# Feedback amplifiers

## 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 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

- Positive feedback is used in making oscillator

**Also Read**