Why does the phase of the opamp increases for a short while and then decreases?

i am designing an opamp but the phase of the opamp looks very weird....please see the attached file. Why does the phase of the opamp increase for a short while and then decreased? Normally, the phase will keep either keep decreasing or staying constant...but increasing? does that mean that the op-amp is not stable?

Re: phase...

Hi,
It depends on the gain figure of the opamp which you didn't attach, but as the phase figure says you have a zero near the first pole of the circuit (a pole-zero doublet) and if they are close to each other, the time domain behavior would have some problems.
Usually, a doublet arises from mismatches in pole-zero compensations due to process tolerances, or is caused by the frequency limitation of current mirrors when they are used for a differential-to-single conversion, or it comes from the feedforward capacitor in a voltage follower. The presence of a pole-zero doublet does not appreciably affect the frequency response of the closed-loop amplifier, but it can greatly modify the time response.
Nevertheless, pole-zero doublets can be used for compensation of OTAs [1], and also in audio applications in which a large gain is required without having a excessively large unity gain bandwidth [2].

[1] Analysis and compensation of two-pole amplifiers with a pole-zero doublet Palmisano, G.; Palumbo, G., TCAS1 1999.

This paper is here:


[2] A High-Linearity 50-Ck CMOS Differential Driver for ISDN Applications R. Castello et al., JSSC 1991.

I have attached this paper.

Re: phase...

thank you very much for giving a detailed explanation....i think i get it now...