The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> How an amp would oscillate if it's phase margin blew 0
https://designers-guide.org/forum/YaBB.pl?num=1378052180

Message started by xianweng on Sep 1^st, 2013, 9:16am

Title: How an amp would oscillate if it's phase margin blew 0
Post by xianweng on Sep 1^st, 2013, 9:16am

Hi ,everybody

If PM of an amp below 0,it will oscillate nearby Vref or from 0 to VDD or something else?
best regards

Title: Re: How an amp would oscillate if it's phase margin blew 0
Post by raja.cedt on Sep 1^st, 2013, 9:24am

Hello,
Very difficult to ans, in fact phase margin is based on small signal metric. But once if starts oscillating there is it would leave small signal regime and rest i can't predict, i guess it reaches very near to vdd and vss.

One more thing, according to linear system theory If Phase margin is -ve then pole will be in RH plane hence response would go in exponential increasing amplitude sinusoidal way and mostly it will touch rails and gain will drop down.

Thanks,
Raj.

Title: Re: How an amp would oscillate if it's phase margin blew 0
Post by xianweng on Sep 2^nd, 2013, 9:33am

Thank you ,I see

Title: Re: How an amp would oscillate if it's phase margin blew 0
Post by Fortion on Sep 5^th, 2013, 10:15am

hello raj can you please elaborate the reason of the negative margin
?
i always find it hard to understand?

Title: Re: How an amp would oscillate if it's phase margin blew 0
Post by raja.cedt on Sep 5^th, 2013, 11:01am

Imagine you have system with 3 poles and high gain, then 3 poles contribute 270 deg, but to be in -ve feedback region you can't have more than 180 hence -90 is the net margin.read razaavi text book.

Title: Re: How an amp would oscillate if it's phase margin blew 0
Post by buddypoor on Oct 1^st, 2013, 9:32am

xianweng wrote on Sep 1^st, 2013, 9:16am:

Hi ,everybody

If PM of an amp below 0,it will oscillate nearby Vref or from 0 to VDD or something else?
best regards

When the PM is negative, there will be a lower frequency fo that exhibits a PM=0 deg.
This is the frequency where oscillations might occur. However, it depends on the loop gain magnitude at this frequency if you can observe oscillations or some other kind of instability.