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navin_kumar Junior Member Offline Posts: 15	about stability of opamp Aug 11^th, 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
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sheldon

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Posts: 751

Re: about stability of opamp
Reply #1 - Aug 11^th, 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

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vivkr

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Posts: 780

Re: about stability of opamp
Reply #2 - Aug 15^th, 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

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sheldon Community Fellow Offline Posts: 751	Re: about stability of opamp Reply #3 - Aug 17^th, 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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