more than the other due
to some circuit imbalance.
Initially let us assume that Q1 is conducting (ON) and Q2 is cut-off (OFF).
Basic electronic elements – transistor
38
Then VC1, the output of Q1 is equal to VCESAT which is approximately zero and VC2 is equal
to VCC. At this instant C1 charges exponentially with the time constant R1C1 towards the
supply voltage through R1 and correspondingly VB2 also increases exponentially towards
VCC. When VB2 crosses the coupling voltage Q2 starts conducting and VC2 falls to VCESAT.
Also VB1 falls due to capacitive coupling between collector of Q2 and base of Q1, thereby
driving Q1 into OFF state. The rise in voltage VC1 is coupled through C1 to the base of Q2
causing a small overshoot in voltage VB2.
Thus Q1 is OFF and Q2 is ON. At this instant the voltage levels are:
VB1 is negative, VC1=VCC, VB2=VBESAT and VC2=VCESAT.
When Q1 is OFF and Q2 is ON the voltage VB1 increases exponentially with a time
constant R2C2 towards VCC. Therefore Q1 is driven to saturation and Q2 to cut-off.
Now the voltage levels are:
VB1=VBESAT, VC1=VCESAT, VB2 is negative and VC2=VCC.
From the above it is clear that when Q2 is ON the falling voltage VC2 permits the
discharging of capacitor C2 which inturn drives Q1 into cut-off. The rising voltage of VC1
is fed back to the base of Q2 tending to turn it ON. This process is regenerative.
…
Charging and discharging time periods are given by
t1 = 0.693R1C1
t 2 = 0.693R2 C 2
The total time period T is given by T = t1 + t 2
For symmetrical astable multivibrator: R1 = R2 = R , C1 = C 2 = C -> T = 1.386 RC
The free running frequency is given by
f =
1
1
=
T 1.386 RC
[38]"
Basic electronic elements – transistor
39
It is also advisable to use Falstads circuit simulator applet, to visualise the flow of
current in this circuit (there is a built example).
Figure 43. Visualising the flow of current in an astable
multivibrator using Falstads applet (Ref. [29])
Basic electronic elements – transistor
40
7.4 Differential amplifier
The differential amplifier is a very important circuit concept, because it is at the basis of
the operational amplifier, which we are going to discuss separately: "Look under the
hood of most op amps, comparators or audio amplifiers, and you'll discover this
powerful front-end circuit - the differential amplifier. A simple circuit able to amplify
small signals applied between its two inputs, yet reject noise signals common to both
inputs. This circuit has a unique topology: two inputs and two outputs. Although you
can tap the signal from one output only, taking the difference between both outputs
delivers twice the gain! And it improves Common-Mode Rejection (CMR), an essential
function when the common-mode signal is a noise source or DC bias from a previous
stage. [39]"
The design consists of two common emitter amplifiers, placed in two symmetric
branches, powered from the same collector power supply. The branches are further
connected at the emitters, and powered with constant current.
Figure 44. Differential amplifier schematic - left, with constant current source; right, with current mirror used as
current source.
The schematic above shows one possibility of implementing a constant current source
for the emitter contact. "How does this amplifier amplify differential signals and reject
common ones? The bias condition assumes equal voltages at VB1 and VB2, forcing the
bias current IE (set by RE) to split equally between the transistors resulting in IC1 = IC2.
With RC1 = RC2, equal voltages develop at VC1 and VC2. [39]"
Basic electronic elements – transistor
41
Now, if we increase say only Vi2, then Ib2 grows, and therefore Ic2 grows. Since Vo2 =
Vcc - Rc2Ic2, Vo2 must necesarilly drop. However, if Ic2 grows, then also Ie2 grows (Ie2
= Ic2 + Ib2). But - since Ie1 + Ie2 = Ie, the constant current source, growth of Ie2
necesarilly means that Ie1 will drop (since Ie1 = Ie - Ie2) . If Ie1 drops, Ic1 drops as well,
and since Vo1 = Vcc - Rc1Ic1, then Vo1 must rise.
