About this AC response of amplifier

[gaom9]

Hi,
I am designing an amplifier for a DAC output driving application. And the structure I chose is Rail-to-Rail input stage + class AB output stage, and the main ampilfier is folding ampifier with gain boosting assistant amplifier. The output driving ability requied is 400Ω+500pF, but the AC response of the amplifier is quite strange as shown below. Will this amplifier be stability? How to calculate its phase margin? Will it need to be compensated? What cause this kind of response?
Thank you!
Best regards!

 

[mengcy]

it seems that it's not stable.
you have to find why the zero occur at low frequency.have you add your Rload and Cload when you did the simulation.
Usually,the main pole is put inside for a class AB output stage.

 

[gaom9]

it seems that it's not stable.
you have to find why the zero occur at low frequency.have you add your Rload and Cload when you did the simulation.
Usually,the main pole is put inside for a class AB output stage.

Hi, mengcy
I added the Rlaod=400 and the Cload=500pF when simulation.
The ClassAB output stage is the Push-Pull, Common Source Amplifier.
The output node of the main amplifier is good.
But I can not find the reason.

Thank you!
Best regards!

Below is the step response of this ampilier with 500pF Cload. It seems good and has a large slew rate about 80V/us. But I still worried about it.