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Negative feedback

  1. Sep 3, 2009 #1
    While introducing negative feedback, my book gave the following example:

    Let the signal voltage be 101mV and the output voltage of the amplifier be 10V
    Out of this 10V a fraction (0.01) i.e. 100mV was made to feedback to the input circuit. The feedback circuit did not introduce any phase change. Hence, the feedback was negative (there is a 180 deg. phase reversal in amplifier circuit.)
    The input signal now becomes (101-100) = 1mV

    Then it was given that,
    Gain (without feedback) is 10V/1mV = 10,000
    Gain with feedback = 10V/101mV = 100 (approx)
    Feedback factor = 100mV/10V = 0.01

    My doubt is that shouldn't the gain without feedback and the gain with feedback be just the opposite?
    With the feedback the input becomes 1mV and without feedback the input was 101mV
     
  2. jcsd
  3. Sep 3, 2009 #2

    Born2bwire

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    They are keeping the output voltage constant and instead modifying the input signal. So with feedback, they needed a greater input signal to achieve the same output. This is to be expected with negative feedback since you are essentially subtracting off some of the output signal off from the input.
     
  4. Sep 3, 2009 #3
    ohk... this was what was intended.
    Thanks a lot Born2bwire
     
  5. Sep 4, 2009 #4
    See complete derivation of op amp feedback and gain in thumbnail.
     

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