So, we came to the conclusion that when Vi2 is increased (while Vi1 is kept constant),
the output voltage from the symmetric branch - Vo1 - is increased; whereas the output
voltage in own branch Vo2 drops. The same is valid if we switch sides too.
Figure 45.Schematic of differential amplifier, with differential
(Vs) and common mode (Vcm) input voltages (Ref. [39] )
Often, this discussion is given in terms of
differential and common mode signals: "
DIFFERENTIAL GAIN
Now suppose a differential signal is applied to
the inputs. This will incrementally increase and
decrease the base voltages to
VB1 + ∆V and VB2 - ∆V
Because Q1 conducts a little more and Q2 a
little less, IE now splits unevenly creating
IC1 > IC2
This, in turn, forces the voltage at VC1 to decrease and VC2 to increase. The result: a
voltage change at each output due to a differential input.
COMMON-MODE REJECTION
Now suppose a common-mode input signal is applied. We incrementally increase both
inputs to
VB1 + ∆V and VB2 + ∆V
Because the conduction level of neither transistors has changed (both bases and
emitters moved by the same amount), the collector currents did not change.
IC1 = IC2 ≈ IE / 2.
Subsequently, the voltages at VC1 and VC2 remain the same! Therefore, the circuit has
rejected a signal common to both inputs.
Basic electronic elements – transistor
42
Well, the last statement is almost true. Actually, a change in emitter voltage had a small
ill effect. It changed the bias current IE set by RE. And this directly impacted IC1 = IC2
≈ IE / 2, slightly shifting the levels at VC1, VC2. As you can see the rejection is not
perfect. However, it can still be effective at removing a large part of noise or a DC bias
common to both inputs. [39]"
Let us also include the following excerpt: "A differential amplifier is a type of an
electronic amplifier that multiplies the difference between two inputs by some constant
factor (the differential gain). A differential amplifier is the input stage of operational
amplifiers, or op-amps, and emitter coupled logic gates. Note that a differential
amplifier is a more general form of amplifier than one with a single input; by
grounding one input of a differential amplifier, a single-ended amplifier results. [40]"
It is also advisable to use Falstads circuit simulator applet, to visualise the flow of
current in this circuit (there is a built example).
Figure 46. Visualising the flow of current in an differential amplifier using Falstads applet (Ref. [29])
Basic electronic elements – transistor
43
8 Sensing application
Special implementations of the transistor can also be used as sensor elements. The most
typical example is a phototransistor.
8.1 Phototransistor
First, a few introductory sentences: "A phototransistor is in essence nothing more
than a bipolar transistor that is encased in a transparent case so that light can reach the
base-collector junction. The phototransistor works like a photodiode, but with a much
higher sensitivity for light, because the electrons that are generated by photons in the
base-collector junction are injected into the base, and this current is then amplified by
the transistor operation. However, a phototransistor has a slower response time than a
photodiode. [41]"
"Like diodes, all transistors
are light-sensitive. Phototransistors are designed
specifically to take advantage of this fact. The most-common variant is an NPN bipolar
transistor with an exposed base region. Here, light striking the base replaces what
would ordinarily be voltage applied to the base -- so, a phototransistor amplifies
variations in the light striking it. Note that phototransistors may or may not have a base
lead (if they do, the base lead allows you to bias the phototransistor's light response.
[43]"
Here is an application note related to interfacing of a phototransistor - apparently as a
switch: "The common-emitter amplifier circuit (Fig. 1) generates an output which
transitions from a high state to a low state when light in the near-infrared range is
detected by the phototransistor. The wavelength range for light in the near infrared
region is about 700 nanometers (nm) to 1100 nm. The output is
created by connecting a resistor between the voltage supply and
the collector pin of the component. The output voltage is read at
the terminal of the collector. It is called an amplifier circuit
because the current generated inthe component when light is
detected is very small. However, the component has an internal
amplifier (in this case a phototransistor) which magnifies this
current to useful levels."
