Jacki wrote on Jul 26th, 2019, 8:37am:I am pretty sure it is not due to the startup circuit, I have added one to guarantee the bandgap core circuit to avoid the all "zero" state.
In the statistic corner, I can see the mismatch of the input stage of the OTA, threshold voltage mismatch is around 5mV, but I don't think it is such a big issue.
I suggest to check the mismatch case only, where the BG did not work. The information that the input terminals of the opamp goes to ground, suggest me that the start-up circuit fails in that corner, and/or the loop gain is not enough in that corner. Double check that the start-up is really functioning. Just adding some startup circuitry might not work in all corners, and it could give you false sense of confidence.
Triode region for the cascode means that you have - much - smaller output resistance on your gain node, and therefore the gain if the amplifier will be compromised. You should be able to see this in small-signal analyses.