A question about cascode frequency compensation!

[daisyinfall]

cascode frequency compensation

As cascode frequency compensation will bring higher PSRR,I apply it in a telecascode two-stage OPamps. but I find: as the compensation capicitor increasing,the phase curve and the gain curve will be distored. Although I can reach 60 degree phase margin,the gain margin will not be enough.You known,the gain margin is just 5 db......

WHY?
And what can i do to get enough gain margin?
Thanks!

Regards!

 

[xerxer]

cascode frequency compensation

check your current ,it maybe too high ,i think.

 

[daisyinfall]

check what ? compensation capacity?

Is there any paper aboat how to use cascode compensation?

 

[sanjiv]

please see attachement.

 

[rambus_ddr]

I think that since the compensation cap Cc increase, the left plane zero created by the Cc and gm3 of M3 decreases and lower than unit gain frequency, which makes the slope of gain decrease, so the gain margin decrease.