What should be the specifications of cmfb for OTA design?

cmfb consideration

Hi everyone,

I'm designing a full differantial folded cascode OTA with a cmfb. The fd amplifier is doing by one of my friend and I have to design the cmfb. I already choosed a sheme but I don't really know the performance that I have to reach in order to get the best as possible cmfb ??? somebody could help me ??? I know that I have to take care about the stability since we get a feed back. But what about speed, impulse response .... (I'm not a specialist ...

BTW what means OTA?:|

Thanks forward for your help !

jem

Re: cmfb consideration

Hi Jem,

Attached is the design notes of the CMFB which you are looking for. I would suggest you to look on journal papers as well on the topic of CMFB. From there you will get the ideas on the design aspect and the factors like stability, settling time and so on. OTA stand for Operational Transconductance Amplifier which give you the I/V response, in other word it sense the input as voltage and provide output as current. The output impedance of the OTA will be high thus it will be able to provide you constant current for voltage variation. You can get more ideas on OTA from many Opamp design chapters.

Thanks,
Suria

Re: cmfb consideration

Thanks suria3. I allready read this notes but I didn't find a really clear information about how the gain, step-response has to be in order to achieve good performance.

Somethingelse?:|

Re: cmfb consideration

Hi Jem,

If you are looking on the further explanations on the CMFB design, im not sure whether you have read Razavi's CMOS Design book. If you haven't gone through the book, there is a sub chapter on CMFB design, so you can have look on that. Generally, in designing the CMFB circuit, one must design it to have a large gain, thus the offset voltage will be lesser when it comes for voltge detection at 2 points, so the CMFB output will compensate that voltage accordingly, thus the Common mode DC voltage is kept constant. I will see if i have any other good notes on CMFB design, then i will send you.

It will be very nice if other experience people can give feedback on the post about the queries on gain, step response and so on.

Thanks,
Suria

cmfb consideration

There is a chapter in Paul Grey's book about the CMFB and he gives a clear explanation. Hope it's helpful.

Re: cmfb consideration

Thanks mens,

I get this two books and that's right, they give very good explanation about designing cmfb. I'm going to do that soon, and I will certainly have some questions later ...