YCWM Warananagar

1. Department of Physics
B.SC.III
Sem-V.Paper-XII
DSE-E4- Digital and Analog Circuits and Instrumentation
Topic-Transistor Amplifier

2. Transistor Amplifier and
Sinusoidal Oscillator
• What is Transistor: Is the semiconductor
device which transfer the resistor from
input to output circuit.(Base –Emitter is
I/P)and (Base – Collector O/P).
• Amplifier-The electrical process in which is
used to increase or magnify power of ac
signal(or amplitude) without changing
frequency or nature of waveforms is called
as amplification. The circuits used to
perform this function is called amplifiers.
(amplifier used in radio receivers, T.V.
receivers, tape recorders and measuring
instruments, Speakers, signal amplifier)
Device which produces larger output with
similar characteristics, like input parameters
like Waveform, Frequency, shape is also
called amplifier.
In amplifier its output is directly proportional
to or dependent on corresponding input
signal is called as linear amplifier.
When output is not directly proportional to
or independent on corresponding input
signal is called as non-linear amplifier.
Classification of amplifiers-
1.Based on inputs-Small signal, Large signal
Amplifiers 2)Based on inputs- Voltage and
power amplifier. 3)Based on configurations-
CB, CE, CC amplifiers. 4)Based on frequency
Responses-AF,IF,RF. 5)Based on biasing-Class-
A,B.AB. and class C 6) Number of stages-
Single and multistage amplifiers. 7)Based on
coupling used –DC(Direct),R-C, Transformer
coupling.

3. Transistors and biasing-
• A transistor consists of two pn junctions formed by
sandwiching either p-type or n-type
semiconductor between a pair of opposite types.
There are two types of transistors.
• (i) n-p-n transistor : An n-p-n transistor is
composed of two n-type semiconductors
separated by a thin section of p-type as shown in
Fig. 2.1 (i).
• (ii) p-n-p transistor : A p-n-p transistor is formed
by two p-section separated by a thin section of n-
type as shown in Fig. 2.1 (ii).
• Transistor terminals : A transistor has three sections of doped
semiconductors. (i)Emitter : The section on one side is called
emitter. The emitter is always forward biased w.r.t. base, so it
supplies large number of majority carriers. Fig. 2.2 (i) shows
the emitter (p-type) of pnp transistor. Emitter of pnp
transistor is forward biased and supplies note charges to its
junction with the base.

4. Transistor Biasing
• (ii) Collector : The section on other side which
collects the charges is called the collector. The
collector is always reverse biased. Fig. 2.2 (i),
the collector (p-type) of pnp transistor has a
reverse bias and receives hole charges that flow
in the output circuit. Fig. 2.2 (ii) shows the
collector (n-type) of npn transistor, has reverse
bias and receives electrons.
• (iii) Base : The middle section which forms two
pn-j unctions between the emitter and collector
is called base. The base-emitter junction is
forward biased, which allows low resistances for
the emitter circuit. The base-collector junction
is reverse biased and provides high resistance in
the collector circuit.

5. Faithful Amplifier:-
• Introduction:- The basic function of transistor is
to do amplification. The weak signal is given to
the base of the transistor and amplified output
is obtained in the collector circuit. One
important requirement during amplification is
that only the magnitude of the signal should
increased there should be no change in signal
shape. This increase in magnitude of the signal
without any change in shape is known as
faithful amplification. In order to achieve this,
means are provided to ensure that input circuit
(i.e. base-emitter junction) of the transistor
remains forward biased and output circuit (i.e.
collector-base junction) always remains
reversed biased during all parts of the signal.
This is known as transistor biasing. Here we
shall discuss how transistor biasing helps in
achieving faithful amplification.
• Faithful Amplification :The process (if raising the strength of a weak
signal without any change in its general shape is known as faithful
amplification. The theory of transistor reveals that it will function
properly if its input circuit (i.e. base- emitter junction) remains
forward biased and output circuit (i.e, collector-base junction) reverse
biased at all times. This is then the key factor for achieving faithful
amplification. To ensure this, the following basic conditions must be
satisfied (i) Proper zero signal collector current. (ii) Minimum proper
base-emitter voltage at any instant. (iii) Minimum proper collector-
emitter voltage After a transistor has been biased in the active region,
it can work as an amplifier. Apply a small ac signal to the base. This
produces fluctuation in the collector current. But there is no change in
the shape and frequency of the signal. When the input signal is so
weak that is produces small fluctuations in the collector current
compared to the quiescent value, the amplifier is called as small-signal
amplifier or a linear amplifier. Small signal amplifier is used as the
first stage of the amplifier used in radio receivers, T.V. receivers, tape
recorders and measuring instruments.

