Hi RobG,
You are correct. The load has nothing to do with the rise/fall time. I made a mistake when I compared two configurations. The output step of the unity gain feedback was half the amplitude of the step of the gain=2 feedback. The supply is single 3.3V.
Quote:2) The tail current source seems to be very large.
3) The slow rise, fast fall would be consistent with capacitance on the tail current source stealing all the current from the diff pair. Be aware that the diff pair bulk connection to the tail node will add even more capacitance and it isn't modeled.
The rise/fall time becomes more balanced when I replace the tail current source with an ideal current source. Then I tried to reduce the capacitance of the source node of the diff. pair. It does not improve much.
I found the tail current decreases by half when the inp/inm increases. It is curious because that the Vds is still much bigger than Vdsat.
In the following picture, net016 is the source of the differential pair, M3/s is the tail current, M1/S and M2/S is the source current of each MOS transistor of the differential pair.
net016 increases by near 1.3V, so the parasitic capacitor must have steal some tail current. But why M3/S decreases by almost half?
Thanks very much for your help.
Yawei