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Homework Help: Finding output voltage of common-drain amplifier

  1. Aug 6, 2011 #1
    1. The problem statement, all variables and given/known data
    Hi, I have a basic MOS common-drain amplifier as described below:

    - Drain connected to 5 V DC
    - Gate connected to an AC source on top of a 3 V DC source
    - Source connected to a 200 uA current source with a resistor R connected in parallel. The parallel connection goes to ground, and the output voltage (Vo) is taken across resistor R.

    If R approaches infinity, I heard someone explain that Vo > 3 - Vt - (Vgs - Vt)

    where Vt = thermal voltage.

    2. Relevant equations

    3. The attempt at a solution

    I'm not understanding how to get to this expression, because I had simply thought: Vo = 3 - Vgs, since it's just the voltage applied at the gate minus gate-to-source voltage. I'm not sure where the Vt comes from in the expression above.

    Thanks for clearing this up for me!
  2. jcsd
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