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output impedance of common source amplifier

Discussion in 'Electronic Basics' started by tempus fugit, Feb 18, 2009.

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  1. tempus fugit

    tempus fugit Guest


    How do I determine the output impedance of a common source (or other JFET)
    amplifier? I know that it is roughly 1/gms, but how do you deal with the
    units? E.g., if a certain JFET has a gms of 5000 uS, the output Z would be
    0.0002, but surely that's not ohms is it? Or do you divide out the micro and
    get an answer of 200 ohms?

  2. Fred_Bartoli

    Fred_Bartoli Guest

    But the output impedance of a common *source* amplifier isn't 1/gm. That one
    is the output impedance of a common drain stage.

    For a common source stage you use 1/go

    Also note that parameters depend on the bias conditions and that those given
    in datasheets and suffixed by a 's', like gfs, gos, are at VGS=0 bias

    And for a R loaded common source stage the output impedance is often
    dominated by the load resistor value.
  3. tempus fugit

    tempus fugit Guest

    Thanks Fred.

    My mistake - I meant common drain; I was thinking source follower.
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