Feedback amplifiers

In a feedback amplifier the fraction of the amplifier output is fed back to the input circuit
Feedback amplifiers consists of two parts
(i) An amplifier circuit
(ii) Feedback circuit
Figure below shows how a feedback is introduced in an amplifier circuit
Figure(b) shows an amplifier without a feedback if v_i and v₀ are input and output voltage of the amplifier then voltage gain would be
A=v₀/v_i ---(13)
Now consider figure (b) in which a feedback loop is added to the amplifier. If v₀^' is the output voltage with feedback and a fraction of its voltage β is applied to the input voltage then input voltage becomes v_i+βv₀^' depending on whether the feedback voltage is in phase or out of phase with the input voltage
If we consider the case of the positive feedback then input voltage becomes (v_i+βv₀^') .When it is amplified by A times it becomes
A(v_i+βv₀^')=v₀^'
or v₀^'(1-βA)=Av_i
So the amplifier gain A^' with feedback is given by

A^'=v₀^'/v_i =A/1-βA ---(14)
The above equation is positive feedback
For negative feedback
A^'=A/1+βA
and term βA is called feedback factor and A=v₀/v_i is called the internal gain of the amplifier
From equation (14) and (15) we see that gain of amplifier increases when positive feedback is applied and decrease when negative feedback is applied
For good performance of an amplifier ,we generally use negative feedback
Positive feedback is used in making oscillator

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