Chapter 1 : Small Signal Low Frequency Transistor
Q. 1. What is a small signal amplifier ? Draw the diagram and explain function
of each component. Explain effects of coupling and by pass capacitors in a
circuit.
Ans. When the input signal is quite weak and produces small fluctuations in the output
current in comparison to its operating value, the amplifier is called small signal or
voltage amplifier.
The practical circuit of transistor amplifier in CE configuration with self biasing is drawn.
The resistances ,
and
form the biasing and stabilization circuit. The biasing
circuit must establish a proper operating otherwise a part of —ve half cycle of the signal
may he cut off in the output. Resistor
is load resistor.
An electrolytic capacitor called input capacitor,
, of about
is used to couple the
signal to the transistor base. In the absence of this capacitor, the signal source
resistance will come across
and thus change the bias. This capacitor allows only ac
signal to flow but isolates the signal source from .
Another capacitor is called emitter by pass capacitor.
of capacity of about
is
used in parallel with emitter resistance
in order to provide low reactance path to the
amplified signal (ac). In the absence of capacitor, amplified ac signal flowing through
will cause a voltage drop across it which in turn will feedback the input side and reduce
the output voltage.
Four coupling of one stage of amplifier to the next stage another capacitor
called
coupling or blocking capacitor
is used. Because of
, the output across
free
from the collector dc voltage. In its absence
will come in parallel with resistor
of
biasing network of next stage and thereby change the biasing conditions of next stage.
Various currents flowing are:
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 1
Emitter by pass capacitor: It is included in the circuit to provide a low reactance path
to the amplified ac signal. It is connected across the emitter resistance,
. Because it
is used to by pass or shunt ac current away from the emitter resistor
. Or
is used
to prevent negative feedback in the emitter circuit. Removal of
results in an increase
in amplifiers input resistance, a reduction in its voltage gain and an increase in the
output resistance “looking into the collector”.
Q. 2. Explain the phase reversal of output voltage in CE amplifier.
Ans. In a CE configuration, the output voltage increases in the negative direction when
the input signal increases in the positive direction and vice versa. This is phase reversal
and causes 180° phase shift between input signal voltage
and output voltage
.
Consider a CE amplifier, the signal being fed at the input terminals i.e. between base
and emitter and output is taken from the collector and emitter ends.
The instantaneous output voltage
is given as
Reactance of coupling capacitor
considered as short.
is negligible at ordinary frequencies and it is
With increase in signal voltage in positive half cycle,
increases causing increase in
and so the drop
. As
is constant therefore, output voltage
decreases. Thus,
signal voltage increases in positive direction, the output voltage increases in the
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 2
negative direction i.e. there is phase difference of 180 degree between input and
output.
Conversely on negative half cycle of the input signal voltage, lesser current flows.
Though collector resistance
and so voltage drop across it decreases. For this reason
output voltage
increases.
Phase reversal can be shown graphically also using load line.
The base current
and
with
as the zero signal base current.
is maximum in
the positive direction,
becomes maximum in the negative direction and vice versa.
Thus, the input and output voltages are in phase opposition i.e. the transistor has
produced a phase reversal of output voltages w. r. t. the input signal voltage.
Q. 3. What is ac emitter resistance ? Give formulae also.’
Ans. The ac or dynamic resistance of the emitter base junction (or simply emitter)
diode is called ac emitter resistance. It is given as change in base emitter voltage
divided by corresponding change in emitter current i.e.
It is possible to derive formulae for ac resistance of the emitter diode by using calculus
and ac resistance of emitter diode is given as
current at Q point.
where
is the dc emitter
As emitter diode is special in transistor analyses so formulae use is
e indicates emitter and lower case r indicates ac or dynamic resistance and prime;
indicates internal resistance.
Q. 4. What is
model of transistor ? Give advantages of this model. Explain
model of transistor amplifier in CE configuration and its performance
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 3
characteristics.
Ans. Apart from hybrid model of transistor, another model is developed i.e.
model
which uses beta and resistance values. It has following advantages:
1. The procedure followed is quite simple and easy to understand.
2. The required datas are easily available.
3. The results obtained are quite accurate for the study of characteristics of amplifier
circuits.
The
model of a CE transistor amplifier:
Circuit of CE npn transistor amplifier with a base is given in figure below. Its dc
equivalent circuit is also shown.
From dc equivalent circuit
AC resistance as seen by the input signal when looking into the base is
equivalent circuit is given in the diagram.
In the absence of ac voltage source resistance
across
as well as
, whole of the signal voltage
because these are connected in parallel across it.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 4
. AC
ac
Then major part of source current
compared to
flows through
because
is large as
. Hence ac equivalent circuit is further simplified (as shown above)
and
Principal Performance Characteristics:
Effect of
on voltage Gain:
When
is considered, there will be voltage drop across it, thereby reducing
hence it will reduce
Larger
larger will be voltage drop across it, less
lesser internal drop across
Circuit with
is as shown.
is in series with
and
. Input ac signal
drops across
and
and
dud combination of
in parallel.
Q. 5. Draw the circuit of CE transistor amplifier and give its h parameter model.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 5
Find out its voltage gain, current gain, input and output impedance.
Ans. The circuit diagram of CE transistor and it CE amplifier Ii parameter equivalent
circuit (with ) is shown.
Circuit analysis can be done vigorously using Ii parameters derived from manufacturer‟s
data sheets for a given transistor. But parameters are specified only L specific set of
bias conditions and therefore need correction when the circuit bias conditions differ from
data sheet.
1. Input impedance : When looking into the base emitter terminals of the
transistor. . is seen to be in series with
For CE circuit,
is normally very small
quantity., so
feedback from output to input is negligible as compared to voltage
drop across . Thus, input impedance to the transistor base is given as,
With unbypassed emitter resistance
in the circuit,
From circuit,
being neglected, we have
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
is more complicated.