6. Necessity of Amplifier:
1)The properly biased transistor raises the
strength of a weak signal and thus acts as an
amplifier.2) Almost all electronic equipment’s
must includes means for amplifying electrical
signals. For instance, radio receivers amplify very
weak signals , sometimes a few millionth of a volt
at antenna—until they are strong enough to fill a
room with sound. 3)The transducers used in the
medical and scientific investigations generate
signals in microvolt (µV) and millivolt (mV)
range. These signals must be amplified thousands
and million times before they will be strong
enough to operate indicating instruments.
Therefore, electronic amplifiers are a constant
and important ingredient of electronic systems.
4) Our purpose here will be to discuss single
stage transistor amplifier. By a stage we mean a
single transistor with its bias and auxiliary
equipment. It may be emphasized here that a
practical amplifier is always a multistage
amplifier i.e. it has a number of stages of
amplification (or transistor).
5)Practical Circuit of Transistor Amplifier It is
important to note that a transistor can
accomplish faithful amplification only if proper
associated circuitry is used with it. Fig. 2.3
shows a practical single stage transistor
amplifier. The various circuit elements and their
functions are described as fallows.

7. • (i)Biasing circuit:- The resistances R1, R2 and RE form the biasing
and stabilization circuit. The biasing circuit must establish a proper
operating point otherwise a part of the negative half cycle of the
signal may be cut off in the output.
• (ii) Input capacitor Cin:-An electrolytic capacitor Cin ( ≅10 μf) is
used to couple signal to the base of the transistor. If it is not used,
the signal source resistance will come across R2 and thus change
the bias. The capacitor Cin allows only a.c. signal to flow but
isolates the signal source from R2. It may be noted that a capacitor
offers infinite reactance to d.c. and blocks it completely whereas it
a. c. to pass through it.
• (iii) Emitter bypass capacitor CE:- An emitter bypass capacitor
CE(≅100 μf) is used in parallel with RE to provide a low reactance
path to the amplified a.c. signal. If it is not used then amplified
a.c.-signal flowing through RE will cause a voltage drop across it,
thereby reducing , the output voltage.
• (iv) Coupling Capacitor Cc:-The coupling capacitor Cc (.= 10 μf).
couples one stage of amplification to the next stage. If it is not
used, the bias conditions of the next stage will be drastically
changed
• due to the shunting effect of Rc . This is because
Rc will come in parallel with the upper resistance
R1 of the biasing network of the next stage,
thereby altering the biasing conditions of the
latter. In short, the coupling capacitor Cc isolates
the d.c. of one stage from the next stage, but
allows the passage of a.c. signal.

8. It is useful to keep in mind that
𝒊𝑩 = 𝑰𝑩 + 𝒊𝒃, 𝒊𝑪 = 𝑰𝑪 + 𝒊𝒄, 𝒊𝑬 = 𝑰𝑬 + 𝒊𝒆as the transistor
can prevailboth the conditions for ac and dc.

9. Phase Reversal In Common Emitter Amplifier
Mode
• In common emitter amplifier mode connection,
when the input signal voltage increases in the
positive sense the output voltage increases in the
negative direction and vice-versa. In other
words, there is phase difference of 180° between
the input and output voltage in CE mode
connection. This is called as phase reversal.
The phase difference of 1800 between the signal
voltage and output voltage in a common emitter
amplifier is known as phase reversal.
• The total instantaneous output voltage VCE is
given by: 𝑽𝑪𝑬 = 𝑽𝑪𝑪 – 𝒊𝑪𝑹𝒄 −−−−−−− −(1)
• When the signal voltage increases in the positive
half-cycle, the base current also increases . The
result is that collector current and hence voltage
drop 𝑖𝐶𝑅𝑐 increases. As 𝑉𝐶𝐶 is constant
therefore, output voltage 𝑉𝐶𝐸 decreases
Phase Reversal In Common Emitter Amplifier Mode

