I am simulating the open-loop characteristics of a two-stage op amp. The simulation circuit is as follows (using a large resistor R0=1Gohm and large capacitor C2=1KF to form a feedback loop).
I performed a pz simulation. The input is v2, and the output is Vout.
The pz simulation results are as follows.
There are two poles here, which correspond to the pole positions after Miller compensation.
But I think there should be a mirror pole and a pole contributed by the tail current source (M5).But pz simulation does not give these two poles.
Later, I reduced C2 to 1F, as the following circuit.
Then, the pz simulation gives the following zeros and poles.
This time there are several more poles and zeros, including the mirror pole, the pole contributed by the tail current source (M5), and the two very small poles and zeros introduced by R0 and C2.
Why is this so? I think if the topology of the circuit remains the same, the number of poles and zeros should be the same.
Why is the value of C2 different and the number of poles and zeros given by pz are different?
Why does the pz simulation not show the mirror pole and the pole contributed by the tail current source (M5) when C2 is equal to 1K?
Thanks you very much.