Page 6
The actual circuit input impedance is
(2) Output impedance : Since output voltage variations have little effect upon the
input of CE circuit, only output of the circuit needs to be considered in determining the
output impedance. Looking back into collector and emitter terminals, larger
resistance
is seen. Thus device output impedance
Actual output impedance is
in parallel with
i.e.
Since
is typically
or 20,
is usually very much less than
approximately equal to
(3) Voltage gain: It is given as output voltage to the input voltage i.e.
minus sign indicates 180° phase shift in
then
. So
is
in the circuit, ignoring
(4) Current Gain: It is ratio of output current to that of input current i.e.:
Above expression is true for both with or without
circuit current gain.
divides between
equation , we have
. It is but device current gain not
. Using current divider
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 7
5. Power gain:
Q. 6.Draw the circuit of CB transistor amplifier and give its h-parameter model
and drive all gains and impedances for it
Ans. CB transistor amplifier circuit with emitter current bias and its h parameter
equivalent circuit are drawn.
The practical CB amplifier circuit has ac signal source connected at input through
coupling capacitor
is connected to collector with
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Coupling capacitor
Page 8
short
circuits ac from the base to ground. It is assumed in h parameter model that the base of
the transistor is ac shorted to ground through
. In absence of
,
and
appears in series with transistor base terminal and hence
neglected when deriving approximate expressions.
1. Input impedance:
When bypass capacitor
is
at base terminal is omitted.
and input impedance to transistor emitter,
Actual circuit input impedance is
2. Output impedance : Since output voltage variations have little effect upon the input
of 03 circuit, only the output of circuit need be considered in determining the output
impedance. Looking back into collector and base terminals of transistor, a large
resistance
is seen. Thus device output impedance
Actual output impedance is parallel combination of
so
is approximately
3. Voltage Gain:
Because of negative value of
When
are in phase.
is omitted,
4. Current gain:
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 9
This expression is true for both circuits (with or without
gain, not the circuit gain. As
is divided between
divided between
Thus overall current gain,
). But this is device current
and the collector current is
5. Power gain:
Q. 7. Draw the circuit of CC transistor amplifier and give its h-parameter
model. Derive all gains and impedances also.
Ans. The practical circuit of CC amplifier using emitter current bias is shown and CC
amplifier h-parameter equivalent circuit is also shown.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 10
The ac signal source is connected to base terminal through coupling capacitor
external
capacitor
divider
load
connected
to
emitter
terminal
of
transistor
. Base bias voltage is derived from dc supply
and
through
,
coupling
by means of potential
. Now emitter bypass capacitor is used.
When the signal applied to the base terminal goes positive, forward bias
causing increase in emitter voltage. This is because emitter voltage
less base emitter voltage
and
is increased
is equal to
remains substantially constant i.e.
=
—
. Since emitter is the output terminal, it is seen that the output voltage of a cc
circuit is almost the same as its input voltage.
Because the output voltage at the emitter terminal follows the input signal applied to
base terminal, the cc circuit is also known as emitter follower.
In the parameter equivalent CC circuit, the current directions and voltage polarities are
indicated for a positive going input signal.
1. Input Impedance : The important point in cc amplifier is that
the
= 1 i.e. all of
is feedback to input .Then,
The input impedance to the transistor base,
2. Output impedance: In CC circuit, any variation in voltage at the output has a
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 11
significant effect on the input circuit. So for finding output impedance at transistor
emitter, the signal voltage is assumed to be zero and emitter current
in terms of
is determined
.
Above equation gives the output impedance of the device only. For the circuit output
impedance,
is in parallel with
so
3. voltage gain:
4. Current gain:
The signal current divides between
Current gain for cc circuit is,
For CC circuit,
is usually very much small than
significant lesser effect upon the circuit current gain.
5. Power Gain:
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
, so that
has a
Page 12
So, cc provides current and power gain but no voltage gain. The input impedance is
very high and output impedance is low. It is suitable for a low impedance load, which is
to be supplied with a signal from a high impedance source.
Q. 8. Explain frequency response of an amplifier.
Ans. The voltage gain of an amplifier varies with signal frequency due to effect of
variation in circuit capacitive reactance with signal frequency on the output voltage. The
curve drawn between the voltage gain and signal frequency of an amplifier is known as
frequency response.
If the input voltage of an amplifier is kept constant but its frequency is varied, it is
observed that the amplifier gain remains practically constant over a sizeable range of
mid frequencies. Amplifier gain falls off at low as will high frequencies.
Q. 9. Explain the effect of emitter bypass capacitor, coupling capacitor, emitter
resistance and shunt capacitor on the frequency response of an amplifier?
Ans. The performance of an amplifier depends to a considerable extent upon its
frequency response. In the design of an amplifier, appropriate steps are taken to ensure
that gain is essentially uniform over some specified frequency range.
A typical graph of amplifier output voltage or power plotted versus frequency is drawn.
It is found that output normally remains constant over a middle range of frequencies
and falls off at low and high frequencies. Gain over this middle range is termed the midfrequency gain. The low and high frequency at which gain falls by 3 db are
designated
and
respectively. This range is normally considered the useful range of
operating frequency for the amplifier and
and
is termed the amplifier bandwidth.
are sometimes called half power or 3db points because the power output is —
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 13
3db from its normal level when
is half of
. When, amplifier output is expressed as
a voltage on the graph of frequency response, 3db points occur when
.
The fall off in amplifier gain at low frequency is due to the effect of coupling and bypass
capacitors. At medium and high frequencies, capacitive reactance
being equal
to
is very small and, therefore, all coupling and bypass capacitors behave as short
circuits. At low frequencies , capacitive reactance of capacitors X increases and some of
the signal voltage is lost across capacitors. Thus with decrease in frequency, the
reactances of capacitor increase and, therefore, gain of the circuit falls.
