Question 8.10: The open-loop gain of an amplifier is 10,000. The input and ...
The open-loop gain of an amplifier is 10,000. The input and output impedances without feedback are 100 KΩ and 100 Ω, respectively. The feedback fraction is 0.01. Calculate (a) the gain with feedback; (b) input impedance with feedback; and (c) output impedance with feedback.
Given,
A = 10,000
β = 0.01
Z_{in} = 100 KΩ
Z_{o} = 100 Ω
=99
Z_{in} with feedback = (1+βA) Z_{in}
= (1+0.01×10,000)×100 KΩ
= 101×100 KΩ
= 10,100 KΩ
=10.1 MΩ
Z_{0} with feedback =\frac{Z_{o} }{1+βA}=\frac{100}{1+0.01×10,000}
=\frac{100}{101}=0.99The result shows that the value of input impedance has become very high due to the introduction of negative feedback and the value of output impedance has become quite low. This is the advantage of negative feedback. Amplifier gain has, however, been reduced.