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Small signal analysis of a cascode amplifier

  1. Jul 29, 2016 #1
    Hello there.

    I am stuck trying to figure out the voltage gain of a cascode amplifier from a small signal analysis. The circuit seems to be under-determined for reasons I do not understand or maybe I am missing the perfectly obvious.

    Here's my small signal model:
    ?temp_hash=c064e565f691e7abb408df22a4274b09.png
    The only relation I can deduce without using further unknown quantities is the KCL for the vx node:
    %7Bds2%7D%7D%20%3D%20v_%7Bin%7D%5Ccdot%20g_%7Bm1%7D%20+%20%5Cfrac%7Bv_x%7D%7Br_%7Bds1%7D%7D.gif
    However, I need another relation to eliminate vx in order to solve for vout/vin. The small signal analysis without the cascode works just fine. May I kindly ask somebody to clarify this?

    Best regards,
    M
     

    Attached Files:

  2. jcsd
  3. Jul 29, 2016 #2

    The Electrician

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    Gold Member

    Since you have a current source supplying the M2 FET, you can eliminate M2 and the voltage gain is the same as it would be without M2, namely Vout/Vin = -gm1*Rds1.

    The small signal at Vx is the same as it is at Vout as long as the load on Vout is infinite, which is the impedance of the current source feeding the drain of M2. With a finite load on Vout, this will not be the result.
     
  4. Jul 30, 2016 #3
    Could you explain why this is the case?
     
  5. Jul 30, 2016 #4

    The Electrician

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    Gold Member

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