NPN BJT in reverse active mode - base current direction

Discussion in 'Homework Help' started by xxxyyyba, Feb 21, 2015.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
    2
    Hi!
    I was wondering what happens with Base current direction if NPN BJT works in reverse active mode? In forward active and in saturation mode currents are like this:

    for.png

    Collector and emitter currents in reverse active mode would be in opposite direction:

    inv.png
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,796
    If the device were constructed symmetrically, then it would behave just like the same NPN with the collector and emitter swapped. Most MOSFETs implemented on an IC are this way and the "drain" and "source" are interchangeable. But most BJT transistors, even on an IC, are not symmetric and are designed to allow high reverse bias voltages on the base-collector junction. A consequence of this is that the reverse beta tends to be very low. It IS an NPN transistor even when reversed, just a pretty crappy one by the metrics we generally use.
     
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  3. ramancini8

    Member

    Jul 18, 2012
    442
    118
    The only advantage that the reverse transistor has is it's very low Vce. This trick was used long ago to minimize offset.
     
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