Hi,
Steve_IC wrote on Jan 28th, 2006, 5:24pm:what i mean is Vcm is the desired output CM voltage, Voc is the CM output voltage without cmfb which is set by input common mode, my question is if we have to make sure Vcm and Voc pretty close.
As you will define Vcm so that the output swing is maximized, you definitely should have Voc close to Vcm... I still don't see exactly the point, because once you implement the CMFB, there will be no direct relationship between the input CM and the output CM, unless you drive some devices of the CMFB out of saturation. I assume in this statement that your amplifier input goes to a MOS gate, of course.
Quote:My thought is just to get max output swing, first we may do a common mode input dc sweep to find the middle point of the output swing without cmfb, which we have to choose it as our desired output CM voltage in our cmfb loop.
I would suggest to first implement the CMFB, then determine the input CM so that your differential pair is in a correct region of operation. Once this is done, sweep Vcm in simulation to determine the optimum output CM for your specifications (gain, swing, ...). This corresponds to the real mode of operation of your amplifier, while the input CM sweep is not so realistic. It may drive out of saturation some devices which are not directly related to the output swing of your two-stage amp.
Paul