10. Phase Reversal In Common Emitter Amplifier Mode
In other words, as the signal voltage is increasing
in the positive half-cycle, the output voltage is
increasing in the negative. sense i.e. output is 180°
out of phase with the input. It follows, therefore,
that in a common emitter amplifier, the positive
half cycle of the signal appears as amplified
negative half-cycle in the output and vice-versa. It
may be noted that amplification is not affected by
this phase reversal.
The fact of phase reversal can be readily proved
mathematically.
𝑉𝐶𝐸 = 𝑉𝐶𝐶 – 𝑖𝐶𝑅𝑐
𝑑𝑉𝐶𝐸 = 𝑑(𝑉𝐶𝐶 – 𝑖𝐶𝑅𝑐)
𝑑𝑉𝐶𝐸 = 𝑑𝑉𝐶𝐶 – 𝑑𝑖𝐶𝑅𝑐
𝑑𝑉𝐶𝐸 = 0 – 𝑑𝑖𝐶𝑅𝑐
𝑑𝑉𝐶𝐸 = – 𝑑𝑖𝐶𝑅𝑐 −−− −(2)
• The negative sign shows that output voltage
is 180° out of phase with the input signal
voltage.
• 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑔𝑎𝑖𝑛 = 𝐴𝑉 =
𝑂𝑢𝑡𝑝𝑢𝑡 𝑎𝑐 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝐼𝑛𝑝𝑢𝑡 𝑎𝑐 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
=
𝑉𝑜
𝑉𝑖
=
𝑅𝐶
𝑟𝑒

11. The output characteristics indicates the relation
between VCE and IC. However, the same information
can be obtained in a much simpler way by representing
IC and VCE graphically.
The relationship between VCE and IC is linear which
can be represented by a straight line on the output
characteristics. This is known as a load line.
The points lying on the load line give the possible
values of VCE and IC in the output circuit. As in a
transistor circuit both d.c. and ac. conditions exist,
therefore, there are two types of load line
DC. load line, A.C. load line
Before to draw the dc and ac load line we have to draw
the equivalent circuits for single stage common emitter
transistor amplifier circuit
:
In a transistor amplifier, both d.c. and a.c. conditions prevail.
The D.C. sources set up DC currents and voltages whereas
the A.C. source (i.e. signal) produces fluctuations in the
transistor currents and voltages.
Therefore, a simple way to analyze the action of a transistor
is to split the analysis into two parts viz,
d.c. analysis and an ac. analysis.
In the D.C. analysis, we consider all the D.C. sources at the
same time and work out the D.C. currents and voltages in the
circuit.
On the other hand, for ac. analysis, we consider all the A.C.
sources at the same time and work out the A.C. currents and
voltages.
By adding the D.C. and A.C. currents and voltages, we get
the total currents and voltages in the circuit. For example,
consider the amplifier circuit shows in Fig. 2.5. This circuit
can be easily analyzed by splitting it into D.C. equivalent
circuit and A.C equivalent circuit.

12. It follows, therefore, that in order to draw the equivalent d.c.
circuit, the following two steps are applied to the transistor
circuit (a) Reduce all ac. sources to zero. (b) Open all the
capacitors. Applying these two steps to the circuit shown in
Fig. 2.5. we get the d.c. equivalent circuit shown in Fig. 2.6.
We can easily calculate the d.c. currents and voltages from
this circuit.
Single Stage Common Emitter Transistor Amplifier
Circuit.
In the D.C. equivalent circuit of a transistor amplifier, only D.C.
conditions are to be considered
i.e. it is presumed that no signal is applied.
As direct current cannot flow through a capacitor, therefore, all
the capacitors look like open circuits in the D.C. equivalent
circuit.
D.C. equivalent
circuit

13. i)Reduce all d.c. sources to zero (i.e. VCC = 0).
ii) Short all the capacitors. applying these two steps to the
circuit shown in Fig. 2.6, we get the a.c. equivalent circuit
shows in Fig. 2.7. We can easily calculate the ac. currents and
voltages from this circuit. It may be seen that total current in
any branch is the sum of d.c. and ac. currents through that
branch. Similarly, the total voltage across any branch is the
sum of d.c. and ac. voltages branch.
Single Stage Common Emitter Transistor Amplifier
Circuit.
In the ac. equivalent circuit of a transistor amplifier, only a.c.
conditions are to be considered. Obviously, the d.c. voltage is
not important for such a circuit and may be considered zero.
The capacitors are generally used to couple or by-pass the a.c.
signal. The designer intentionally selects capacitors that are
large enough to appear as short Fig. 2.7 circuit to the a.c.
signal. It follows, therefore, that in order to draw the a.c.
equivalent circuit, the following two steps are applied to the
transistor circuit
A.C. equivalent circuit

