Folded cascode opamp 130 nm

[rramnani]

I am designing a PMOS input folded cascode OTA, specs are VDD = 1.2 V, current limit = 1mA, Gain>=60dB, Output Swing = 1 V (differential), UGBW >= 800 MHz.
V,CM (output) =0.6 V. Can anybody suggest how do I go about improving the UGBW from 400 MHz(present) to 800 MHz ?
Please let me know if I missed something.
Thanks

 

[Dominik Przyborowski]

You can used gain boosted cascodes to increase open loop gain (without degrading a bandwidth), feedforward compensation technique to compensate second pole and improve transconductances of input pair (for 1st order amp GBP = gm/Cload). Check this papers:
Feedforward compensation
a few words about cascode loads

//edit: At the last 4 years peoples increasing bandwith and transconductance by recycling techniques. Look for "recycled folded cascode" and "improved recycled folded cascode" papers

 

[yuvan]

Hi, I agree Gain boosted cascoded is gonna increase the gain but atleast I guess the Gain bandwidth product will remain the same. And also the for regulated cascode, the problem of pole zero pair will also be not very far from UGB.

 

[rramnani]

Hi, thanks Dominik & Yuvan.
Dominik, I am allowed to use the same configuration without using gain boosted cascodes (Don't you think they'll introduce extra poles from the tiny amplifiers?)
I will look into these papers.

I am trying to increase I (tail current) and W/L by twice of the input pair. (gm = sqrt (2*I*Kn*W/L)).
But It doesn't seem to help. Any suggestions?

Thanks.

 

[yuvan]

Hi,
Can you tell us about the pole locations? And as Dominik suggested you can also try out the recyclic folded cascode architecture.I used it in the past and it indeed improves.

 

[sudheerkm]

forget the current limit in the spec for a while;

increase bias current 4 times , i think u will be there at 800MHz

 

[rramnani]

Yuvan,
f1= f3dB = 642.19 KHz
f2 = 700 MHz
OK I will try that approach, Thanks.

Sudheer, I tried what you said, but doesn't work :|
Any suggestions welcome.

 

[Dominik Przyborowski]

What is a configuration of your ota? Is it a voltage, transimpedance, switched capacitor amplifier? CMFB is continuous or SC? I'm asking because I want to try to check if I'm able to meet a specification of your OTA;-)
Maybe I'll be able to find some sugestions...

//edit: And also tell sthing about load of your ota.

 

[rramnani]

Its a transimpedance amplifier (Vi --> Id using the input stage, then Id--> Vo using the pmos cascode load). CMFB is SC. Load is 675 fF at each output.