Page 46 - Semester-IV-Electronics

 Negative feedback causes a loss in the magnitude of gain of the

amplifier but it has many advantages so that negative feedback is

specially applied in amplifiers.

 Positive feedback increases the gain of the amplifier but it becomes

unstable as an amplifier.

 With an appropriate value of positive feedback, gain of the amplifier

can become infinite, i.e. the input signal in the amplifier is totally

supplied by itself. In such a state, the amplifier behaves as an

oscillator.

 If a part βV of the output voltage V0 is fed back, the resultant input
0
becomes, V = V + V f
i
if
 But, V = AV , where A is the gain of amplifier with open loop and V
0 if f
= βV .
0

 V /A = V + βV 0
i
0

 V *(1 - Aβ) = AV
0 i

 So that the resultant gain of feedback amplifier, A = V /V = A/1-Aβ
i
0
f
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