Negative feedback and stability

Discussion in 'Feedback and Suggestions' started by LvW, Jul 31, 2013.

  1. LvW

    Thread Starter Active Member

    Jun 13, 2013
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    Reference is made to
    http://www.allaboutcircuits.com/vol_3/chpt_4/12.html
    (Volume III-Semiconductors, Biplar Junction Transistors, Feedback)

    „...negative feedback drives it the opposite direction: toward a point of stability“.

    „An amplifier circuit equipped with some amount of negative feedback is not only more stable, but....“

    “...the advantages of stability, lower distortion, and greater bandwidth are worth the tradeoff in reduced gain for many applications.“

    Also reference is made to:
    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html
    (Volume III-Semiconductors, Operational Amplifiers, Positive feedback )

    First paragraph:
    „Negative feedback makes all these circuits stable and self-correcting.“


    Correction:
    When using the term „stability“ it is necessary to discriminate between static and dynamic stability - that is between dc stability of the operating point (against tolerances and other uncertainties) and dyamic stability (against oscillations).
    This is important because - in contrast to the above quoted statements - negative feedback DECREASES dynamic stability properties.
    (Otherwise, no internal phase compensation would be necessary for opamps).

    In fact, each negative feedback turns into positive feedback for increasing frequencies. Based on this observation the classical definitions for stability margins are formulated.
    In summary: Negative feedback improves dc stability BUT deteriorates dynamic ac stability at the same time.
     
  2. Georacer

    Moderator

    Nov 25, 2009
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    This is true. However, then, we would need to discuss previously on phase margins and feedback stability, which is a more advanced topic (and which lies in control theory) than simple negative feedback loops.

    I hope you understand my reasoning.

    However, if you believe you can propose a simple way to integrate this detail into the existing flow of the e-book, you are welcome to do it.
     
  3. LvW

    Thread Starter Active Member

    Jun 13, 2013
    674
    100
    Georacer, thanks for replying.
    I think it would be sufficient simply to mention that because of rising phase shifts the negative feedback effect will continuously migrate into a positive feedback. However, in case the gain is already sufficient small (below unity) the problem is not too big and causes - most probably - just a step response with a small ringing effect.
    I will try to propose a corresponding reformulation.
     
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