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21st September 2015, 16:35 #1
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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?

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22nd September 2015, 01:07 #2
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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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22nd September 2015, 12:24 #3
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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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22nd September 2015, 14:00 #4
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Re: stability and phase bode plot
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).
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27th September 2015, 09:34 #5
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