I am a newbie coming across a opamp designed by a guru:
Are those caps on the right-top corner used for cancelling zeros?
And C0-C3 for Miller compensation?
What are those transistors connected by 1.15V used for? they connect cpp/cnp to the input transistors' source.
Well, this circuit is a simple two stage differential amplifier with the first stage being folded cascode. The caps C0 - C3 are indeed for compensation but not miller. They are for Cascode compensation technique.
Coming the transistors connected to 1.15 V, I am not sure where the sources are connected. Mostly they will be for providing the lead compensation and hence might act as resistors.
The output class-AB control transistors M25-M28 have the same size. Is it a design flaw? It seems to me that the PMOS & NMOS should equally devide the current in quiescent?
You are right, the CLASS AB control should be ratioed as per the current flowing. Then the PMOS and NMOS have to be sized as per the current, giving similar VGS values
The output transistors are in right ratio. I was talking about the two pairs of NMOS/PMOS connected in parallel before the gates of the output transistors. They are used to control the voltage difference between the output gates. If you still feel like to answer the question, you may need to check the schematic. Thanks.
M25-28 are for level control. I just read from some books. It's suggested pmos/nmos should be equal to the output transistor ratio. But in the schematic it's not.
C0-C3 they call Cascode Miller Compensation.
I have doubts with the caps on the up-right corner. They are connected to some switches. I doubt they are like resistors with caps for zero cancellation. But I am not sure.
ss is the source of the two input nmos transistor. Check out the sch. I am still not clear why it's connected to 1.15V through transistors.