Connect with us

forward voltage, reverse current

Discussion in 'Electronic Basics' started by Jacky Luk, Aug 27, 2004.

Scroll to continue with content
  1. Jacky Luk

    Jacky Luk Guest

    I'm trying so hard to imagine of such a transistor circuit when it is driven
    into saturation."The collector-base pn junction acquires a forward voltage
    bias even though there is still a reverse current flowing through it", I'm
    trying to visualize it, how can a foward voltage applied across to the
    terminals while there is a reverse current over it. Any hints would be
    greatly appreciated!
    Thanks
    Jack
     
  2. Hi,
    Digging into my creaky old memory banks, I seem to remember that under
    saturation minority carriers pile up in the base region. The distribution of
    these being proportional to the emitter current at one end and the collector
    current at the other. As the first is greater than the latter, a density
    gradient forms across the base region down which the carriers diffuse into the
    collector. In other words, diffusion currents are not necessarily respecters
    of bias polarity.


    Cheers - Joe
     
  3. John Larkin

    John Larkin Guest


    In "transistor action" the collector-emitter current is arguable
    backwards as far as the collector is concerned. The transistor action
    shoots charge into what would have been a back-biased c-b diode. So a
    normal non-saturated conducting transistor has a back-biased c-b
    junction but still has collector current. So somebody applies the
    words "reverse current" to this situation (personally, I wouldn't,
    because there's no c-b junction current. There's transistor current
    from c to e.)

    OK, let Vc get very low, in saturation. Now the collector voltage is
    below the base voltage, so the b-c junction gets forward biased. Now
    base current is flowing into *both* the emitter and the collector. So
    some wise guy can claim the the collector is getting forward and
    reverse current at the same time. But the b-c current is positive
    (talking NPN here, of course) in saturation, and there is positive c-e
    current, too, but the statement

    "The collector-base pn junction acquires a forward voltage
    bias even though there is still a reverse current flowing through it"

    is silly.


    John
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-