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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> gain_margin https://designers-guide.org/forum/YaBB.pl?num=1309405810 Message started by vijayakumar.M on Jun 29th, 2011, 8:50pm |
Title: gain_margin Post by vijayakumar.M on Jun 29th, 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 29th, 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 30th, 2011, 12:37am vijayakumar.M wrote on Jun 29th, 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). |
Title: Re: gain_margin Post by Frank Wiedmann on Jun 30th, 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 30th, 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" . |
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