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lnarayan

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Common Mode Feedback
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

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SATurn

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Re: Common Mode Feedback
Reply #1 - 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

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lnarayan New Member Offline Posts: 2	Re: Common Mode Feedback Reply #2 - 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
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mc66 New Member Offline Posts: 6	Re: Common Mode Feedback Reply #3 - 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
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