Download Single Stage Transistor Amplifiers Introduction

Introduction
When only one transistor with associated
circuitry is used for amplifying a weak signal, it is
called “single stage transistor amplifier”. Although a
weak amplifier consists of a number stages, yet such
a complex circuit can be conveniently split up into
separate single stage. By analyzing carefully only a
single stage and using this single stage repeatedly,
we can effectively analyze the complex circuit.
Therefore, single stage amplifier analysis is of great
value in understanding the practical amplifiers
circuits.
Circuits
Elements of
Transistor
Amplifiers
A transistor can accomplish
faithful amplification only if
proper associated circuitry is
used with it. The various circuit
elements of a single stage
transistor amplifiers are. . .
(i)
A biasing circuit
which establish the proper
operating point. This ensures
faithful amplification of the
signal. Generally potential
divider method of bias is
employed for this purpose.
(ii) An input capacitor Cin which
couples the signal to the base of the
transistor. If its not used, the signal
source resistance will change the
bias conditions of the circuit. This
capacitor only allows a.c. signal to
flow but isolates the signal source
from the biasing circuit.
(iii) A coupling capacitor Cc which
couples one stage of amplification
to the next. If it not used, the bias
conditions of the next stage will be
drastically changed. This capacitor
isolates the D.C. of one stage from
the next but allows the passage of
A.C. signal.
(iv) An emitter by pass capacitor CE in
parallel with emitter resistance RE . This
capacitor provides a low reactnce path
to the amplified A.C. Signal. If it not
used, then amplified A.C. signal flowing
through RE will cause a voltage drop
across it, thereby reducing the output
volage.
D.C & A.C. Equivalent Circuits. In
a transistor amplifier, both D.C
and Ac. Conditions prevail. The
D.C. source set up D.C currents
and voltages whereas sources
(i.e signal) produces fluctuations
in transistor currents and
voltages.
Therefore. A simple way to
analyze the action of a transistor
amplifier is to split the analysis
into two parts viz d.c. equivalent
circuits and A.C. Equivalent
circuits
(i) In drawing the D.C. equivalent
circuit, only D.C. conditions are
considered i.e. it is presumed that no
signal is applied. For this purpose, all
A.C. sources are reduced to zero and all
the capacitors are considered open. If
we carefully apply these steps to a
transistor amplifier, we shall get the
D.C. equivalent circuit.
(ii) In drawing A.C. equivalent
circuit, only A.C. conditions
are considered. For this
purpose, all D.C. sources are
reduced
to
zero
and
capacitors are considered
short.
LOAD LINE ANALYSIS. The output
characteristics are determined
experimentally and indicate the
relation between VCE
and IC
graphically . It is well know that
relation between VCE and IC linear
so that it can be represented by a
straight line on the output
characteristics.
This is known as “load-line”. As in a
transistor both D.C. and A.C.
Conditions exist, therefore, there
are two types of load lines namely;
D.C Load Line and A.C. Load Line.
The former determines the locus of
IC and VCE in the zero signal
conditions while the later shows
these values when signal is applied.
Voltage Gain. The
voltage gain of an
amplifier is the ratio
of output voltage to
the input voltage i.e.
Voltage = Output Voltage
Signal
For CE amplifier, it can be easily proved that:
Voltage gain= b x RAC
RIN
where:
RAC = effective collector load
RIN = input resistance of the transistor
Power Gain. The ratio
of power output to
input power of an
amplifier is called
power gain i.e.
Power Gain = Output Power
Input Power
For CE amplifier, it can be proved that:
Voltage gain=
2
b
x RAC
Rin