How to improve PSRR+ in a two stage op amp?

[ray.deng]

This is a two stage op amp. The first stage is a folded cascode and second stage is common source stage. I'm directly using the second stage driving the output resistance. I'm measuring the PSRR+ by connecting the output to the negative input while grounding the positive input. See the schematic.

With a singal source at the positive voltage supply, I got a PSRR+ at about 22dB at 1Mhz. However, the project spec requires 40dB at 1Mhz. What should I do to improve the PSRR+? By the way, PSRR- seems to be fine. It is about 60dB at 1Mhz. Thanks!

 

[amriths04]

Re: How to improve PSRR?

One simple way is to shift the frequency response of your open loop amplifier upwards over the entire frequency range, till it's UGB. This way you increase the loop gain and hence the supply noise would be rejected by the loop.

Another way is to put big caps at the supply, bypassing the supply noise to ground.

 

[dick_freebird]

Re: How to improve PSRR?

One thing is, to bypass current-mirror gate nodes to their
reference rail (vV+ or V-) to ensure that the bias is as
supply-rejecting as practical. V+ pulls directly on the
front end and common-mode current deviation on the
diff pair pushes through the whole rest of the part.