Question about one-stage full differential Op amp

[yushen_yang]

Hi all,

Thanks for those who gave me advice on the topic I posted couple days ago. Unfortunately, my problem is still there.

I post my question again and also I upload the schematic and wave picture. Hope it can give you more clearly understand where my problem is.

Question as below:
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

 

[Btrend]

1. From your waveform, I see that ur v(4) is too high to make the CMFB PMOS in active region. because if your common mode voltage is 2.5V , then the Vdssat of top CMFB PMOS is (5-2.5-|Vtp|)~=1.5V, but the waveform show that u only have 0.2V around, so ur CMFB loop gain is such small that it can't not set the correct CMFB voltage.
2. From ur waveform, ur common voltage is not 2.5V. it is above that. the formulat for common voltage is Vcom=(Vout+Vout1)/2. u can use awave to calculate that value.