Amplifier Feedback

Discussion in 'General Electronics Chat' started by Shagon, Jun 14, 2011.

  1. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    Hey guys... I have been doing research on negative feedback trying to understand it how it works and how does it reduce distortion within an amplifier. And I just cant seem to understand this thing.

    What I do understand is that with negative feed back the output voltage would oppose the input voltage

    So lets say the input voltage is 1V and the output voltage is 5V would that introduce 4V back into the system or what?

    And how does that help to reduce distortion?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. Shagon

    Thread Starter Active Member

    Aug 19, 2009
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    I would read your first link... I have already gone through the wiki link already... still don't quite get it.

    How does it do that? How would the negative feedback help to ensure the output follows the input signal?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    It might help you to look at an opamp circuit that is wired as an emitter follower.

    The noninverting (+) input presents a high impedance to the signal source.
    The inverting (-) input is connected to the output.
    The inverting input adjusts the output so that it "sees" the same thing as what the noninverting input is doing in an active manner (this sentence is true regardless of the configuration of the circuit).

    Without the negative feedback, the opamp output would be slamming from one rail to the other, depending upon the + input signal.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    What we most often simply call "distortion" is amplitude distortion, where the amplification is not the same for all signal levels. As the input varies, different levels are amplified by different amounts, so that the input and output wave forms are not the same.

    An important effect of negative feedback is that it can set up a (reduced) level of gain, which is mostly dependant on the amount of the feedback, and not so much on the amplifier's gain. The feedback "dosage" can be set using items such as resistors which can be made very stable, so gain variation and hence distortion can be made very much less.

    The Wikipedia link posted by SgtWookie shows the maths behind this idea. From this it can be seen that if the gain of the amplifier without feedback is sufficiently big, a more stable gain closely defined by the feedback can be obtained.
     
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