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ashwin87

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Folded Cascode OTA design - Help
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

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niwin

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Re: Folded Cascode OTA design - Help
Reply #1 - 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...

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Folded Cascode OTA design - Help Reply #2 - Oct 24^th, 2011, 3:14pm A schematic-picture of your circuits and test configuration would be useful
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Folded Cascode OTA design - Help Reply #3 - 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.
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