US03CPHY22 Unit 2 Small Signal Amplifier

Physics Department Bhrijesh N Patel
1 | P a g e
Shree P M Patel College of Electronics and Communication, Anand
S.Y. B.Sc. Semester - III
US03CPHY22 Basic Solid State Electronics
Unit – 2 Small Signal Amplifier
Introduction
Almost no electronic system can work without an amplifier. It is only
because of the enlargement or the amplification of the signal picked up by
microphone that we can enjoy a music orchestra. We are able to hear the
news or the cricket commentary on our radio, simply because the amplifier
in the radio amplifies the weak signals received by its antenna. The signal
can only be of any use if it is amplified to give a suitable output.
SINGLE·STAGE TRANSISTOR AMPLIFIER
Almost all amplifiers use this biasing circuit, because the design of the
circuit is simple and it provides good stabilization of the operating point. If
this circuit is to amplify ac voltages, some more components must be added
to it.
The capacitors Cc are called the coupling capacitors. A coupling
capacitor passes an ac signal from one side to the other. At the same time, it
does not allow the de voltage to pass through. Hence, it is also called a
blocking capacitor. For instance, it is due to the capacitor Cc (connected
between collector and output) in Fig. l b that the output voltage v0 across the
resistor R0 is free from the collector's de voltage.
The capacitor CE works as a bypass capacitor. It bypasses all the ac
currents from the emitter to the ground. If the capacitor CE is not put in the
circuit, the ac voltage developed across R E will affect the input ac voltage.
Such a feedback of ac signal is reduced by putting the capacitor CE.

Physics Department Bhrijesh N Patel
2 | P a g e
(a) Voltage-divider biasing circuit (b) same circuit converted into an amplifier
If the capacitor CE is good enough to provide an effective bypass to the
lowest frequency of the signal, it will do so better to the higher frequencies.
We, therefore, select such a value of capacitor CE that gives quite a low
impedance compared to RE at the lowest frequency present in the input
signal.
To what extent an amplifier enlarges signals is expressed in terms of its
voltage gain. The voltage gain of an amplifier is given as
The other quantities of interest for a voltage amplifier are its current gain
(Ai), input impedance (Zi), and output impedance (Z0). The amplifier can be
analysed for its performance by the following two methods:
1. Graphical method
2. Equivalent circuit method

Physics Department Bhrijesh N Patel
3 | P a g e
GRAPHICAL METHOD
For analyzing an amplifier by this method, we need the output
characteristics of the transistor. These are supplied by the manufacturer.
When the ac voltage is applied to the input the base current varies. The
corresponding variations in collector current and collector voltage can be
seen on the characteristics. This method involves no approximating
assumptions. Hence, the results obtained by this method are more accurate
than the equivalent circuit method. One can also visualize the maximum ac
voltage that can be properly handled by this amplifier. In fact, for large-
signal amplifiers (power amplifiers) this is the only suitable method.
Is de Load Line same as ac Load Line?
In the amplifier circuit of Fig.1, the resistors R 1 and R 2 form a voltage
divider arrangement for fixing a certain de base voltage. This base voltage
and the resistor RE fix the emitter current. The collector current is almost the
same as the emitter current. The resistor Re then decides the value of V CE.
Writing the KVL equation for the output section of the circuit, we get
Vcc = lcRc + VCE +IERE
= VCE + lc(Rc +RE) [since Ic = IE]
This is the equation of the de load line. By plotting this line on the output
characteristics, the de collector voltage and current can be determined for the
given value of base current. As regards the de currents and voltages, the
amplifier circuit of Fig. 1 (b) behaves like the circuit shown in Fig. 2a. This
is obtained by opening all the capacitors in the original circuit. The
capacitors are as good as open circuits for dc.

Physics Department Bhrijesh N Patel
4 | P a g e
Fig. 2 Amplifier circuit of Fig. 1b
Suppose we had changed the de bias, giving a different value of base
current. The collector current and collector voltage both will change. As a
result, the Q point will shift on the de load line. This is what roughly
happens when we apply an input ac signal. But, in the ac signals, the
variations occur very fast. The capacitors can no longer be considered as an
open circuit. In fact, the variations in the currents and voltages occur so fast
that the capacitors in the circuit may be treated as short circuits. Also while
dealing with ac currents and voltages, we need not consider the de supplies.
CALCULATION OF GAIN
To understand how to calculate the current gain and voltage gain by the
graphical method, we consider a typical amplifier circuit. One such circuit is
shown in Fig.3.
The output characteristics of the transistor used in this circuit, are shown
m Fig. 4.
We first plot the de load line on the output characteristics. The equation
of this dc load line is given by Eq.