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Design >> Analog Design >> CMFB circuit for Two stage Opamp
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Message started by mateenfl on Sep 29^th, 2003, 8:01am

Title: CMFB circuit for Two stage Opamp
Post by mateenfl on Sep 29^th, 2003, 8:01am

Hi,

I am simulating a two-stage opamp (first stage is folded cascode and the 2nd stage is common source) with class A output stage. I am using cascode compensation scheme.

I am looking for a CMFB circuit for this opamp. Any links/papers describing the CMFB circuit in detail,will be welcomed.

Kind Regards,
Faisal Mateen

Title: Re: CMFB circuit for Two stage Opamp
Post by Paul on Sep 30^th, 2003, 11:41pm

Hi Faisal,

for CMFB it should be sufficient if you act on the CM voltage in your folded cascode 1st stage while measuring your CM output at the output of the 2nd stage. The following paper shows an example for CMFB in a folded-cascode amplifier:

"A high-speed fully differential CMOS Opamp", by A.Younis, presented at the Midwest Symposium on Circuits and Systems 2000. You can get it from the IEEE website.

There's also some examples in "Design of Analog Integrated Circuits and Systems" by Laker and Sansen, McGraw-Hill in chapter 6. You may find some info in any advanced CMOS design textbook targeting amplifier design.

Paul

Title: Re: CMFB circuit for Two stage Opamp
Post by Frank Wiedmann on Oct 1^st, 2003, 12:36am

You can also find some information about CMFB circuits in section 12.5 of "Analysis and Design of Analog Integrated Circuits" by Gray et al. (http://www.amazon.com/exec/obidos/tg/detail/-/0471321680/) and in section 6.6 of "Analog Integrated Circuit Design" by Johns and Martin (http://www.amazon.com/exec/obidos/tg/detail/-/0471144487/).

Title: Re: CMFB circuit for Two stage Opamp
Post by EVI on Dec 1^st, 2003, 6:24am

Hi Faisal,

To my experience it is very difficult to design a single stable CMFB circuit covering both stages of the opamp because you will effectively have three inverters in the loop that form a ring oscillator and it is hard to make such a circuit stable. It is possible to make it stable but by the expence of the CMFB bandwidth. By using two CMFB circuits (one for each stage) you will avoid this problem. B. Razavi in "Design of Analog CMOS Integrated Circuits" on page 324 also confirms this: "It is important to note that fully-differential two-stage opamps require two CMFB networks, one for the output of each stage".

Eugene.