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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> about stability of opamp https://designers-guide.org/forum/YaBB.pl?num=1123744244 Message started by navin_kumar on Aug 11th, 2005, 12:10am |
Title: about stability of opamp Post by navin_kumar on Aug 11th, 2005, 12:10am hai please give the exact answer for this normally for opamps we will specify the minimum phase required is .,>60deg is it from settling point of u (am i correct ) please share ur thoughts thanks and regards naveen |
Title: Re: about stability of opamp Post by sheldon on Aug 11th, 2005, 5:10am Navin, Phase Margin is a figure of merit for the frequency response of the op-amp and it can not be calculated from time domain characteristics of the circuit, for example, settling time. Phase Margin is the measure of how close the phase of the loop gain is to inverting at unity gain. Or the phase margin is the difference between the phase of the loop gain and inversion, phase=180 degrees for unity gain of the loop gain. It is described in detail by the books in the Reference books section, for example, Gray and Meyer. If you have access to Spectre, stability analysis will calculate it for you directly. Best Regards, Sheldon |
Title: Re: about stability of opamp Post by vivkr on Aug 15th, 2005, 10:51pm Hi Sheldon, A bit of digression here. I think that it would not be entirely correct to say that the phase margin cannot be deduced from time-domain characteristics as it is related to the damping factor. For a second-order system, a simple formula may be derived directly from the overshoot seen when a step input is applied to the system. A rough calculation shows that the relative overshoot for a stable second-order system is exp(-pi*tanPM) As real systems always have many more poles and a different kind of damping, this approximate formula should be applied carefully to deduce the phase margin from transient characteristics in case an AC analysis is not satisfactory enough for whatever reason. And a final word of caution: Phase margin may be a false indicator of stability, especially in certain forms of opamp compensation (e.g. Ahuja compensation). Cheers Vivek |
Title: Re: about stability of opamp Post by sheldon on Aug 17th, 2005, 2:07am Vivek, I agree that phase margin can be extracted from time domain response and have posted references to the methodology in the past. However, phase margin is not extracted from settling time. For reference see "CMOS Analog Circuit Design" by Allen and Holberg, first edition, pages 685-693. Sheldon |
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