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Bipolar transistors in reverse

Discussion in 'Electronic Design' started by E, Feb 21, 2010.

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  1. E

    E Guest

    Got just two nice lab psu (100W and weight only 13 kg) off Ebay.
    So to put them in some use I measured how well some bipolars work
    with collector and emitter swapped.

  2. Tauno Voipio

    Tauno Voipio Guest

    That's an old trick to get a very small saturation voltage
    and tiny beta.

    In the time before FET's (late 1960's) we used 2N3904's
    in reverse in R/2R ladder switches in a DAC.
  3. Tim Williams

    Tim Williams Guest

    I once found some 2SC2878's in some junk. Vcbo = 50V, Vebo = 25V, Ic =
    300mA, forward hFE 200~1200, reverse hFE 150 typical. Good luck finding
    more, though.


    Deep Friar: a very philosophical monk.

    One of the problems with using "regular" transistors in the inverted
    connection is that the applied voltage between "collector" (really the
    emitter) and base is limited to about 7V, the Vbeo rating of the
    transistor. Texas instruments used to make a line of PNP transistors
    that were symmetrical in that the forward and inverse betas were
    equal, and the Vbeo and Vceo ratings were equal. They were designed
    for chopper applications. I'm not sure that they are still available,
    since FETs are far superior in this application.
  4. Tim Williams

    Tim Williams Guest

    Hmm, Datasheet Archive lists the 2SC3327 as a cross to the 2SC2878. Data
    looks pretty close, 25Vebo and such. A few others have moderate Vebo, like
    12-15V, and super hFE, like 2SC3622 and 3836, but no reverse hFE listed.

  5. whit3rd wrote...
    Yes, inverted, good memory. Now totally gone, sadly.
  6. Jim Thompson wrote...
    Hmmm. No sign in my records.

    There is a PNP chopper transistor still for sale.
    It's Motorola's old mmbt404a, with Vebo = 25V, and
    it's offered by LRC and by Planeta (in Russia).
    I dunno where to (easily) buy it!

    But I have observed that many discrete PNPs have
    high Vebo, and lots (if not all?) PNP BJTs in ICs
    seem to as well. As for their inverted-mode gains,
    well that's another matter.
  7. Tim Williams

    Tim Williams Guest

    Cool! Kind of expensive, and you never know when stock runs out. Better
    than nothing though.

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