Re: how to decide the bias-voltage of the folded-cascode opa
Well, simply put, when in a branch, there is a PMOS and an NMOS, they have to have their, β's matched. It is mostly a bipolar concept. But it applies for MOSFETS well. So, when you are designing your circuits, you have to look for the variations in a NMOS and PMOS for a given W/L. The best example is to run a test simulation of a common source amplifier with PMOS load. Then see how the variations will affect the vds,sat of the PMOS. Then you can do it reverse way. Use a PMOS common source stage, and then with NMOS load. Try and run the same simulation. The important part is that, the currents in both the simulations should be equal. Try to run this over corners, you will get an idea about the sizing of your MOSFETs. This will help you in understanding the FS and SF corners.