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some questions about pz analysis

zuiying

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.

  • ShawnLogan

    Dear zuiing,

    zuiying said:

    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.

    Unless I overlooked it in your post, I am not sure what you have specified in the pole-zero analysis GUI options panel shown in Figure 1. There are options to filter out poles and zeroes above a certain frequency as well as to filter out poles and zeroes that are close to one-another (i.e., pole-zero cancellation). The latter are also dependent on the accuracy settings absdiff and reldiff.

    Filtering can also be performed by the Direct Plot form following the pole-zero analysis, and I am not sure if you specified a filter in that GUI.

    zuiying said:

    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?

    Clearly the poles and zeroes will vary in location with your value of C2 and hence if you are doing any filtering of the poles and zeroes, you could potentially filter them out if you have multiple values for C2 - especially if C2 is varying by a factor of 1000.

    Once again, I am not sure of the details of your pole-zero analysis and hence this may not be applicable. You might, however, consult the Spectre manaul to review the default values for its analysis as they may not be appropriate for your very large values of C2 and R0 - clearly these values are not realizable and may present challenges to spectre's default accuracy settings.

    Shawn

    Figure 1

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