All transistors have capacitances between their terminals. There are also stray
capacitances
which are capacitances between wiring and ground. All these
capacitances are very small and therefore at low and medium frequencies their
reactances are very high. With increase in frequency, reactances of the
fall and
when these reactances become enough, they begin to shunt away some of the input
and output currents. With the increase in frequency current gain continues to decrease
until it becomes too small to be useful. The 3db point for the amplifier can occur when
the reactance of the
becomes equal to the
Q. 10. What are different transistor cut off frequencies?
Ans. If no external stray capacitances were present, the device internal capacitances
and transit time of charge carriers across the transistor junction and through the
semiconductor material limit the circuit frequency response. The limitations are
expressed as
(a) Alpha cut-off frequency :
is defined as the frequency at which the
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 14
current gain
falls to
or 0.707 of its low frequency value
corresponding to 3dB
loss. Thus at cut off frequency,
is inversely proportional to square of base width and directly proportional to
minority carrier mobility.
(b) Beta cut off frequency :
decrease to
is the frequency at which the current gain
or 0.707 of its low frequency value. Thus at
Alpha and beta frequencies are related as
(c)
parameter of transistor: It is another high frequency characteristics of
transistor and is defined as short circuit common emitter current gain falls to unity at
this frequency.
where
is low frequency value and
Q. 11 Explain Miller effect.
Ans. Transistor internal capacitances are assumed to be very much smaller than the
external or stray capacitances. These capacitances (internal) are very important and
tend to have their maximum effect at input terminals of the transistors.
When input signal of voltage
collector voltage,
where
is applied to the base of transistor, the change in
is circuit voltage gain. The negative sign is due to phase shift
inherent in a CE amplifier. As seen, collector voltage
base voltage is increased by
is reduced by
, when
. Thus total reduction in collector base voltage,
This also shows the change voltage across
, capacitor between collector and base.
Now charge supplied to the input of the circuit,
Thus looking into the base, the
appears to be
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
is capacitance i.e.
Page 15
amplified by a factor of
This amplification is known as Miller effect.
The total input capacitance
Thus
to the transistor is
in parallel with
.
At high frequencies, the value of
reduces the input impedance
of the circuit and
affects the frequency response.
Miller effect occurs only with a circuit in which there is a phase reversal such as a CE
configuration. It does not occur in CB or CC circuits.
Q. 12. What is gain-bandwidth product?
Ans. For any amplifier, gain-bandwidth product is constant and is equal to
i.e.
frequency at which short circuit common emitter current gain,
=1,
at a frequency 6MHz, then
Thus for given
bandwidth.
falls to unity. If
= 6 MHz i.e. gain-bandwidth product is 6MHz.
of an amplifier, gain can be increased only at the expense of
Q. 13. Describe the difference between
transistor.
and hybrid equivalent of a BJT
Ans. For transistor modeling, hybrid parameter equivalent circuit continues to very
popular but now it shares the spotlight with an equivalent circuit derived directly from
the operating conditions of the transistor i.e.
model. Hybrid equivalent model suffers
from being limited to a particular set of operating conditions if it is to be considered
accurate. The parameters of the other equivalent circuit can be determined for any
region of operation within the active region and are not limited by the single set of
parameters provided. But
model fails to have a parameter defining the output
impedance level of the device and the feedback effect from output to input.
Q. 14.For each model, list the conditions under which it should be applied.
Ans. The obvious advantage of
model is that only
is required to perform an
approximate analysis of an ac configuration. The value of
can be determined from
instrumentation such as curve tracer negating any need for a data sheet. The input
impedance can be determined. Then, following a calculation of
the system and an application of the equation
But
model
fails
to
include
effects
of
from a dc analysis of
for CE configuration.
the
feedback
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
element
and
output
Page 16
impedance
effect of
Once
. So hybrid equivalent is used for analysis. But if conditions are that
and
can be ignored, the basic format of two models is same.
is known one-half the approximate equivalent model is known for each
configuration. If the
model is used,
can be determined directly by dc analysis.
For hybrid model, the average value of the provided range of
can be used although
the range is normally provided for a particular Q-point. Of course a measured value
of
under operating conditions is better choice than the calculated
value.
Q. 15. Explain how coupling capacitor affects the operation of an amplifier.
Draw required diagrams also.
Ans. Effect of Coupling Capacitors:
1. The capacitors
and
are coupling capacitors used for blocking the dc part and
allowing only the ac part of the signal to pass through.
2. Reactance of a capacitor is given by,
3. Thus, the capacitive reactance will increase with, decrease in frequency and it will
decrease with increase in frequency f.
4. Therefore „the coupling capacitor will offer a very low reactance in mind and high
frequency regions and they can be replaced by short circuit, without any problem. So
they do not have any effect at medium and high frequencies.
5 At low frequencies however, the reactance of coupling capacitors will be large. Due to
this, the voltage drop across them increases with reduction in frequency.
6. This increased voltage drop will reduce both the output voltage and gain of the
amplifier in the low frequency region of the frequency response.
Q. 16. Write short note on Emitter Follower.
Ans. It is also a negative current feedback circuit. This circuits exhibits a large input
impedance, a small output impedance, and a voltage gain of approximately unity.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 17
Further the output voltage tends to be in phase with the input voltage-hence the term
“follower”.
At first glance, one might be tempted to dismiss such a circuit as useless because of its
unity voltage gain. However, its input and output impedances promote its use as input
and output buffer stages in amplifier systems.
The circuit of an emitter follower is shown in figure. The main difference between an
emitter follower and conventional amplifier is the absence of collector load and emitter
bypass capacitor. The emitter resistance
itself acts as the load and the ac output
voltage
is taken across it. The biasing is provided either by base resistor method or
potential divider method. The circuit consists of a resistance of the order of
in the
emitter itself and the collector is connected directly to the supply. Stabilization of
operating point is exceptionally good because of high value of emitter resistance
Operation. When input signal
develops an output voltage
.
is applied to the base, the resulting emitter current
equal to
voltage opposes the ac signal voltage
across the emitter resistance
as it is in-phase opposition to
. This
. Thus it
provides negative current feedback. Moreover, this voltage (
) feedback to the input
is proportional to the emitter current hence this circuit is called a negative current
feedback circuit.
