Are JFETs the same as MOSFETs?

Discussion in 'General Electronics Chat' started by lll, Jan 7, 2015.

  1. lll

    Thread Starter New Member

    Mar 7, 2012
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    We never learned about JFETs in my analog circuits classes, only BJTs and MOSFETs.

    Can I treat them like MOSFETs when I see them in circuit diagrams? Are gain, output impedance, biasing, etc., calculated the same way for JFETs as they are for MOSFETs?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Not Quite. The Gate to Channel region is essentially a Si Junction Diode, normally operated reverse-biased, so the depletion region modulates the Drain to Source channel conduction. They have to be biased differently than an enhancement-mode FET
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Also, jfets normally allow some current flow from S to drain when the gate is at the Source voltage. You will have to pull below source to reach cutoff (no current flow). That is why MikeML said biasing is different.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    To add to the above comments:
    JFETs are normally depletion-mode and carry their maximum current when Vgs is 0V. Vgs must be reverse biased to turn them off (voltage the opposite polarity as the normal drain voltage).

    Most MOSFETs are enhancement-mode and carry no current when Vgs=0V. Vgs must be forward biased to turn them on (voltage the same polarity as the normal drain voltage).
     
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