what is the usage of the filter-like block in the opamp ?

[heverlee]

Hi:

I have a small question about this opamp compensation as shown below. the feedback is not shown but you can see C2 is used as the compensation cap. on the other hand, there is a RC filter (R1-C1) used at the output of the 1st stage folded-cascode amplifier. This block indeed improves the compensation result. Can any one help explain me the principle of using this block to aid opamp compensation?

thank you very much

 

[diemilio]

Re: what is the usage of the filter-like block in the opamp

What do you mean by "improves the compensation result"?

 

[LvW]

Re: what is the usage of the filter-like block in the opamp

..............there is a RC filter (R1-C1) used at the output of the 1st stage folded-cascode amplifier. This block indeed improves the compensation result. Can any one help explain me the principle of using this block to aid opamp compensation?

It´s a so called "snubber circuit" which, indeed, improves stability.
It introduces a pole as well as a zero (at a somewhat higher frequency) - thereby reducing loop gain at the upper frequency region (without making the slope of the loop gain steeper !) and thus improving phase margin.

 

[heverlee]

Re: what is the usage of the filter-like block in the opamp

thank you for your kind reply

..............there is a RC filter (R1-C1) used at the output of the 1st stage folded-cascode amplifier. This block indeed improves the compensation result. Can any one help explain me the principle of using this block to aid opamp compensation?

It´s a so called "snubber circuit" which, indeed, improves stability.
It introduces a pole as well as a zero (at a somewhat higher frequency) - thereby reducing loop gain at the upper frequency region (without making the slope of the loop gain steeper !) and thus improving phase margin.

 

[drDOC]

I think we are also looking at a sort of cascode Miller frequency compensation trick. Or active Miller zero compensation.

A normal miller cap would go from gate M11 to vout.

But I think in this case there is a cascode M1 and then C2 goes to the output. Now the miller feedback has a gain of C2/C1 for high frequencies. This means that your bandwidth increase by this amount. The resistor R1 works as a snubber giving more phase margin.