Electronic Circuit Analysis

Multistage Amplifrers 39
The equivalent circuit for the above transistor configuration is,
hIe
C
Fig. 2.3 Equivalent circuit
Suppose the value of C c is very large. Then at the low and medium frequencies, the impedance
is negligible. So the equivalent circuit is as shown. We shall consider the effect of C E later.
From the equivalent circuit,
Vo = - h fe · i b· RL
negative sign is used since it is NPN transistor. The collector current is flowing into the transistor.
Input current, Ib = Vi Rs + R j
For a transistor amplifier circuit in common emitter configuration,
R j = hje + (1 + h fe ) ZE'
Where ZE = RE in parallel with C E =
Ay at low frequencies, (LF)
1+ jroC E
RE
Vs
Vo
A=y
~
S
Vo
Ay(LF) = V.
S
When ro is large,
(l+hfe)RE
1+ jro C E RE
can be neglected. So in the Mid Frequency range, (M.F.),
-hfeRL
Av (M.F) = R h
s + ie
In this expression, there is no for ro term. Hence in the mid frequency range, Ay is independent
off or the gain remains constant, irrespective of change in frequency.