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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 Feedback amplifiers
  • Figure(b) shows an amplifier without a feedback if vi and v0 are input and output voltage of the amplifier then voltage gain would be
    A=v0/vi ---(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 vi+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 (vi+βv0') .When it is amplified by A times it becomes
    A(vi+βv0')=v0'
    or v0'(1-βA)=Avi
    So the amplifier gain A' with feedback is given by

    A'=v0'/vi =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/vi 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


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