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 v_{0} are input and output voltage of the amplifier then voltage gain would be
A=v_{0}/v_{i} ---(13)

Now consider figure (b) in which a feedback loop is added to the amplifier. If v_{0}^{'} 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}+βv_{0}^{'} 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}+βv_{0}^{'}) .When it is amplified by A times it becomes
A(v_{i}+βv_{0}^{'})=v_{0}^{'}
or v_{0}^{'}(1-βA)=Av_{i}
So the amplifier gain A^{'} with feedback is given by

A^{'}=v_{0}^{'}/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=v_{0}/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