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Design >> Analog Design >> opamp/bias from CMFB
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Message started by ontheverge on Dec 7^th, 2010, 8:42pm

Title: opamp/bias from CMFB
Post by ontheverge on Dec 7^th, 2010, 8:42pm

Hi,

In designing opamp circuit, is it desirable to bias output common mode different from target Vout_CM to have consistent feedback current to the input pairs, or should ckt be biased so that Vout_cm is almost the same as its target value, what are the considerations here?

thanks,

Steve

Title: Re: opamp/bias from CMFB
Post by raja.cedt on Dec 7^th, 2010, 10:21pm

try to keep same but across corners it might be tough, but whatever the designed common mode , CMFB loop will try to make it equal to Vref just by introducing systematic offset.

Thanks.

Title: Re: opamp/bias from CMFB
Post by carlgrace on Dec 8^th, 2010, 1:15pm

Echoing what Raja said, usually the desired common-mode level is a the point of maximum swing, and generally it is a convenient voltage to generate. (for example, sometimes it is mid-supply). But, it really doesn't make much difference because the feedback loop will take care of any offsets.