question about one-stage full differential Op amp

[yushen_yang]

Hi all,

I am designing a one-stage full differential cascode op amp which are stacked with two pmos and two nmos in each path. the lower nmos pair is the input. and then a extra nmos acting as a current source connects ground and the lower nmos pairs. a pair of pmos triode device are used to be the CMFB.

My problem is the output common is at the middle value of power supply. and I check thousand times that all the transistor are working on the right mode.(even if the maximum swing won't change the trasistor working mode) however, the output swing is unbalance where the positive swing are relatively smaller than the negetive swing. did any one have this problem before?

spec:
0.5um process
Power supply 5V
ouput common mode: 2.5V
gain 54dB
oupout swing +/- 1V

 

[hr_rezaee]

hi
send your netlist and model.

 

[suria3]

Hi,

What i can assume is your tail transistor could go to triode region which will effect your gm, gain, thus the output signal is not amplified accordingly and unbalanced. Is that for all the range of input swing (small to large) your amplifier gives unbalance out. If only for large swing it gives unbalance swing, then it could be because of the tail transistor working in triode regime, but if it didnt work for both range, then could be other factor.

Regards,
Suria

 

[nxing]

Did you swap the input polarity? What's the result. If the output swing don't swap accordingly, that means something wrong with you output leg.

 

[huojinsi]

ur circuit operating point isn't correct.Pls check operating state of ouput transistor carefully!