# Phase margin is 90 degrees but transient step response shows overshoot

The bode plot of the loop is as shown here

The transient step response looks like this

The block diagram looks something like this - Basically a single stage opamp(OPAMP in fig) with high gain(cascode structure) biasing a fet(M1) such that its drain is equal to Vbias. Small signal step is applied at I1 Ibias and transient current through the fet M1 drain is observed.

Am I missing something here?

• What kind of system are we dealing with? Is it completely linear over the operating range? Sep 12 '19 at 20:30
• It’s a second order system with an opamp in it. Used some miller compensation to split poles apart. When a step current is applied at a point in the loop, I’m observing an overshoot inspite of PM being close to 90 degrees
– SBO
Sep 12 '19 at 21:06
• Show your circuit? It certainly seems like it should settle nicely. Sep 12 '19 at 21:58
• Also would be good to show how you're measuring the open loop response. How you're "breaking" the loop, injecting the disturbance and where you're measuring the response. Sep 12 '19 at 22:09
• It doesn't look to me like your loop gain measurement technique is accounting for the FET's gain. Sep 13 '19 at 7:06

## 1 Answer

I suspect you're looking at an artifact of the propagation delay through the op amp. The output overshoots because the effects of the negative feedback aren't felt until a finite time period after the response to the step.

If there's a propagation delay of N ns, the step response won't produce an effect on the output until T=N, and thus the feedback won't produce an effect until T=2N.

• Surely, loop analysis characterizes precisely all of that..? Sep 13 '19 at 14:38
• @AdilMalik - No, loop analysis essentially looks at the response to a CW sine wave after all perturbations have died out. You are looking at a step response, where the effects of the higher-frequency non-linearities are still in force. Sep 13 '19 at 17:32
• Yes ofcourse. I agree that frequency response is done on a linearised model. However, this is not what this answer is about and what I am questioning. Christobol refers to delay thru the opamp which should be characterised in the linear model etc? Sep 13 '19 at 18:28