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?