Figure 47. Common emitter phototransistor amplifier (Ref. [42] )
For more, see [44], [45]
Basic electronic elements – transistor
44
9 PE Questions
•
•
•
What is the composition of a (bipolar junction) transistor ?
What is the main behavior of a transistor in electric circuits?
Which states do we use to model the behavior of transistor?
Basic electronic elements – transistor
45
Resources and references
[1].
Transistor, http://en.wikipedia.org/wiki/Transistor
[2]. Bipolar junction transistor,
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
[3]. Current source, http://hyperphysics.phyastr.gsu.edu/hbase/electronic/npnce.html#c4
[4]. Testing Transistors with Ohmmeter, http://hyperphysics.phyastr.gsu.edu/hbase/electronic/trantest.html#c1
[5]. The junction transistor, http://hyperphysics.phyastr.gsu.edu/hbase/solids/trans.html
[6]. habil.dr. Vladimir Gavryushin. “Bipolar Junction Transistors.” Functional
combinations in solid states.
http://www.mtmi.vu.lt/pfk/funkc_dariniai/transistor/bipolar_transistor.htm
[7]. Technolab SA - Techical Education Equipment. “HAKO Section 6 Overhead
Models.” Hako - Movable Overheadmodels, Cut-away models.
http://www.technolab.org/Hako/Katalog-e/Section6.htm
[8]. “Hydraulic analogy - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Hydraulic_analogy
[9]. SatCure. “How do Transistors Work.” Hobby Electronics Tutorial.
http://www.satcure-focus.com/tutor/page4.htm
[10]. Dr. Robin N. Strickland. “Course Notes.” ECE 220 Basic Circuits.
http://apache.ece.arizona.edu/~ece220/Course_Notes/courseNotes.html
[11]. “Transistor models - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Transistor_models
[12]. “Transistor (Java calculator).” http://www.ee.ucl.ac.uk/~amoss/java/trans.htm
[13]. Bill Wilson. “Small Signal Model for Bipolar Transistor.” Connexions .
http://cnx.org/content/m11349/latest/
[14]. Andy Collinson. “The Transistor as a Switch.” Circuit Analysis, Design & Theory.
http://www.zen22142.zen.co.uk/Design/bjtsw.htm
[15]. Tecat: Technical Writing and Educational Consulting. “Visualization Techniques
/ Understanding Transistors.” http://www.tecat.ca/Visualization.htm
[16]. Murat Aşkar. “TABLE OF TRANSISTOR MODELS.” EE 312 Digital Electronics.
http://www.eee.metu.edu.tr/~askar/EE312/Transistor_Models.pdf
[17]. Dr. Lloyd Bumm. “Lecture Notes on BJT & FET Transitors v1.1.1.” Electronics
Lab (Phys2303).
http://www.nhn.ou.edu/~bumm/ELAB/Lect_Notes/BJT_FET_transitors_v1_1.html
[18]. John Hewes. “Transistor Circuits.” The Electronics Club.
http://www.kpsec.freeuk.com/trancirc.htm
[19]. Chuck McManis. “Bipolar Junction Transistor Theory.”
http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/bjt_theory.html
[20]. John Hewes. “Multimeters.” The Electronics Club.
http://www.kpsec.freeuk.com/multimtr.htm
Basic electronic elements – transistor
46
[21]. Allaboutcircuits.com. “Meter check of a transistor .” Volume III Semiconductors » BIPOLAR JUNCTION TRANSISTORS .
http://www.allaboutcircuits.com/vol_3/chpt_4/3.html
[22]. Kilowatt Classroom, LLC. “Introduction to Transistors.”
