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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 1st, 2013, 9:16am |
Title: How an amp would oscillate if it's phase margin blew 0 Post by xianweng on Sep 1st, 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 1st, 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 2nd, 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 5th, 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 5th, 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 1st, 2013, 9:32am xianweng wrote on Sep 1st, 2013, 9:16am:
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. |
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