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JFET-JFET cascode amplifier

  1. Mar 26, 2008 #1
    Hi. I'm not actually doing an electronics course (Maths & Physics), but I have just finished building a 140W BJT guitar power amplifier and cabinet. I'm looking to build a FET-FET cascode preamplifier now, but am not too familiar with JFETs - So far, I've got:

    JFET2 (2N3819)
    JFET1 (2N3819)
    Resistor & capacitor (parallel)

    with the input on the gate of JFET1, output from the source of JFET2, and the base of JFET 2 tied to ground.

    I've tried to derrive equations for the JFETs, in order to work out the values for the resistors and capacitors, but with no success - can anyone here help me, or point me to any good books/websites?

    Last edited: Mar 26, 2008
  2. jcsd
  3. Mar 26, 2008 #2
    When biasing Jfets, Gate has to be at a lower potential than the source. That's when drain to source current stars flowing. Drain to source current depends on gate voltage, and can be calculated using a quadratic equation.

    When doing biasing calculations on jfets, two parameters are needed, the pinchoff voltage Vp, and the saturation current Idd. You can obtain these from the datasheet, or measure them easily.

    I googled jfet biasing and there is a plenty of tutorials.


    Hope that helps.
  4. Mar 28, 2008 #3
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