Stability of Gain Factor
The introduction of negative feedback makes the amplifier insensitive to variations in the values of the
components and parameters of the active devices used in building the amplifier. The expression that relates
percentage variation in the gain parameter of the amplifier with feedback to the percentage variation in
the same without feedback can be derived as follows.
A=
Af
+ βA 1
Differentiating the above equation with respect to A we get,
dAf =
Af
+1 β A X dAA
1
|(1+βA)| is called the desensitivity parameter D. thus, percentage variation in gain with feedback
is equal to the percentage variation in gain without feedback divided by the desensitivity parameter D.
Effect on bandwidth
Bandwidth increases with introduction of negative feedback. Increase in bandwidth results from the fact
that amplifier exhibit a constant gain-bandwidth product. Reduction in gain is, therefore, accompanied by
increase in bandwidth. Bandwidth increases by the same desensitivity factor D = 1+βA by which the gain
reduces. Bandwidth of amplifier with feedback is given by
(BW)f = BW X (1+βA)
Effect on Non-Linear Distortion
It can be proved that non-linear distortion decreases by the desensitivity factor D = 1+βA. Let us assume
that the distortion levels without and with negative feedback are D2 and D2f, respectively. D2 is the
distortion contributed by the active device. In the presence of feedback, D2f appears as –βAD2f at the input
of the amplifier. The following gives the expression for D2f.
D2-βAD2f = D2f =
D+
2βA
Effect on Noise
1
Introduction of negative feedback acts on the noise generated in the amplifier in the same manner as it
does on the non-linear distortion. Let us assume Nf and N are the noise levels with and without negative
feedback respectively. Reduction in noise is governed by
N =
Nf
+ βA 1
Effect on Input Resistance
Input resistance in the case of an amplifier with negative feedback is affected depending upon how the
feedback signal is connected to the source of external input signal. The input resistance increases if the
feedback signal is connected in series with the source of input and decreases if the feedback signal is
connected across it in shunt.
Thus in the case of voltage-series and current-series feedback, Ri is the input resistance without feedback
and the input resistance without feedback Rif is given by
Rif = Ri X (1+βA)
Input resistance in the case of voltage-shunt and current-shunt feedback is given by
Ri
Rif =
+
βA
1
Effect on Output Resistance
Output resistance in the case of an amplifier with negative feedback is affected depending upon how the
feedback signal is connected to the source of external input signal. The output resistance increases in the
case of output is current and decreases in the case of output is voltage.
Thus in the case of current-series and current-shunt feedback, Ri is the input resistance without feedback
and the input resistance without feedback Rif is given by
Rof = Ro X (1+βA)
Input resistance in the case of voltage-series and voltage-shunt feedback is given by
Ro
=
Rof
+ βA
1