Gain with feedback

Posted by AJ × 05:23

• Gain without feedback is A

• Feedback factor β

• Gain with feedback is (1+A β)

Voltage series feedback

• With zero feedback then Vf=0 the voltage gain of amplifier stage is

A=Vo/Vs=Vo/Vi

• If feedback of Vf is connected then,

Vi=Vs-Vf

Vo=AVi=A(Vs-Vf)=AVs-AVf=A(Vs-A(βVo)

Then, (1+ βA)Vo=AVs

Overall gain with feedback is

Af=Vo/Vi=A/(1+A β)

This shows that gain of feedback has reduced by factor (1+A β)

Voltage shunt feedback

• Af=Vo/Is=A Ii / (Ii+If)=AIi/(Ii+ βAIi)

• Af=A/(1+ βA)

Input impedance with FB

• Ref to fig(1)

Ii=Vi/Zi=(Vs-Vf) / Zi = (Vs- βVo) / Zi

Ii Zi= Vs- βAVi

Vs=Ii Zi+ β A Vi = Ii Zi+ β A Ii Zi

Zif = Vs/Ii=Zi+(βA)Zi=Zi(1+ βA)

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