Phase margin is 90 degrees but transient step response shows overshoot

Question

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?

Cristobol Polychronopolis · 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.

Phase margin is 90 degrees but transient step response shows overshoot

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