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Design >> Analog Design >> Stability issue
https://designers-guide.org/forum/YaBB.pl?num=1160990196

Message started by rf_man on Oct 16^th, 2006, 2:16am

Title: Stability issue
Post by rf_man on Oct 16^th, 2006, 2:16am

Hi gurus,

Please see the attached bode plot of the loop gain of a certain circuit, it's made by spectre stb analysis.
From the plot, it can be seen that at 0db points, the loop phases are respectively 96 degrees and -100 degrees, so the phase margins are 276 degrees (or -84 degrees), -80 degrees, respectively, right? The phase margins are both greater than 60 degrees, therefore the loop should be stable. But from transient simulation, the circuit oscillates, what's wrong?

I don't have experience with stability issues, if the question is stupid, please don't laugh at me. Thank you in advance for your feedback.

Title: Re: Stability issue
Post by ACWWong on Oct 16^th, 2006, 2:28am

your plot shows positive loop gain between point A and B, when the phase is 0, ie positive feedback... so you should expect your circuit to oscillate at a frequency between A&B (roughly where the phase crosses 0).

Title: Re: Stability issue
Post by rf_man on Oct 16^th, 2006, 3:06am

Thank you very much for your reply. But do you agree with me that the phase margins are both greater than 60 degrees? So my conclusion is that in practice, we shouldn't always believe the phase margin, right?

Title: Re: Stability issue
Post by ACWWong on Oct 16^th, 2006, 7:19am

please see vivkr comment in:
http://www.designers-guide.org/Forum/YaBB.pl?num=1141283321/2#2

so blindly reading the PM number given in a stb is dangerous unless you inspect the full bode plot properly.

i think this forum has quite alot of good discussions re: phase margin, loop stability etc. use the search facility for more info.

cheers
aw