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Design >> Analog Design >> gain_margin
https://designers-guide.org/forum/YaBB.pl?num=1309405810

Message started by vijayakumar.M on Jun 29^th, 2011, 8:50pm

Title: gain_margin
Post by vijayakumar.M on Jun 29^th, 2011, 8:50pm

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

Title: Re: gain_margin
Post by raja.cedt on Jun 29^th, 2011, 9:19pm

hi,
The main motivation behind this gain margin is "if at all system goes into +ve feedback region it shouldn't have loop gain so that it won't do any harm to us". Now coming to your question when phase becomes -180deg system will be in +ve fB mode so check there how much loopgain below zero.

Thanks.
Raj.

Title: Re: gain_margin
Post by buddypoor on Jun 30^th, 2011, 12:37am

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

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

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).

Title: Re: gain_margin
Post by Frank Wiedmann on Jun 30^th, 2011, 5:20am

See also http://www.designers-guide.org/Forum/YaBB.pl?num=1124688329. I also comment on this subject at the beginning of http://sites.google.com/site/frankwiedmann/loopgain.

Title: Re: gain_margin
Post by buddypoor on Jun 30^th, 2011, 8:43am

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" .