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PM and GM of stability analysis

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Usman Hai

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Hi,
I designed a simple cascode opamp and did the stability (stb) analysis. Mag and Phase plot is perfect. However when I get the stability analysis summary I am getting negative values for Phase margin and Gain Margin.
The phase when my loop gain crosses 0dB is -93 degrees. However, in stability analysis I am getting phase "margin" as -93 degrees. whats wrong here?
 

Usman Hai

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Hi,
I found the answer. Actually in the actual implementation of stb analysis the loopGain is taken as -T(x), that is, 180 degrees phase difference from the usual definition.
So correct way is to use the calculator and plot PhaseMargin(getData("loopgain" ?result "stb-stb").
Hope it will help people in future.
 

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