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    stability and phase bode plot

    As my attached picture, if we have negative feedback system, we should make unity gain be within -180 phase not to oscillate our system, considering the number of pole in amp.

    And I think,

    upper unity gain frequency, my Aol(in the picture) already has -180 phase because amp has negative gain. so without considering the number of pole in amp, -180 by gain + -180 by negative feedback.. = -360 phase so. my system could osciilate.
    But in real, amp has several pole. so it is not oscillate.

    Is my view right? I'm so confused.

    Plus, why does amp have -90 phase at the pole?

    Click image for larger version. 

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    Re: stability and phase bode plot

    An opamp has a frequency compensation capacitor inside that rolls off its high frequency gain. At the frequency where the internal phase shifts of the opamp cause its output to have a phase shift of 180 degrees then the gain is less than 1 so it does not oscillate when negative feedback is added.


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    Re: stability and phase bode plot

    op amp already has negative voltage gain. It means that op amp already has 180 phase shift.

    Is phase shift by pole in op amp additonal?

    or phase shift by pole in op amp make totally negative voltage gain(180 phase shift)

    I'm so confused..
    Last edited by greenjuice; 22nd September 2015 at 12:27. Reason: edit



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    Re: stability and phase bode plot

    Quote Originally Posted by greenjuice View Post
    op amp already has negative voltage gain. It means that op amp already has 180 phase shift.
    Negative feedback has 180 degrees phase shift.

    Is phase shift by pole in op amp additonal? or phase shift by pole in op amp make totally negative voltage gain(180 phase shift).
    The pole that rolls off the high frequencies produces 90 degrees phase shift. Its cutoff frequency is low enough to dominate the phase shifts because at a frequency where another phase shift in the opamp occurs, this dominant pole reduces the gain to less than 1 so the opamp does not oscillate when any amount of negative feedback is used.


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    Re: stability and phase bode plot

    Quote Originally Posted by greenjuice View Post
    As my attached picture, if we have negative feedback system, we should make unity gain be within -180 phase not to oscillate our system, considering the number of pole in amp.

    And I think,

    upper unity gain frequency, my Aol(in the picture) already has -180 phase because amp has negative gain. so without considering the number of pole in amp, -180 by gain + -180 by negative feedback.. = -360 phase so. my system could osciilate.
    But in real, amp has several pole. so it is not oscillate.

    Is my view right? I'm so confused.

    Plus, why does amp have -90 phase at the pole?

    Click image for larger version. 

Name:	Block_Diagram_for_Feedback_svg.png 
Views:	8 
Size:	62.3 KB 
ID:	121620
    Discussing stability properties and tendency to oscillations you have to consider LOOP GAIN only - that is the gain of the complete feedback loop, including the negative sign.
    In your case: Lop gain LG=- Aol*beta.



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