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Frank Wiedmann

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Munich, Germany

Re: What's the phase margin in this casde?
Reply #15 - Jun 25^th, 2007, 12:37am

The step response will be different. The easiest and most precise way to determine its shape in all such cases is probably to simply perform a transient simulation of the closed-loop circuit with a step at the input and look at the result. Even seemingly small changes in the Bode plot of the loop gain can lead to significant differences in the step response, see e.g. figures 5.3 and 5.4 of http://www.elec.ucl.ac.be/enseignement/ELEC2650/chapter5.pdf.

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Asmodeus Junior Member Offline Posts: 22	Re: What's the phase margin in this casde? Reply #16 - Jan 22^nd, 2010, 3:57am What i feel is that the circuit is clearly unstable. At frequency around A it is unstable. Practically speaking any noise around this frequency will oscillate and won't let the circuit operate !
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: What's the phase margin in this casde? Reply #17 - Jan 22^nd, 2010, 8:27am hi all, seems this system has 2poles after that one zero, so how is it possible to have phase more than -180? and even if it is crossing -180 also you can't say it is unstable..strictly speaking Phase Margin applicable for all pole system, so better start using Root locus. Thanks, Rajasekhar.
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Frank Wiedmann

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Posts: 678
Munich, Germany

Re: What's the phase margin in this case?
Reply #18 - Jan 22^nd, 2010, 1:25pm

Asmodeus wrote on Jan 22^nd, 2010, 3:57am:

What i feel is that the circuit is clearly unstable. At frequency around A it is unstable. Practically speaking any noise around this frequency will oscillate and won't let the circuit operate !

I'm afraid that your feeling is misleading you here. Conditions for circuit stability can be rather non-intuitive. I would suggest that you get familiar with the Nyquist stability criterion that has already been mentioned in this thread.

If you don't believe me, build a circuit with a loop gain as shown in your favorite simulator (using ideal controlled sources etc.) and do a transient simulation. There will probably be some ringing in the step response but no self-sustained oscillation; if you simulate long enough, the output voltage will be constant at the end.

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