It may be noted that when the input signal voltage
goes through its positive half
cycle the output voltage
is also seen to go through its positive half cycle. Hence
the output voltage is in phase with the input signal voltage i.e. the output voltage
(emitter voltage) just follows the input voltage and hence the name emitter follower.
Advantages
1. High input impedance and low output impedance, so it can be used for impedance
matching. Sometimes it is called impedance transformer.
2. Because of 100 percent feedback, output is distortionless and bandwidth is very
large.
3. Relatively high current gain and power gain.
4. Output and input ac voltages are in phase and also approximately equal in magnitude
i.e. emitter output voltage closely follows the input and so it is called the emitter
follower.
Applications
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 18
Because of high input impedance and low output impedance an emitter follower is
capable of transferring maximum power from the high impedance source to low
impedance load. When an emitter is employed for this purpose, it is called a buffer
amplifier. Though impedance matching can be achieved by employing a step-down
transformer but emitter follower is preferred. It is because emitter follower is not more
convenient than a transformer but it also provides better frequency response.
There are many instances (especially in digital electronics) where an increase in current
is required but no increase in voltage is required. For such situations emitter follower,
because of its high current gain, can be employed.
Q. 17. What do you understand by class A, B and C power amplifiers.
Ans. Transistor power amplifiers handle large signals. Many of them are driven so hard
by the input large signal that collector current is either cut-off or is in saturation region
during a large portion of the input cycle. So such amplifiers are generally classified
according to their mode of operation. This classification is based on the amount of
transistor bias and amplitude of the input signal. It takes into account the portion of the
cycle for which the transistor conducts. They are classified as below:
1. Class A Power Amplifiers. In this case, transistor is so biased that the output
current flows for the entire cycle of the input signal. Thus the operating point is so
selected that the transistor operates only over the linear region of its load line. So such
an amplifier can amplify input signal of small amplitude. As the transistor operates over
the linear portion of load line, the output waveform is exactly similar to output
waveform. So class A amplifiers are characterised by a high fidelity of the output. Such
amplifiers are used where freedom from distortion is prime aim. Operation is restricted
only over a small central region of the load line so such amplifiers can be used for
amplifying signals of small amplitude. Also ac power output per transistor is small. The
maximum possible overall efficiency with resistive load is 25%. The maximum possible
collector efficiency resistive load is 50%. In case an output transformer is used, both of
these efficiencies are 50%.
2.Class B Power Amplifiers. In this case, the transistor bias and signal amplitude are
such that out-put current flows only during positive half cycle of the input signal. At zero
signal, the collector current is zero and no biasing system is required in class B
amplifiers. The operating point is selected at collector cut-off voltage, Because of total
absence of negative half cycle from the output the signal distortion is high. Zero signal
input represents the best condition for class B amplifiers because of zero collector
current. The transistor dissipates more power with increase in signal strength. In
comparison to class A amplifiers average current is less, power dissipation is less. So
overall efficiency is increased. The theoretical efficiency in class B operation is about
78.5% while it is only 50% in class A operation.
3. Class C Power Amplifiers. A class C power amplifiers is biased for operation for
less than 180° of the input signal cycle and will operate only with a tuned or resonant
circuit which provides a full cycle of operation for the tuned or resonant frequency. Such
power amplifiers are, therefore, employed in special areas of tuned circuits, such as
radio or communications.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 19
Q. 18. Draw the block diagram of a multistage amplifier having n-stages. Write
expression for its gain (A).
Ans. A multi-stage amplifier can be represented by a block diagram, as shown in
Figure. It is to be noted that the output of the first stage makes the input for the second
stage, the output of second stage makes the input for third stage and so on. The signal
voltage
is applied to the input of the first stage and the final output
at the output terminals of the last stage.
is available
Input to the first stage,
= Signal voltage
Output of first stage or input to the second stage
is the voltage gain of first stage
Output of second stage or input to the third stage
is the voltage gain of second stage
Similarly the output of nth stage (or final output)
is the voltage gain of the last
stage.
Overall gain of the amplifier is given as:
(visualising the multi-stage amplifier as a single amplifier with input
voltage
and output voltage
)
i.e. the gain of a multi-stage amplifier is equal to the product of gains of individual
stages. It is worthwhile to mention here that in practice total gain A is less
than
due to the loading effects of the following stages.
When the gains are expressed in db, the overall gain of a multi-Stage amplifier is given
as the sum of gains of individual stages in decibels (db).
Taking logarithm (to the base 10) of equation (1) and then multiplying each term by 20,
we have
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 20
In the above equation, the term to the left is the overall gain of the multi-Stage
amplifier expressed in decibels. The terms on the right denote the gains of the individual
stages expressed in decibels. Thus,
Q. 19. Draw the circuit diagram of a push-pull amplifier. Explain its operation.
Discuss advantages and disadvantages.
Ans. Push-Pull Amplifiers. The push-pull amplifier and is frequently used in the
output stages of electronic circuits. It is employed whenever high output at high
efficiency is required. The audio power amplifiers used in transistor receivers, taperecorder, record-player, public address systems etc. make use of such circuits. These
systems are usually operated by batteries or cells and, therefore, amplifier efficiency is
of prime importance.
1. Basic Principle of Working of Push-Pull Amplifier. The basic principle on which
push-pull amplifier operates is that the input signal is converted before amplification1
into two separate signals which are identical except for a 180° phase difference. Each of
these signals is applied as the input to one of the transistors are 180° out of phase, so
the output of the transistors. The output transformer is connected in the collector
circuits of the transistors in such a way that the collector currents of the two transistors
combine to provide an overall signal having the same waveform as the original input
signal. The magnitude of this combined output signal is larger than the input signal.
Push-pull amplifiers can be operated in class A, class AB or class B mode.
Class A push-pull amplifier is discussed below.
