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I've learned in school to calculate gain margin by first finding the phase crossover frequency at -180 deg, but sometimes I see people calculate gain margin by finding the phase crossover frequency at 0 deg which doesn't make sense to me. I'm confused as to why that method is used? I can't find any explanations online so any help is appreciated.

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  • \$\begingroup\$ it must be different methods as one includes the 180 deg inversion \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 '20 at 3:38
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What causes instability problems is positive feedback, i. e., feedback with the same (or close to the same) phase as the input. In other words, 0˚ for the loop gain.

So where does the 180˚ come from?

Typically, it's assumed that the feedback signal is subtracted from the input signal to create the error signal. The negative sign implicit in the subtraction represents a phase rotation of 180˚. So, if you're looking at the phase shift of the loop gain without taking the subtraction into account, then you want to avoid phase shifts close to 180˚ instead.

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    \$\begingroup\$ ..."looking at the phase shift of the closed loop..." . No - the phase as well gain margin are found by analyzing the loop gain - and NOT the closed loop. \$\endgroup\$ – LvW Jan 3 '20 at 10:02
  • \$\begingroup\$ @LvW - you are absolutely correct, thanks for pointing that out. I’ve edited my answer. \$\endgroup\$ – joribama Jan 3 '20 at 12:23

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