Positive Feedback Amplifier

FEEDBACK AMPLIFIER
FEED BACK:
The process of applying a fraction of output energy of some device back to the input is known as feedback.
FEEDBACK AMPLIFIER:
The amplifier using feed back process is called feedback amplifier.
TYPES OF FEEDBACK AMPLIFIER:
There are two types of feedback amplifier: a)Positive feedback amplifier b)Negative feedback amplifier
POSITIVE FEEDBACK AMPLIFIER:
>The amplifier in which feedback energy is applied in the same polarity or in same phase is called as positive feedback amplifier.
>It increases the voltage gain of amplifier.
>In this distortion is more.
>It has increased instability.
>It seldom used as an amplifier. Mostly used in oscillator.
NEGATIVE FEEDBACK AMPLIFIER:
>The amplifier in which feedback energy is applied in the opposite polarity or out of phase with input is called negative feedback amplifier.
>It decreases the voltage gain.
>In this distortion is less.
>It is more stable.
>It is mostly used as an amplifier.
PRINCIPLE OF NEGATIVE FEEDBACK AMPLIFIER:
>Block diagram of negative feedback amplifier:

>A=Voltage gain without feedback. Af=voltage gain with feedback. Vi=Input signal to be amplified.
Vo= Output signal of amplifier
Vd=Actual input signal to the amplifier.
Vf= Feedback signal applied to input side. m=feedback fraction
>From block diagram it is clear that, m=Vf/Vo i.e. Vf=mVo Vd=Vi-Vf=Vi-mVo
Again A=Vo/Vd i.e. Vo=AVd=A(Vi-mVo) i.e. Vo=AVi-AmVo i.e. Vo+AmVo=AVi i.e. Vo(1+mA)=AVi Vo/Vi=1/(1+mA) Af=1/(!+mA)
It is clear that voltage gain with feedback decreases by a factor (1+mA).
ADVANTAGES OF NEGATIVE FEEDBACK AMPLIFIER:
1-Resultant gain is independent of transistor parameters or supply voltage fluctuation.