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? on biasing JFETs

Discussion in 'Electronic Design' started by Dave, Aug 9, 2007.

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  1. Dave

    Dave Guest

    Are JFETs biased differently from standard bipolar transistors? Them must
    be, or the schematic I have is crap. The gate/drain junction is not biased
    with reverse polarity, like I am used to with, say, 2N3904s. It looks like
    it is forward based just like the source/gate junction. Any words of wisdom
    are welcome. Obvious newbie here...

    Thanks,

    Dave
     
  2. Eeyore

    Eeyore Guest

    Totally. Because they work on an entirely different principle.
    Actually, bipolar transistors are *forward* biased. You need some base current
    for them to operate as linear amplifiers (as opposed to switches)

    An n-channel jfet will however have 'negative bias' (wrt the source) on it's
    gate. When Vgs = 0 the fet will pass the maximum current possible for that
    device (Idss).

    Graham
     
  3. Dave

    Dave Guest

    AAAhhhhh. Okay, I had no idea that Vgs=0 would pass *maximum* current for
    that device. I obviously have a lot to learn... Thank you. Now I have
    some idea as to what I am looking for. Much appreciated.

    Dave
     
  4. neon

    neon

    1,325
    0
    Oct 21, 2006
    yes you do.
     
  5. Guy Macon

    Guy Macon Guest

    Don't assume that the two are mutually exclusive. It could be that
    JFETs are biased differently from bipolars (they are) *and* the
    schematic you have is crap.

    I hope this helps. :)
     
  6. Tim Wescott

    Tim Wescott Guest

    If the gate is a higher voltage than the drain (assuming the schematic
    isn't crap) check to see if the JFET is P-channel (analogous to a PNP
    bipolar).

    Generally a JFET will conduct serious current when the gate-source
    voltage is zero, so you will often see N-channel JFETs biased with a DC
    path to the negative rail and the source connected to the same rail
    through a small resistor.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
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