http://web.archive.org/web/20060526125209/http://www.kilowattclassroom.com/Arc
hive/Transistor.pdf
[23]. Toy Mayeda, Allan Weber. “Transistor Packages.” EE 459L - Embedded Systems
Design Laboratory - Library.
http://ee.usc.edu/library/ee459/datasheets/Transistors.pdf
[24]. C.R.Wie, SUNY-Buffalo. “BJT-FET Fabrication Applet.” Educational Java Applet
Service. http://jas.eng.buffalo.edu/education/fab/BjtFet/index.html
[25]. Joe Wolfe . “How do diodes and transistors work?” FAQ in high school physics.
http://www.phys.unsw.edu.au/~jw/FAQ.html#diodes
[26]. Raymond W.N Chan. “Investigation of the Characteristics of a Transistor (A).”
Show One Experiment - Experiment E19.
http://www.radian.com.hk/alphyprac/showOneExpt.asp?exptcode=E19
[27]. Bryan Audiffred. “Bipolar Transistor Introduction.” EE 2230 Electronics I Topics. http://baudiff-2.lsu.edu/~bryan/notebook/S07/EE2230/pages/28.html
[28]. Christian Wolff . “npn Transistor Operation.” Radar Basics - Theory of
Semiconductors. http://www.radartutorial.eu/21.semiconductors/hl18.en.html
[29]. Paul Falstad. “Circuit Simulator Applet.” Math, Physics, and Engineering
Applets. http://www.falstad.com/circuit/
[30]. Hyperphysics. “NPN Common Emitter Amplifiers.” http://hyperphysics.phyastr.gsu.edu/hbase/electronic/npnce.html#c2
[31]. Doug Gingrich . “The Common Emitter Amplifier.” PHYS 395 ELECTRONICS
LECTURE NOTES .
http://www.phys.ualberta.ca/~gingrich/phys395/notes/node81.html
[32]. Chiu-king Ng. “Amplifier (Applet).”
http://www.ngsir.netfirms.com/englishhtm/Amplifier.htm
[33]. “Common emitter - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Common_emitter
[34]. Allaboutcircuits.com. “The common-emitter amplifier .” Volume III Semiconductors » BIPOLAR JUNCTION TRANSISTORS .
http://www.allaboutcircuits.com/vol_3/chpt_4/5.html
[35]. Innovatia. “Amplifier Circuits.”
http://www.innovatia.com/Design_Center/Amplifier_Circuits.htm
[36]. “Current mirror - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Current_mirror
[37]. “IC power supply pin - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/IC_power_supply_pin
[38]. Visionics. “Astable Multivibrator.”
http://www.visionics.ee/curriculum/Experiments/Astable/Astable%20Multivibrator1.h
tml
[39]. eCircuit Center. “BJT Differential Amplifier.”
http://www.ecircuitcenter.com/Circuits/BJT_Diffamp1/BJT_Diffamp1.htm
[40]. “Differential amplifier - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Differential_amplifier
Basic electronic elements – transistor
47
[41]. “Photodiode - Wikipedia, the free encyclopedia.”
http://en.wikipedia.org/wiki/Photodiode
[42]. Fairchild Semiconductor Corporation. “Design Fundamentals for Phototransistor
Circuits.” Application Note AN-3005. http://www.fairchildsemi.com/an/AN/AN3005.pdf
[43]. Eric Seale. “Phototransistors.” The EncycloBEAMia.
http://encyclobeamia.solarbotics.net/articles/phototransistor.html
[44]. Radio-Electronics.Com. “Photo transistor (Phototransistor).” http://www.radioelectronics.com/info/data/semicond/phototransistor/photo_transistor.php
[45]. PerkinElmer, Inc. “Typical Phototransistor and IRED Applications.”
http://optoelectronics.perkinelmer.com/content/ApplicationNotes/APP_IREDAndPho
totransistorTypicalApps.pdf
[46].
Basic electronic elements – transistor
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