2. Circuit Arrangement. The circuitry of a typical push-pull amplifier is shown in
Figure. As already mentioned, push-pull amplifier uses two identical transistors,
say
. The emitter terminals of the two transistors
connected
together. The input signal is applied to the inputs of two transistors through centreJ Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 21
tapped step-up transformer
which provides opposite polarity signals to the two
transistors. The collector terminals of both the transistors are connected to end
terminals of centre-tapped primary of output transformer
The power supply
is
connected between the emitter terminals and the centre-tap of primary of output
transformer. Resistors
load
and
are used to provide the proper bias for the circuit. The
is connected across the secondary of the output transformer
. The turn-
ratio
of the output transformer is chosen so as to match the load with the
output impedance of the amplifier and therefore, transfer maximum power. The
quiscent currents of the two transistors, which are equal in magnitude, flow in opposite
directions through each half of primary of the output transformer
so no saturation of
the magnetic core occurs.
3. Circuit Operation. When the base current of one transistor is being driven positive
with respect to the quiescent point Q, the collector current increases, thus causing a
decrease in collector potential relative to ground. At the same time, however, a reverse
action takes place in the base circuit of the second transistor, i.e. base current
decreases causing a drop in the collector current with a consequent rise in collector
potential w.r.t. ground. This means that the. ac current flowing through the transformer
primary winding is in the same direction. As
increases (i.e. pulls), the current
decreases (i.e. pushes). Hence the name push-pull amplifier. The overall operation
results in ac voltage induced in the secondary of the output transformer and thus ac
power is delivered to the load. The difference of two collector currents is illustrated in
Fig.
.
4. Distortion. In view of the fact that the load current in the secondary of the outputtransformer is proportional to the difference of collector currents. It follows that the
harmonic content will be less than it is in either of the collector currents because of
cancellation effects. Thus the push-pull arrangement yields much less distortion in the
output. This is illustrated below.
The base currents
of transistors
respectively are expressed as:
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 22
Their collector currents are expressed as:
The output voltage induced in the output transformer secondary, being proportional
to
From the above equation for output voltage
it is obvious that all even harmonics
are eliminated. This conclusion was reached on the assumption that the two transistors
are identical. If their characteristics differ appreciably, the appearance of even
harmonics is not ruled out.
5. Advantages. 1. Because of absence of even harmonics in the output of the pushpull amplifier, such a circuit gives more output per active device for a given amount of
distortion or less distortion for a given power output per transistor.
2. As already mentioned earlier, the decomponents of the collector current oppose each
other magnetically in the transformer core. This eliminates any tendency toward core
saturation and consequent non-linear distortion that may arise from the curvature of the
transformer magnetization curve.
3. Another advantage of this system is that the effects of ripple voltage that may be
contained in the power supply because of inadequate filtering will be balanced out. This
cancellation results because the currents produced by the ripple voltage are in opposite
directions in the transformer winding, and so will not appear in the load. Of course, the
power supply hum will also act on the voltage-amplifier stages, and so will be part of
the input to the power stage. This hum will not be eliminated by the push-pull circuit.
6. Disadvantages. The drawbacks of push-pull amplifiers are : (i) requirement of two
identical transistors (ii) need of use of driver stage to furnish two equal and opposite
voltages at the input and (iii) need of bulky and expensive transformer.
Q. 20. What do you mean by coupling of two amplifier stages. Explain with
requisite circuit diagrams the resistance-capacitance coupling scheme.
Ans. In a multistage amplifier, the output of first stage is combined to the next stage
through a coupling device. The process is known as coupling. The coupling device is
used to:
(i) transfer the ac output of one stage to the input of next stage.
(ii) Block the dc to pass from one stage to the next stage i.e. to isolate the dc
conditions.
R-C (Resistance-Capacitance) Coupled Transistor Amplifier
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 23
A two-stage R-C coupled amplifier using N-P-N transistors in CE configuration is shown
in Figure. The two transistors used are identical and use a common power supply
The resistors
form the biasing and stabilization network. In this
arrangement, the signal developed across collector resistor
of the first stage is
coupled to the base of the second stage through the coupling-capacitor
. As the
coupling from one stage to the next is obtained by a coupling capacitor followed by a
connection to a shunt resistor, therefore, such amplifiers are called resistancecapacitance coupled or R-C coupled amplifiers. The input capacitor
voltage to the base of transistor
parallel with the resistor
function of
input circuit.
. In the absence of
couples ac signal
the signal source will be in
and the bias voltage of the base will be affected. Thus the
is to allow only the alternating current from signal source to flow into the
The emitter-bypass capacitor
, offers low reactance path to the signal. If it is not
present, then the voltage drop across
will reduce the effective voltage available
across the base-emitter terminals (the input voltage) and thus reduces the gain.
The coupling capacitor
transmits ac signal but blocks the dc voltage of the first
stage from reaching the base of the second stage. Thus the dc biasing of the next stage
in not interfered with. For this reason, the coupling capacitor
is also called the
blocking capacitor.
1. Operation. When ac signal is applied to the base of the first amplifier, it appears in
the amplified form across collector load
. The amplified signal developed across
is
transmitted to the base of next stage of the amplifier through coupling capacitor
This is further amplified by the next stage and so on. Thus the cascade stages amplify
the signal and thus the overall gain is considerably increased. The phase of output is the
same as that of input because the phase is reversed twice by two transistors as they are
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 24
in CE configuration.
It may be mentioned here that the overall gain is less than the product of the gains of
individual stages. This is because when a second stage is made to follow the first one,
the effective load resistance of the first stage is reduced because of the shunting effect
of the input resistance of the second stage. This reduces the gain of the stage which is
loaded by the next stage. For example, in a 4-stage amplifier the gains of first three
stages will be reduced due to loading effect of the subsequent stage. However, the gain
of final or last stage (.4th stage in this case) which has no loading effect of subsequent
stage, remains unaffected. Thus the overall gain shall be less than the product of the
gains of four stages.
2. Analysis. For drawing approximate model of the circuit shown in Fig. following
assumptions are made.
1.
is so small that the voltage sources
2.
is so large that it can be considered as an open-circuit.
3. The bias resistors
can be neglected.
are very large in comparison to
4. The reactance of emitter-by pass capacitor
small that parallel combination of
and
.
for any given input frequency is so
can be considered as a short-circuit.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 25
Q. 21. Draw the block diagram of negative feed back amplifier. Derive an
expression for the voltage gains of an amplifier of gain (A) when
subjected to negative feedback(B).
