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OMG can i use NPN substitute to FET?

Discussion in 'General Electronics Discussion' started by electronoobz, Oct 26, 2012.

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  1. electronoobz


    Jan 14, 2012

    i broke my 2n5458 pin. I only have npn here.

    im making an amp. I need to pass this tommorow.. I got my npn in a broken toy.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Generally: no.

    A FET is voltage controlled whereas an NPN is current controlled. Some Changes in your circuit will be necessary. How much, that depends on the specific application of the transistor.
  3. electronoobz


    Jan 14, 2012
    if it is current controlled, then i would just put put a resistor before the npn right?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Not necessarily.
    - Does the driving stage deliver enough current?
    - a bipolar transistor needs a diffferent biasing than a FET.
    - a bipolar transistor has Vbe~0.6V, the 2n5458 is characterised by a treshold voltage VT=-6V This means the FET is conducting for Vgs=0V! An NPN will not conduct for Vbe=0V
    etc. etc.

    If you're very desperate, try to reattach the pin to the transistor. use a sharp tool to remove some plastic around the stub of the pin that is now inside the housing. Using a small soldering iron try to solder a piece of wire to the pin. Do no overheat the transistor and do not mechanically stress the wire. It might work.
  5. electronoobz


    Jan 14, 2012
    yeah im desperate. But if it's that hard, i'll just ask for an excuse to my prof. Hahaha. Thanks anyway.
  6. screwball


    Jan 9, 2012
    Try fixing a wire as said to the broken pin to the FET, it shouldnt be too hard
    Im sure your prof would appreciate you atleast trying to do something about it rather than not bothering ;)
  7. Raven Luni

    Raven Luni

    Oct 15, 2011
    Are these components that you have to supply yourself or were you issued them for a specific project? If they're issued then tell him Raven Luni said he is a moron for giving you defective components and no spares (because something's always going to go wrong right?)
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    The 2N5458 is a depletion-mode JFET. You can't replace it with a bipolar transistor.
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