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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Folded Cascode OTA design - Help https://designers-guide.org/forum/YaBB.pl?num=1318871130 Message started by ashwin87 on Oct 17th, 2011, 10:05am |
Title: Folded Cascode OTA design - Help Post by ashwin87 on Oct 17th, 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 24th, 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 24th, 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 25th, 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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