The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Two Stage Op Amp Pole Splitting Compensation
https://designers-guide.org/forum/YaBB.pl?num=1488332040

Message started by DanielLam on Feb 28^th, 2017, 5:34pm

Title: Two Stage Op Amp Pole Splitting Compensation
Post by DanielLam on Feb 28^th, 2017, 5:34pm

Hi all,

Has anyone run into a situation where pole splitting was not recommended to compensate a 2 stage opamp? I suppose the alternative compensation scheme would be dominant pole by putting a big fat capacitor to ground at the intermediate node.

Thanks,
Daniel Lam

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by ULPAnalog on Mar 2^nd, 2017, 7:29am

I have once used feedforward compensation to compensate a two stage opamp in an high bandwidth application, the architecture of which is similar to the one reported in "A 56 mW Continuous-Time Quadrature Cascaded SD Modulator With 77 dB DR in a Near Zero-IF
20 MHz Band" by L. Breems et al.
As claimed by the authors, this topology gives the gain of a two stage op-amp and speed of a single stage amplifier.

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by deba on Mar 2^nd, 2017, 8:56am

In analog LDOs with pMOS pass transistor, some times miller compensation is not used as the PSRR spec is to demanding. In such cases, putting a cap at the intermediate node connected to the supply is used for improved PSRR performance.

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by DanielLam on Mar 6^th, 2017, 11:51pm

Thanks for the replies. Learned quite a bit.

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by AnilReddy on Mar 9^th, 2017, 4:38am

Hi Daniel,

could you let me know what were the conditions in which a pole splitting is not recommended?

i think it depends on what load cap we are driving. if the load cap is too large, then we may end up with even bigger comp cap which may not be ok.

thanks,
anil

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by DanielLam on Mar 9^th, 2017, 10:14am

Hey AnilReddy,

The question just popped in my head one day. I didn't have a specific situation. But looking at the other 2 replies, they are some good examples. I think I saw an article mention the PSRR the other day.

Regards,
Daniel Lam

Title: Re: Two Stage Op Amp Pole Splitting Compensation
Post by DanielLam on Mar 31^st, 2017, 7:23pm

Thought I'd just give some results on PSRR of a conventional 2-stg opamp with/without the compensation capacitor.

The device lengths were 1 um. The PSRR defined here is the AC gain from the worst case rail to the output. I know there may be some other PSRR definitions.

With miller cap: -38 dB
W/o miller cap: -85 dB