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 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
  

Also Read

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• Notes
  • Semiconductor Materials
  • Bipolar Junction Transistor
  • Transistor as an Amplifier
  • Feedback Amplifier
  • Transistor as an Oscillator
  • Logic Gates
• Assignments
  • Semiconductor MCQ
  • Semiconductor Questions

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