raja.cedt wrote on Nov 22nd, 2011, 3:29am:@Robg ....due to sourse follower CMRR will be bad, how can you say that CMRR is still better when you are at midrail, becaz untill unless both source followers turn off CMRR is bad i guess
Raj - I didn't mean to say it was better at midrail, just that it was still good (I don't know for sure - but "eyeballing" the circuit I think it should be acceptable). The reason is that as one source follower (SF) shunts current, the other SF is debiased by approximately the same amount, thus the combined top+bottom tail current is about the same. Depending on how you sized the SFs you could optimize CMR or constant gm.
Hai should be aware that mismatched offsets in the diff pairs gives dual diffs bad CMRR. That is to say that when input is low, the offset is from the pDiff, and when the input is high the offset is from the nDiff. Thus the difference in offset results in an output voltage error that is dependent on input common mode. I expect that this error is at least as large as the one caused by the SFs.