Ans. When the feedback voltage (or current) is so applied as to weaken the input
signal (i.e. when the feedback is inphase opposition to the input signal), it is called
negative, inverse or degenerative feedback. Negative feedback reduces the
amplifier gain but it has numerous advantages (such as gain stability, reduction in
non-linear distortion, reduction in noise, increase in bandwidth or improvement in
frequency response, increase in input impedance and decrease in output
impedance) and is frequently used in amplifier circuits. The reduction in gain due to
negative feedback, can always be compensated by increasing the number of
stages.
Principle of Feedback in Amplifier For an ordinary amplifier (i.e. one without
feedback), the voltage gain equals the ratio of output voltage and input voltage, as
illustrated in Fig. 2.
.e. Voltage gain,
The gain is called the open-loop gain.
A block illustrating the principle of feedback in an amplifier is given in fig. 1. Let a
fraction
of the output voltage
the open-loop gain
be supplied back to the input and A be
Now the input voltage becomes
in case of positive feedback
…(1)
in case of negative feedback
…(2)
depending on whether the feedback voltage is in phase with or in
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 26
phase opposition to the input signal voltage.
Consider a negative feedback case. The actual input voltage to the amplifier is the
signal voltage
less feedback voltage
i.e. Actual input voltage to amplifier,
The output voltage
must be equal to the input voltage
by amplifier gain A. So,
But
is called the voltage gain of the amplifier with feedback,
referred to as closed-loop gain.
multiplied
. This is also
Thus voltage gain with negative feedback,
Similarly voltage gain with positive feedback,
The term
ratio and
A is called the feedback factor whereas
is known as the feedback
is known as loop gain.
In general both A and
are phasor quantities (having magnitude as well as
phase). Every amplifier stage introduces a phase shift of 180° (assuming CE
configuration, as usual). Thus for a 3-stage amplifier, total phase shift between the
output and input will be 3 x 180° = 540°or 180°. is usually arranged to have a
phase value of 00 or 180° (i.e. either inphase with input voltage or in phase
opposition to it).
Since
is a complex quantity, it can be observed that
1. If
is less than unity for equation (5), then
exceeds A. This condition
corresponds to positive feedback because voltage feedback adds to input signal
voltage and increases input voltage
. Positive feedback, though, increases the
gain but it reduces the stability and increases the distortion and so it is usually
avoided.
2. If
is equal to zero then the gain
becomes infinite. This is only
possible when input is zero. Thus the amplifier is then capable of giving output
voltage even with zero signal. Under such situation the circuit operates as an
oscillator.
3. If
is greater than unity then
smaller than A. This means the feedback
voltage
becomes smaller than input signal voltage
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
. This corresponds to
Page 27
negative feedback in an amplifier. Though negative feedback reduces the gain of
the amplifier but improves its performance in several aspects, given in the
succeeding Articles. It may be noted that negative voltage feedback does not affect
the current of the circuit.
Note: The assumptions made in investigating the effect of negative feedback on
the amplifier characteristics are:
1. Basic amplifier is unilateral from input to the output (this is not valid if
for a CE transistor amplifier).
2. Passive feedback network is unilateral and transmits a signal from the output to
the input but not in the opposite direction.
3. Feedback network presents no loading on the output of the basic amplifier.
4. There is no forward transmission through the
feedback network.
Q. 22. Write short note on Distortion in amplifier circuits.
Ans. Under ideal conditions, amplified signal must have exactly the samewaveform as the input signal. But in practical amplifiers, this ideal condition is
never achieved.
Some changes in the wave-form may take place in addition to the increase in the
amplitude. Such a change is called the distortion and is undesirable because it may
change the intelligence (useful information carried by the signal, as illustrated in
figure 1.
There are three basic types of distortion in amplifiers viz amplitude (or non-linear)
distortion, frequency distortion and phase distortion.
1. Amplitude or Non-Linear Distortion. For a small swing of input signal, only a
small part of the trans-conductance curve, as illustrated in figure 2, is used. This
small part is almost linear and operation over this region of the curve is called the
linear operation because the changes in the output current are proportional to the
changes in input voltage. In such a case, the shape of the amplified waveform is
the same as the shape of the input waveform and therefore no distortion occurs.
But for large input signal, the operation becomes non-linear.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 28
Because of non-linearity of the trans-conductance curve, the resulting collector
current
is no longer truly sinusoidal. The upper half of the wave is elongated.
Since this output current flows through a load resistance, the output voltage will
also have non-linear distortion. Such a distortion produces new frequency
components in the output which are not present in the input signal. These
frequencies are usually harmonics of the input frequency. Hence this distortion is
sometimes referred to as the harmonic distortion.
2. Frequency Distortion. Frequency distortion is caused in an amplifier when
signal components of different frequencies are amplified differently. It may occur
even with - small signal operation. Fig. 3 illustrates this type of distortion. The
input signal spectrum
contains many equal amplitude sinusoidal components. Now if the cut off frequency
is less than the highest sinusoidal frequency, the higher frequencies in the output
spectrum are attenuated (cut off). Frequency distortion may be caused either by
the electrode capacitances of the amplifying device or by the reactance elements
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 29
associated with the circuit. In a practical case, frequency distortion is always
present and the gain is constant only in the middle frequency range but falls off for
low and high frequencies. The quality of speech or music signals is affected by such
distortion.
3. Phase Distortion. Such a distortion is said to be present when signals of
different frequencies suffer unequal phase shift, as illustrated in fig. 4. Because of
the phase distortion, the third harmonic component changes its phase with respect
to the fundamental at the output. Phase distortion is caused by the electronic
component‟s used in the amplifier circuit.
The amount and kinds of distortion that can/cannot be tolerated depend on the
particular application of the amplifier. An amplifier that is entirely satisfactory for
one application might not be so for another because of the type of distortion
introduced by it.
