why we are getting gain margin at a phase of 0 degree instead of -180 degree?????

vijayakumar.M wrote on Jun 29^th, 2011, 8:50pm:


The answer to your question is simple:
Some authors plot the BODE diagram for the product gain*feedback (without consideration of the minus sign at the summing point) and some other plot the BODE diagram for the complete loop gain (including the sign inversion).
In the first case, the phase starts at zero deg and the gain margin is defined for a phase shift of -180 deg. (knowing that additional -180 deg have to be considered).
In the 2nd case, the phase starts at -180 deg (a "must" for negative feedback) and the gain margin is defined for -180-180=-360=0 deg.
I prefer the second case as it is based on the real loop gain (and is more logical, for my opinion).

By the way: Both alternatives (loop gain T(s) or -T(s)) can be observed also in Nyquist plots. In some books/publications the "critical point" is located at "-1" and in some others at "+1" .