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Design >> Analog Design >> Phase margin definition
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Message started by mohanps on Mar 1^st, 2006, 11:08pm

Title: Phase margin definition
Post by mohanps on Mar 1^st, 2006, 11:08pm

Basic question on the Phase margin definition. Why the PM is only
looked at the gain cross over point? If the phase goes down
to 20 degrees and come back to 60 degrees (due to Zero) at 0 crossing, is it
not dangerous as the phase is close to 360 degrees when the
gain is still positive (leading to oscillations)

Thanks
Mohandas

Title: Re: Phase margin definition
Post by ACWWong on Mar 2^nd, 2006, 2:09am

sounds dangerous to me... perhaps you can see the difference in the transient behaviour of your feedback system when compared to a classical (ie PM always > 60 deg for >0dB gain) 60deg PM system. you might find a tendency for ringing at a frequency of poistive gain and low PM... especially if you purposely stimulate at that frequnecy...

Title: Re: Phase margin definition
Post by vivkr on Mar 2^nd, 2006, 2:27am

Hi Mohandas,

The above definition of PM implicitly assumes that the gain curve monotonically decreases.
The PM criteria is just a simplification of the more general Nyquist Stability Criterion.
Whenever in doubt, refer to the latter.

Regards
Vivek