14. It is the line on the output characteristics of a
transistor circuit which gives the values of Ic and
VCE corresponding to zero signal or D.C.
conditions. Consider of CE transistor amplifier
shown in fig.2.6. In the absence of signal, DC.
conditions prevail in the circuit as shown in Fig.
Applying Kirchhoff's voltage law to output loop of fig 2.9
(i) we get, 𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶 – 𝐼𝐸𝑅𝐸
𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶 + 𝑅𝐸 −− −(1)

15. The two end points of the D.C. load
line can also be determined in another way. Consider
equation.
Dividing throughout by VCC
The equation of a line having intercepts a & b on x-axis and
y-axis respectively is given by
Comparing equation 2.3 and 2.4, we have Intercept on
x - axis Intercept on y – axis =
From the above, it is clear that D C. load line gives
complete information about the output circuit of
transistor amplifier in the zero signal conditions.
All the points showing zero signal IC and VCE lie on
the d. c. load line.
Also IC and VCE conditions in the circuit are
represented by the output characteristics.
Therefore, actual operating conditions in the circuit
will be represented by the point where D. C. load
line intersects the base current curve.

16. This is the line on output characteristics of a transistor
circuit, which gives the values of IC and VCE ,
when signal is applied. Consider a. c. equivalent
circuit shown in Fig. 2.10 (i). To add a. c. load line to
the output characteristics, we require two end points :
i.e. (i) Maximum Collector - emitter voltage point,
(ii) Maximum Collector current point. Maximum
Collector - emitter voltage = 𝑉𝐶𝐸 + 𝐼𝐶𝑅𝐴𝐶 .This locates
point C of a. c. load line on the collector-emitter
voltage axis.
Maximum Collector Current = IC +
𝑉𝐶𝐸
𝑅𝐴𝐶
Where,𝑅𝐴𝐶 = 𝑅𝐶‖ 𝑅𝐿 =
𝑅𝐶 𝑅𝐿
𝑅𝐶+ 𝑅𝐿
This locates the point D of a.c. load line on the
collector-current axis. By joining points C & D, the
a.c. load line (CD) is constructed as shown in Fig. 2.10
(ii).

17. The voltage gain of an amplifier varies with
signal frequency. It is because reactance of the
capacitors in the circuit changes with signal
frequency. Hence, it affects the output voltage.
The curve between voltage gain and signal
frequency of an amplifier is known as frequency
response curve.
shows the frequency response of a typical amplifier. The
gain of the amplifier increases as the frequency increases
from zero, till it becomes maximum at 𝑓𝑟.
This maximum frequency at 𝑓𝑟is called resonant
frequency. If the frequency of signal increases
beyond fr, the gain decreases.
The performance of an amplifier depends upon its
frequency response. When amplifier is to be
designed, ensure that gain is uniform over some
specified frequency range. For instance, in case of
audio amplifier, it is necessary that all the
frequencies in the sound spectrum (i.e. 20 Hz to
20 kHz ) should be uniformly amplified,
otherwise speaker will give a distorted sound
output.

18. :The process
of injecting a fraction of output energy of
some device back to the input is known as
feedback. Feedback is useful to reduce the
noise in amplifier. It also makes amplifier
operation stable. Depending upon whether
the feedback energy aids or opposes the input
signal, there are two basic types of feedback
viz, positive feedback and negative feedback.
: When the feedback
energy (voltage or current) is in phase with the input
signal and thus aids it, it is called positive feedback.
Both amplifier and feedback network introduce a
phase shift of 180°. It causes a 3600phase shift around
the loop which causes the feedback voltage Vf to be in
phase with the input signal Vm.
The positive feedback increases the gain
of the amplifier. But positive feedback
increases distortion and instability.
Therefore, positive feedback is rarely
employed in amplifiers. Important use of
positive feedback is in oscillators.

19. When the feedback energy
(current or voltage) is out of phase with the input
signal and thus opposes it, it is called negative
feedback. shows that the amplifier introduces a
phase shift of 1800 into the circuit. Here feedback
network is designed in such a way that it
introduces no phase shift (0° phase shift). So the
feedback voltage Vf is 180° out of phase with the
input signal V. Negative feedback reduces the
gain of the amplifier.
Advantages of Negative Feedback :
(i) Reduction in distortion
(ii) Stability in gain (iii) Increased Bandwidth
(iv) Improved input and output impedances.
Because of these advantages, the negative
feedback is frequently employed in amplifiers.