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Homework Help: Differential Amplifier with negative feedback

  1. Nov 6, 2017 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations
    • Adiff(single-ended)= 1/2gm*ro||Rd
    • β=R2/(R1+R2)

    3. The attempt at a solution
    I am really confused as to how to find A. I know the feedback of the circuits around the R2 and R1, but I dont understand how the open loop gain can be found. Can someone assist me in deriving it? First question is what gain are they asking for: common mode or differential? I assume differential, but I am honestly lost.
  2. jcsd
  3. Nov 7, 2017 #2


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    Do you know the meaning of the various given quantities like Vov, VA, ro, RD ?
    Note that the gain is Adiff=0.5*gm*ro||RD.
    Last edited: Nov 7, 2017
  4. Nov 7, 2017 #3
    Yes; my concern is whether the open loop gain(A) is just this equation.

    Also, thanks for the reply!
  5. Nov 7, 2017 #4


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    I have a question to you:
    The first of the "relevant" equations (gain) - did YOU write down the formula or were it given as part of the original text?
    My question is because I do not think that we need this equation.
    So I don`t think we need ro. By the way - what is RD?

    In any case: We need the transconductance gm. From where do we get this value?
    Last edited: Nov 7, 2017
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