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Design >> Analog Design >> CMFB question
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Message started by 4analog on Feb 23^rd, 2006, 7:00am

Title: CMFB question
Post by 4analog on Feb 23^rd, 2006, 7:00am

Hi,

I have a question about the common mode errors. Why is the common mode error not a problem in single ended diff pairs...?

The way I understand the common mode errors is that it occurs when there is a mismatch in the PMOS and NMOS current sources which cause the output at each end of the diff pair to change to match there currents.

This case can also occur in a single ended dif pair, but why do we have CMFB ckts only for fully-differential amplifiers..?

Any reponse is apprecited.

Thanks

Title: Re: CMFB question
Post by ACWWong on Feb 23^rd, 2006, 8:09am

i think when you have a drain of a pmos meeting an nmos you do generally have to have some way of defining the DC point... which in a single ended case is usually done by the next stage or feedback or something.
in differential circuits, with differential outputs, a convenient way to do this is CMFB (detect the commonmode by averaging the differential output and use that in a control loop).
In the single-ended output case there is no "CM" as such, just the output DC level which might need controlling, so it these cases it is not termed CMFB as your are not feedback the common-mode.

Title: Re: CMFB question
Post by zhonghan on Feb 25^th, 2006, 12:39am

For single ended differential amps, the load of the diffrential pair is a current mirror but not current source. When the current of the tail current sourse is increased, the current mirror can make a response. For full differential amps , the upper current sourses are also biased , which can induce mismatch.