one stage foled cascode OTA

[coxstreet]

Hi, I am using the classic folded cascode one stage OTA for the specification: gain=70dB, phase margin=60 degree and Unit gain frequency=90M Hz.
load capacitor =50p F

1. How to increase phase margin without decreasing gain, unit gain frequency and also without increasing load capacitor?

My simulation result: dc gain=70dB, Unit gain frequency=98M Hz, phase margin = 42 degree.

2. How to increase gain without decreasing unit-gain frequency?

My simulation result: dc gain=63dB, unit gain frequency=98M Hz, phase margin=60degree

3. How to increase -3dB frequency without inceasing load capacitor and also without decreasing gain and unit-gain frequency? Thanks!

COX

 

[Dominik Przyborowski]

Second pole which decreasing your PM is bounded with cgs of cascode devices, so You can try to decreases dimensions of cascode (holding the same W/L ratio). Other method for fast single stage opamps is to use a feedforward compensation method.

Check this classic paper - W. Sansen, Z.Y. Chang: "Feedforward compensation techniques for high-frequency CMOS amplifiers", IEEE Journal of Solid State Circuits, vol. 25, no 6, pp 1590 - 1595.

//edit: If your technology provides high enough current gain factor You can also add some degeneration to current sources - this is helpfull sometimes ;-)

 

[coxstreet]

Hi Dominik, yeah decreasing the size will make the second pole larger but smaller L degrades gain. I will read the second method you mentioned. Thank you!

Cox