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JFET question

Discussion in 'Electronic Design' started by Bo, Apr 21, 2008.

  1. Bo

    Bo Guest

    I have a common source n channel JFET circuit--

    with R1/R2 bias divider R1 = 420K, R2 = 180K

    RD = 2.7K
    Rs = 2.7K
    RL = 4K

    cap Cs in || with Rs.

    Vp = -4
    early voltage = 125
    Idss = 12mA

    How can I find small signal voltage gain @ mid freq?

    I try to compute gm-- but need Vgs to do so. Do I assume Vs = 0 since
    Cs short @ mid freq and then use dc value of Vg (6V) to say Vgs = 6v?

    I'm thinking there has to be additional info to figure this out...

    Any clues appreciated...

    Bo
     
  2. Hello Bo,

    It's easy to calculate the gain.

    G = gm*RD||RL||rd

    Just forget the output resistance rd, because
    it's much higher than RD||RL.

    Calculate the DC-current Id.
    Go to the datasheet and look for gm(Id).

    G = gm*RD||RL

    That's all

    Helmut
     
  3. Did you try google?

    http://www.ele.auckland.ac.nz/archives/design/tools/ampfet.htm
    http://www2.eng.cam.ac.uk/~dmh/ptialcd/spice/simulation.htm
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/fet.html

    and many more...
     
  4. Bo

    Bo Guest

    Thanks to all for your replies...

    I finally found something via google that helped me. Ended up
    calculating Vgs from a quadratic equation...

    I could not look up in datasheet because P/N is unknown...

    Thanks again,

    Bo
     
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