Why can't I swap emitter and collector?

Discussion in 'General Electronics Chat' started by hp1729, Oct 18, 2016.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    If an NPN transistor is just a layer of P type material between two N types why can't I swap the emitter and collector in a circuit? I remember doing this with some JFETs, source and drain could be swapped.
     
  2. MrChips

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  3. hp1729

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  4. Bordodynov

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    See
    Draft345.png
     
  5. Morvan

    New Member

    Jun 24, 2014
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    Good morning.
    All responses before me seem to achieve purpose which be to elucidate transistor operation. But, to help even further, if I could, I say transistors are like a water pump, in the sense they have pre-conditions to work (think about when you invert lids you impede they work like project itself was thought and, either, in correct current flow).
    To clarify even more, you can read this article, from T. E. (Collin Mitchell).
     
  6. ramancini8

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    Jul 18, 2012
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    You can swap emitter and collector leads in a junction resistor. The swapped circuit has much less gain because the target is so small, and the collector offset voltage is very small. Before the advent of FETs, inverted transistors were often use to reduce offset.
     
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  7. hp1729

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    "Before FETs"? < :) > Dang, I thought I was old. </ :) >

    Thanks for the insight.
     
  8. OBW0549

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    Back during the Late Jurassic when I started as a hobbyist, I was playing with germanium transistors such as the 2N107, 2N170, CK722 and the 2N1305, as well as vacuum tubes like the 12AX7, 6AU6 and the 6L6. Good stuff.
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    With a real world bipolar transistor you actually can swap the C and E and it will still work as a transistor, albeit with a much reduced gain of about one.

    The input stage of evert TTL gate works in this mannor, so there are real applications for this.
     
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  10. crutschow

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    Some of the first commercial transistors were fairly symmetrical and would work nearly the same in the inverted configuration as the normal one.
     
  11. OBW0549

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    There will also be a much lower effective BVcbo, as the new "collector" won't be able to stand much more than 5-7 volts with respect to the base. This didn't used to be much of a problem with the old germanium transistors, since BVebo was often almost as high as the BVcbo.

    Still, the gain did suck badly.
     
  12. tindel

    Active Member

    Sep 16, 2012
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    My understanding and test results show that jfets are still often times symmetrical. The drain and source can be swapped with no adverse effects usually. Some datasheets even say they are symmetrical.

    I know that its a different transistor type than asked about... but still on topic with the general meanderings.
     
  13. nsaspook

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    How well swapping works depends on the manufacturing process, with older usually meaning better.

    Diffused-base transistors.
    [​IMG]

    Improved Mesa transistors.
    [​IMG]
    Planar EPITAXIAL transistors.
    [​IMG]

    Bipolar diffusion IC process.
    [​IMG]

    Bipolar implantation IC process.
    [​IMG]
     
  14. hp1729

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    I remember them all fondly. OA2 rectifiers, Magic eye tubes used for tuning. Radios that stood on the floor. The 6L6s that powered my Ameco 40 and 80 meter transmitter kit I built. Wired chassis before there were circuit boards.
    (singing) Those were the days.
     
  15. nsaspook

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    I will never understand the fondness for tubes. They were hot, bulky and unreliable in rough service. The day they replaced all but one of the R390 era receivers was a day of celebration for having a few less vacuum tube era devices to maintain when I served on my first ship.
     
  16. OBW0549

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    The OA2 wasn't a rectifier; it was a gas-discharge voltage regulator tube used more or less the way we use Zener diodes today.
     
  17. hp1729

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    R390? Ah, yes. 6HA5's, 6C4's, 6BA6's. Remember them. I wasn't Radioman. I was Data System technician back before computers had ICs. When Univac was king. The Navy had some good schools back then. I spent half my enlistment in schools.
    That sounds right. Thank you for correcting my failing memory. :)
     
  18. ramancini8

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    Don't knock tubes. They are rugged, radiation resistant devices. I repair antique auto radios, and almost never have to replace a tube. I flew as radar tech is a super constellation in the late 50s, full of tubes, but the biggest failure item was the circuit boards not the tubes.
     
  19. nsaspook

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    I was a UYK-* repairman too.
     
  20. nsaspook

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    Tubes had their place, in their time just like core memory did. We still have a few for HV regulation in machines making transistors today.
    http://forum.allaboutcircuits.com/threads/tube-creep.113198/
     
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