How to get high UGB(600 MHz) for a two-stage fully differential telescopic OPAMP ?

[TheDoodlebug]

Hey guys! Desperately need help regarding a design assignment.

I should use a fully differential, two-stage [telescopic + gain stage] opamp of 0.35um technology and Vdd 3.3V.

The specifications to be met are UGB>600 Mhz, P.M = 60deg and Settling time < 20 ns.

I have read several research papers, but most of them deal with increasing gain or swing for a small UGB. BUT gain is not a concern for me and I can't find any paper dealing with high UGB.

Some sources suggest using a nullifying resistor compensation method. Does it work for this topology and is it worthwhile reading about it ?

Do you guys have any other suggestions in meeting the high UGB specification ?

 

[Dominik Przyborowski]

It depends to your power consumption and loading capacitance constraints. With high enough current (2mA) and resonable Cload (10pF), the simple design should met your specification.

 

[TheDoodlebug]

Thanks for replying. The power consumption should be <3mW and the capacitances >500fF. After some consideration, I've decided to go ahead with the nullifying resistor compensation.

 

[Dominik Przyborowski]

For better PSRR and GBW You should use cascode compensation (look for the classic Ahuja, Copeland and Ribbner and Fischer IEEE SSC papers from 1983-1985) especially that your first stage is a telescopic cascode. For load capacitance in order of 1 pF You are able to choose small current ratio between 2nd and first stage, i.e. 2-3 to obtain stable opamp. In this thread You can found some usefull informations also.