Basic condition for oscillation

Discussion in 'General Electronics Chat' started by athulascc, Mar 1, 2015.

  1. athulascc

    Thread Starter Member

    Aug 15, 2014
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    I have learned that there is two basic conditons for sustain oscillation

    1. A(w)b(w)=1 (loop gain=1)
    2.phase shift of 0 or multiple of 360

    but to start A(w)b(w)>1

    why those conditions? how to understand that?
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    For an oscillation to start an amplifier has positive feedback .
     
  3. GopherT

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    Nov 23, 2012
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    @athulascc

    Because all other conditions will either dampen the oscillation or cause a stable saturation condition at the positive or negative rail.
     
  4. bertus

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  5. athulascc

    Thread Starter Member

    Aug 15, 2014
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    yes @Dodgydave , but if A(w)b(w)>1 at the start then how it becomes =1 when oscillation?
    when we design we can't make actually A(w)b(w)=1 , but how it oscillate?

    @GopherT I am trying to find theory behind these two conditions?
     
  6. athulascc

    Thread Starter Member

    Aug 15, 2014
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    @bertus thanks.that contains what I am looking for.
     
  7. Brownout

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    Jan 10, 2012
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    You don't really need to worry about that. The amplifier's gain will, at some point, decrease to the point where loop gain will = 1. At that point, you have sustained oscillation.
     
  8. GopherT

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    Nov 23, 2012
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    I thought it was obvious.
    Any phase shift not = 360 degrees (or multiples) will cause the gain of 1 condition to not be true at various times in the oscillation period.
    Any gain below 1 will cause the signal to dampen each oscillation.
    Any gain over 1 will cause signal to saturate after some number of oscillations.
     
  9. ErnieM

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    A gain of 1 will sustain an oscillation, but that is of little use at start up when there is no oscillation. So a gain >1 is required to start.

    Once an oscillation begins a gain >1 will indeed produce saturation; saturation is a non-linear condition with a side effect of limiting gain, so the now distorted (or clipped) signal will be running about with a gain of precisely 1.
     
  10. athulascc

    Thread Starter Member

    Aug 15, 2014
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    great. that means gain depends on frequency. and when w comes to A(w)b(w)=1 it sustains.
    I am getting more and more knowledge.
     
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