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Transistor backwards... But it works ?_?

Discussion in 'Electronic Design' started by Slater, Jul 11, 2012.

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  1. Slater

    Slater Guest

  2. Guest

    You sure it's not reading them out backwards?
    Bipolar transistors are somewhat symmetrical; it's an N-P-N structure so the
    base-collector and base-emitter junctions are similar. The transistor won't
    work as well backwards but it's still a transistor. The "reverse" beta isn't
  3. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    I once measured one BC547C(or 548C) from old stock that was pretty good:

    Forward biased it still had beta > 800 @ 1nA Ic (yup, no mistake, that
    hardly more than 1pA base current) and reverse biased it still had circa

    I think I still have it, carefully stored, somewhere. Well, somewhere...
  4. The old Ge ones must have been even better- you could get tens of uA
    collector current with 1pA of base current.
  5. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    It was of course checked for leakage :)

    If can I can find it I'll check it and post the results...
  6. Tauno Voipio

    Tauno Voipio Guest

    NPN is NPN even in reverse. The properties usually differ from
    the forward case.

    In the time before FET's, the reversed 2N3704's were used as
    analog switches, as the saturation voltage was very low, at the
    cost of a lousy beta. For more information, look for Ebers-
    Moll transistor model, which explains the behavior pretty well.
  7. Tim Williams

    Tim Williams Guest

    I once put in a 2N3904 emitter follower "backwards" (the EBC pinout is
    nicely symmetrical). Didn't even notice a difference.

    Occasionally, it's intentionally useful. You can't get a >30V input
    differential any other way. (All the classic 30V bipolar analog chips, such
    as LM358, LM393, etc., with 30V differential inputs, essentially did this.
    In fact, their PNP input transistors were composed of two back-to-back
    collectors -- go figure!)

  8. Fred Abse

    Fred Abse Guest

    I've used that to identify B and E on an unknown pinout. The way round
    that gives the highest beta, and highest BVceo is the correct one.
  9. legg

    legg Guest

    As with other posters, the parts will work inverted. I see a funtional
    gain requirement of slightly more than 0.5 for basic function, though
    the output of the parallel port may be unpredictable, given hardware
    designer's leeway.

    Of greater importance is the limited supply voltage, below the
    normally-expected Veb breakdown values.

    I'm not sure you'd get usefull inverted beta if the EB jn were

  10. Guest

    It is if they're inserted backwards.
  11. That's the way you can usually tell C from E with a handheld
    multimeter (on the diode range). If it has an HFE function, that's a
    confirmation (but not so convenient to use on SMT transistors).
  12. Fred Abse

    Fred Abse Guest

    If it has a diode range.

    I was spoiled, I had access to curve tracers before meters with diode
    ranges became common.
  13. Jamie

    Jamie Guest

    I kind of thought that destroyed the beta in the device when using
    it in forward mode afterwards? This was something that took place over
    time of use in the reverse mode.

    I guess it wouldn't matter if you never intend to extract the unit for
    another job.

    I remember a circuit years ago that had a R on the emitter side and if
    the R got opened and the circuit had been operating for while in reverse
    state, the transistor had to be replaced because the beta was no longer
    where it should of been.

  14. Fred Abse

    Fred Abse Guest

    The classic cross-coupled astable multivibrator drives each base negative
    wrt each emitter while each coupling capacitor discharges. Not a problem
    unless the junctions zener, just something to watch out for.
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