First slide

Transistor

Question

The voltage gain of an amplifier with 9% negative feedback is 10. The voltage gain without feedback will be

Moderate

A

90
B

10
C

1.25
D

100

Solution

The process of injecting a fraction of output energy of some device back to the input is known as feedback. When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it, then it is called negative feedback. Voltage gain with feedback is $A_{vf} = \frac{A_{v}}{1 + {βA}_{v}}$ where $A_{v}$ is voltage gain without feedback and $β$ is negative feedback.
$Given A_{vf} = 10, β = 9 % = \frac{9}{100} 10 = \frac{A_{v}}{1 + \frac{9}{100} A_{v}} or 10 + \frac{9}{10} A_{v} = A_{v} or 0.1 A_{v} = 10 or A_{v} = 100$

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