Q.23. Write short note on Effect of —ve feedback on output resistance of
an amplifier.
Ans. (c) Effect of negative feedback on output impedance. Just an high input
impedance is advantageous to an amplifier, so is low output impedance. With lower
output impedance, the amplifier is better suited to drive a low impedance load.
Such a desirable characteristic can be had by employing negative feedback. The
effect of negative feedback on the output impedance of an amplifier is explained
below.
The voltage-series feedback circuit given in Figure provides sufficient circuit detail
for determining output impedance with feedback. The input terminals are shortcircuited so that
is now the only input voltage to the amplifier. Now a voltage
sources
is applied at the output terminals so that
the applied source.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
current is drawn from
Page 30
Thus series voltage negative feedback reduces the output impedance of an
amplifier by a factor
reduced.
This is the same factor by which voltage gain is
Q. 24. Explain the Miller theorem and what is its importance?
Ans. Consider an arbitrary circuit configuration with N distinct nodes 1, 2, 3,……..N.
Let node voltages be
……..
where
since N is the reference on
ground, node. Node I and 2 are interconnected by impedance Z‟. Let
Designate the ratio
known.
by K, which in sinusoidal state will be complex number
and will be a function of laplace transform variable s. Current
drawn from
through Z‟ can be obtained by disconnecting terminal 1 from z‟ and by bridging an
impedance z‟/(l-k) from
Current
to ground.
is given by,
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 31
were
shunted
across
terminals
,
the
current
drawn
from
would be the same as that of original circuit. This theorem will be useful
in making calculations if K is known.
Q. 25. Explain the role of coupling capacitor in an amplifier circuit?
Ans. For coupling of one stage of amplifier to the next stage a capacitor
called
coupling or blocking capacitor is used. Because of it, the output across the load
resistance
is free from the collector dc voltage.
Q. 26. Write the characteristics in ideal voltage amplifier.
Ans. Main characteristics of voltage amplifiers is to present a high resistance to the
input transducer to minimize the loading effects and to provide a large enough
voltage signal to the large signal amplifier stages to operate such output devices
like loudspeaker, sservo-meteretc.
Q. 27. Define bandwidth and gain of an amplifier?
Ans. The frequency response of an amplifier signifies different range of
frequencies. The low frequency and high frequency at which gain falls at 3db are
designated as
and
. The frequency difference
is termed as amplifier
bandwidth.
An amplifier amplifies small signal into large signals at output The change on
amplification can be in voltage, current or power. So output change with respect to
input change is calculated as ratio and termed as gain of amplifier.
Q. 28. What will happen to output ac signal if the dc level is insufficient in
an amplifier? Sketch the effect of wave form.
Ans. As mentioned the dc bias is insufficient or the amplifier is insufficiently
biased. That means that the amplifier is in cutoff. In such a situation of biasing, if
an ac input
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 32
signal is applied the output would be only switching wave form, since as the input
rises above the threshold of cutoff, the output will switch to low voltage, otherwise
the output will remain in high voltage. Hence the output voltage would be in out of
phase by 180° w.r.t input but would be switching between high and low voltage
i.e.
and GND.
Q. 29. What do you mean by Gain-Bandwidth product?
Ans. For any amplifier, when the bandwidth is multiplied by gain at mid
frequencies, it is known as gain-bandwidth product. For any amplifier, gain
bandwidth product is constant. For example if
gain band-width product is 6 MHz.
=1 at a frequency of 6 MHz, then
Q. 30. In the circuit given in figure,
and input signal is of 2mV (rms.)
Determine input resistance, output resistance, current gain, voltage gain
and power gain
(i) taking
into account
(ii) neglecting
.
What would be rum value of signal voltage across the load? Assume a =
0.98.
Ans.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 33
Q. 31. A transistor amplifier is shown. The parameters of the transistor
are
=
,
and the output load resistor dissipates a
signal power of 10mW. Determine the power gain of the stage and the
input signal emf E. .The power of 10mW. Determine the power gain of the
stage and the input signal emf E. The reactances of the capacitors may be
neglected. Draw the equivalent circuit of the amplifier.
Ans.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 34
The ac equivalent circuit of the amplifier is:
Q. 32. Explain Miller theorem. Explain emitter follower using this.
Ans. Consider an arbitrary circuit configuration with N distinct nodes, 1,2,3……..N,
as shown in diagram.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 35
Let node voltages be
where
= 0 since N is the reference ,on
ground, node. Node I and 2 ar interconnected with an impedance Z‟. Assumed
that
is known = K, which, in the sinusoidal steady state will be complex
number and will be a function of Laplace transform function variable S. Now
current
drawn from
through Z‟ can be obtained by disconnecting terminal 1
from z‟ and by bridging an impedance z‟/(I-k) from
The current
to ground.
is given by,
Therefore is Z1 = Z‟/(1 — k) were shunted across terminals
, the current
drawn from
would be same as that from the original circuit. So
same in both circuits.
Now
drawn from
between
remains
may be calculated by removing Z‟ and by connecting
and ground an impedance
is given by
Since identical node equations (kCL) are obtained from the configurations of both
figures, these two networks are equivalent. It is useful to use this theorem only if it
is possible to find value of K by some independent means.
Q. 33. For the amplifier shown in figure, calculate
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 36
Ans. Miller‟s theorem is applied at 200k resistance R‟ so that the following figure is
obtained, where
is the voltage gain from base to collector. Assuming this
gain is much larger than unity
The effective load resistance is
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 37
Assumed that
Q. 34. In a small signal amplifier shown below,
are negligible.
(a) Draw the h parameter equivalent circuit for the amplifier. Calculate the
input and output impedance and the voltage gain.
(b) Give the DC load line of the circuit and find Q point.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 38
Ans.
is
so
negligible,
so
is close to
infinity,
hence open
is negligible,
is short
(b)
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 39
Q. 35. Given
(a) Common-emitter hybrid equivalent circuit
(b) Common base
Determine
model.
Ans.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 40
(b) Common base
model.
