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JFET question

  1. Nov 16, 2007 #1
    I was just reading about JFETs. I decided to try a circuit in multisim.
    Connected a N-channel jfet(2N4222), the source is connected to ground and the drain is connected to +12V.
    The gate is not connected. But there is a drain resistor(1K) connected between gate and ground to drain any gate capacitance.
    When I turn on the power, the jfet should start conducting. correct?
    But it doesn't. can some one please explain why this is happening.
  2. jcsd
  3. Nov 17, 2007 #2


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    What is the gate for? How can you get any conduction if you don't provide any gate bias?
  4. Nov 17, 2007 #3


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    I'm not sure why you're putting this "drain resistor" to drain the gate capacitance. You'll only have to worry about capacitance affecting your circuit at high frequencies.
    But with the simple gate bias (gate directly grounded), you should get IDSS. And as you reverse the gate with respect to the source (approaching VGS(off), whats suppose to happen?
    Mayb you could provide the schematic?
  5. Nov 18, 2007 #4
    Here's what I am trying to do -
    Turn ON a LED using a JFET. I have attached the ckt. I changed the JFET to J308.
    Without the LED, the Idss is about 40mA which is close to what the datasheet says.
    But when I add the LED between the source terminal and ground, it doesn't light up.
    If I forward bias the gate with 1V, the LED lights up.
    The LED needs only 20mA to light up.
    I have attached the schematic.

    Attached Files:

  6. Nov 18, 2007 #5


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    http://en.wikipedia.org/wiki/LED_circuit" [Broken]
    Last edited by a moderator: May 3, 2017
  7. Nov 18, 2007 #6
    In my ckt, LED drop is 2V.
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