how to decide the bias-voltage of the folded-cascode opamp??

[ronanchang]

How to decide the bias-voltage of the folded-cascode opamp??
My folded-cascode opamp is correct in FF-mode,but incorrect in FS-mode and SF-mode.
Is there any efficient method to make the opamp correct in all mode ???

 

[jiangwp]

Re: how to decide the bias-voltage of the folded-cascode opa

Maybe you can give a more margin for the saturature voltage for the load and current transistors (or the overdrive), so you can reduce the bias level for PMOS and increase the bias voltage for NMOS .

The best way is that you can calculate the size for every transistors by hand , then regualte it iteration.

 

[Vamsi Mocherla]

Re: how to decide the bias-voltage of the folded-cascode opa

Well, the problems seems like that your devices are less matched β-wise, where β = K. W/L. Hence to make them more β matched, you have to increase the VDsat values in your design calculations or try and increase the sizes of the devices. that will take care in Fast -Slow and Slow - Fast corners. I hope that I am clear on this.

 

[Analog_IC]

Re: how to decide the bias-voltage of the folded-cascode opa

Vamsi,
I donot understand about beta matching .. can u suggest some document...

 

[Vamsi Mocherla]

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.