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Design >> Analog Design >> Common Mode Feedback
https://designers-guide.org/forum/YaBB.pl?num=1174965451

Message started by lnarayan on Mar 26^th, 2007, 8:17pm

Title: Common Mode Feedback
Post by lnarayan on Mar 26^th, 2007, 8:17pm

Hi All,
Greetings to every one on board. I'm a new member here and this is my first Post.
Could anyone tell me the need for Common Mode Feedback(CMFB) circuit in fully differential op-amps and how they are realised? I went through a few books and articles and I still find the concept very elusive . If anyone could explain the CMFB concept in an understandable manner, it would greatly help.
Thanks in advance
--lnarayan

Title: Re: Common Mode Feedback
Post by SATurn on Mar 29^th, 2007, 12:23pm

Hello,

Actually, the concept of CMFB is very simple. You are just going to set the CM value of two differential high impedance nodes on desired value. If you have not such a feedback, the CM value could have any value and even stick to one of the supplies or put the devices on an undesired regime. The main reason is that it is very difficult to make sure that bias current of PMOS devices is the same as NMOS devices.

You can for a more complete discussion have a look on B. Razavi's book (Analog CMOS Design).

SATurn

Title: Re: Common Mode Feedback
Post by lnarayan on Mar 30^th, 2007, 8:09am

Hi,
Thanx a ton. The explanation in Razavi's book was amazing. It was very helpful.
thanks again
--lnarayan

Title: Re: Common Mode Feedback
Post by mc66 on Apr 4^th, 2007, 7:20am

one node output,you connect one load transistor gate and drain together,that sure the mos work in right region

but tow node output,you need do something to sure all mos work in right region,that's why we nedd CMFB