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Design >> Analog Design >> High speed opamps
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Message started by venkatakalluru on Jul 13^th, 2006, 10:35am

Title: High speed opamps
Post by venkatakalluru on Jul 13^th, 2006, 10:35am

Hi,
I am designing a highspeed opamp for a pipelined ADC application. The required UGB is anywhere between 800Meg to 1G. The design is on 0.35u process. I have chosen folded cascode with gain boosting technique for the amp (A fully differential opamp with SC-CMFB). To meet the high gain spec , I have no other option than to pump more and more current into the opamp. This made my transistors too big. Though the schematic is working fine, I encounter problems when I simulate C-extracted version. The parasitic caps due to huge transistors prove fatal to my phase margin and speed. Now my question is

1)Before proceeding into high speed design, are there any subtle points I need to take care of?
2) Is there anyway I can bypass these huge sizes of transistors?

Any reference or book on this issue will be highly useful.

Thanks
kalluru

Title: Re: High speed opamps
Post by TT on Jul 14^th, 2006, 6:05pm

What do you call a C-extracted version ? If you are talking about parasitic extraction after layout then
I think your design flow may have a bug. The simulated version should use a model which include at least
gate source capacitors. Then if after C extracted version you have a bug then either your interconnect
are too important (which I doubt compared to Cox density) either the parasitic extractor may not
get the correct values.

Usually at these UGB freq, you'd better use large Vgs if possible, even if from a gm/Id pt of view it is not the best
option. (be careful with 1/f noise) Try to use small current and small size first, because using large current requires
large W which gives large cap then need large current, etc ...

Title: Re: High speed opamps
Post by loose-electron on Aug 4^th, 2006, 8:28am

Do you really want an op-amp here?

The stated bandwidth was up to 1GHz. Typically I stop using feedback amplifiers above 100MHz or so (for 0.18 CMOS types of processes)

Consider some common source differential pair amplifiers, without feedback. and you will need to do some correction circutis for gain errors, watch your linearity, and things like that.

Title: Re: High speed opamps
Post by Faisal on Aug 7^th, 2006, 5:48am

Interesting problem, you have on your hands ;D

What is the intended speed of your ADC.. or specifically your switched capacitor gain stage (settling phase in nsec) ??