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Design >> Analog Design >> Folded Cascode OTA design - Help
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Message started by ashwin87 on Oct 17^th, 2011, 10:05am

Title: Folded Cascode OTA design - Help
Post by ashwin87 on Oct 17^th, 2011, 10:05am

Hi,

I am currently designing a folded cascode amplifier. I have designed the cascode transistors with Vov = 100mV. Vdd = 2.5 V. Vinput common mode = 1.25

i simulated the DC transfer curve for Vicm = 0 to 2.5. I got all the transistor in saturation from 1 to 1.8 V (constant slope). When when I simulated for output swing, upon giving differential input signals, my cascode leg transistors go into triode region. range of differential input voltage give is from +- 0.05

Please help me look at the direction to solve this problem.

Thanks
Ashwin

Title: Re: Folded Cascode OTA design - Help
Post by niwin on Oct 24^th, 2011, 2:18pm

You should not measure output swing open loop (well, I guess you could, but I would not recommend it). When you apply a large differential input signal directly, you are causing large signal currents to swamp out the cascode transistors. The cascode transistors are going to try to sink/source this extra current, and when they can't, they will go into triode.

In normal usage, you will employ negative feedback to keep the two inputs at the same potential.

See page 6 of the following document for an example:
http://www.mrc.uidaho.edu/mrc/people/harry/ee415/class_handouts/two_stg_char.pdf

Title: Re: Folded Cascode OTA design - Help
Post by loose-electron on Oct 24^th, 2011, 3:14pm

A schematic-picture of your circuits and test configuration would be useful

Title: Re: Folded Cascode OTA design - Help
Post by raja.cedt on Oct 25^th, 2011, 1:00am

hello,
it is better to test your opamp in closed loop always (at least for high gain amps and to get proper control on o/p bias) just keeping it in unity feedback or some kind of feedback which is having FB at dc and no fb at frequency of interest by keeping low pass filtered FB

Thanks,
raj.