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Opamp stability analysis

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stenzer

stenzer

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Hi,

I'm analyzing a circuitry including an opamp. I'm interested in the stability of the system. By analyzing the loop gain of the opamp by the nyquist criterion (bode plot), I'm able to determine the phase margin and gain margin, and thus if the system is stable or not. But I have two questions concerning the stability by means of its desired operation frequency f_x (e.g. at a single sine wave).

Is it possible to make any stability statement by having a look, at the desired frequency f_x, in the bode plot (phase & gain) of the

(i) loop gain, or
(ii) closed loop (transfer function of the complete system)

BR
 

Hi,

No. Stability does not depend on the desired frequency.
If the circuit is not stable it will oscillate at it´s own frequency.

Klaus
 

An additional way to observe stability is to input a step function and observe the output for overshoot and ringing.
That's a good test to do in addition to a Bode plot, since I've had circuits that look okay on a Bode plot, but still have excessive overshot and ringing (or even oscillation) apparently do to large signal non-linearity.
 

Hi , tank you for the replies!

@ KlausST: I already thought so. So thank you for your confirmation!

@ crutschow: To investigate the step response is a good idea. I will performe a transiente Spice simulation of my system, and look how it behaves.

BR
 

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