Q. 36. In a CE transistor amplifier when signal changes by 0.02 V, the base
current changes by
and collector current by 1 mA. If the collector
resistance
and
(i) Voltage gain
(ii) Current gain
(iii) Input impedance
(iv) AC load resistance and
(v) Power gain.
. Find;
Ans.
(i) Voltage gain,
(ii) Current gain
(iii) Input impedance,
(iv) AC load resistance
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 41
(v) Power gain,
APPENDIX
Q. 1. Draw the practical circuit of complementary symmetry push pull
amplifier and explain its working?
Ans.
This arrangement uses two transistors having complementary symmetry (one npn
and another pnp). That is why it is called complementary. They have symmetry as
they are made with the same material and technology and are of maximum rating.
In the circuit, resistors
provide the voltage divider bias to forward bias
the emitter base junction of transistor
and similarly
provide the
voltage divider bias for emitter junction of transistor
. The resistors are so
selected that under zero signal condition, the operating point is cut-off and so no
collector current flows.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 42
Signal applied at input goes to the base of both
: Both conduct in opposite
half cycles of input signal. This results in a half cycle of output voltage across the
load resistor
due to
and then
is forward biased and conducts and
develops second half cycle.
Since collector current from each transistor flows through the load during alternate
half cycles of input signal, no centre tapped output transformer is required.
One obvious disadvantage is the requirement of two supplies and difficulty of
obtaining matched complementary transistors.
Q. 2. Draw two stages of RC coupled, direct-coupled and transformer
coupled amplifiers and make a chart showing the comparison of three
coupling schemes?
Ans. Two stages RC coupled Transistor amplifier.
Two stage transformer coupled transistor amplifier.
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 43
Two stage direct coupled transistor amplifier.
Comparison of different types of Couplings:
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 44
Q. 3. (a) Draw the circuit of wein bridge oscillator using op amps. Explain
its working principle.
requirement?
What
is its frequency of oscillations and
(b) A certain colpitt oscillator uses a tank circuit with L = 2OmH,
pF,
= 200
= 300 pF. Calculate the frequency of oscillations?
Ans. (a)
An oscillator circuit in which a balanced bridge is used as the feedback network is
wein bridge oscillator. Active element is operational amplifier which has a very
large positive voltage gain
(infinite) input resistance.
, negligible output resistance, and very high
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 45
Two auxiliary voltage
such that
have the same phase angle at the frequency,
And
at this frequency
If
a
null
is
desired,
then
must
be chosen
so
that
(b)
Q. 4. (a) Draw the block diagram of regulated power supply and explain
the role of each block.
(b) Draw the circuit of a class A direct coupled power amplifier. Show that
the dissipation in the transistor is maximum under quiescent conditions.
Obtain the expression for maximum efficiency of the circuit.
Ans. (a)
Figure shows the complete circuit of a regulated power supply with a transistor
serves regulator as a regulating device. The ac voltage is connected to a
transformer which transforms the voltage level for the desired dc output. A bridge
rectifier then provides a full wave rectified voltage that is initially filtered by a
or
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 46
CLC filter to produce a dc voltage. Resulting dc voltage usually has some ripple or
ac voltage variation. A regulating circuit use this dc input to provide a dc voltage
that not only has much less ripple voltage but also remains constant even if the
input dc voltage varies some what on the load connected to the output dc voltage
changes.
(b)
A schematic circuit of a series fed class. A power amplifier using resistive load
.
The term series fed is because from the fact that the load
is connected in series
with the transistor output. Signals handled by this circuit is in the range of volts
and the transistor used in power transistor capable of operating in the range of few
watts. It is used very less because of its poor efficiency of collector. The output
characteristics with operating point Q are important
represents no
signal collector current and collector emitter voltage respectively. When ac signal is
applied, then Q point shifts up and down causing output current and voltage to
vary about it. The output current increases to
the collector emitter voltage increases to
and falls to
Similarly,
and falls to
Input power from collector supply voltage,
The power drawn from the collector supply is used in the following two components
(i) Power dissipated in collector load as heat
(ii) Power supplied to the transistor
Power supplied to the transistor,
(dc) is subdivided into ac power developed
across the load resistor constituting ac power output and is given
as
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 47
With zero signal applied at the input of class A power amplifier, ac power
developed across the load reduces to zero and therefore all the power fed to
transistor is wasted in the form of heat. Thus, a transistor dissipates maximum
power under zero signal condition.
Q. 5. Show that efficiency of class B push pull amplifier is 78.5%?
Ans. The power provided to the load
coupled to the amplifier is drawn from the
dc power supply
and is considered an input dc power. The input dc power is
given as,
is the average or direct current taken from the dc
supply
In class B operation the current drawn from a single power supply is a fuel wave
rectified signal, thus
Total collector power dissipation for the two transistors,
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 48
Hence Proved.
Q. 6. Draw the hybrid parameter equivalent of high frequency analysis of a
transistor.
Ans. For high frequency analysis, a transistor is replaced by hybrid
shown.
is base spreading resistance
model as
is capacitance connected between B‟ and
E.
is there for feedback effect between output and input is taken into account
by connecting it between B‟ and C.
is the current generator.
Q. 7. Write the different types of power amplifiers?
Ans. Power amplifiers are primarily audio and radio power amplifiers.
Audio power amplifiers are small signal amplifiers that raise power levels of signals
that have audio frequency range (20HZ - 20 kHZ).
J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 49
Radio power amplifiers, large signal amplifiers, raise the power level of signals that
have radio frequency range.
Classification according to mode of operation
(i) Class A power amplifier —> The transistor is so biased that the output current
flows for the entire cycle of input signal.
(ii) Class B —> The transistor bias and signal amplitude are such that current flows
only for positive half cycle.
(iii) Class AB —> The output signal swings between 180° and 360°
(iv) Class C power amplifier —> It is biased for less than 180°
(v) Class D power amplifier —> It is on for very short intervals.

J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 50

J Srinivasa Rao –Small Signal Low frequency Transistor Amplifiers
Page 51