Re: forward voltage, reverse current

From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 08/27/04 

Date: Fri, 27 Aug 2004 08:21:37 -0700

On Fri, 27 Aug 2004 19:27:29 +0800, "Jacky Luk" <jl@knight.com> wrote:

>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
>

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

Relevant Pages

  • Re: reversed transistor
    ... Now if we reverse the voltage accross the collector-emitter, I mean negative to the collector instead of the normal positive, how much voltage can it withstand? ... Because the emitter material is doped to a much higher concentration than the collector material is, the breakdown voltage of the emitter-base junction is much lower than the collector-base junction. ...
    (sci.electronics.design)
  • Re: Maximum Ratings
    ... Some LEDs have a maximum reverse voltage rating less than their normal forward voltage. ... The best tool for this test is a transistor curve tracer. ...
    (sci.electronics.components)
  • Maximum Ratings
    ... Some LEDs have a maximum reverse voltage rating less than their normal forward voltage. ... Some transistor spec sheets don't mention the limit; some give something like half the collector current rating, and a small number give currents like 1mA. ...
    (sci.electronics.components)
  • Re: [opensuse] Ballmer: Linux users owe Microsoft
    ... I know that current can flow from + to -, also one can reverse polarity ... In a mismatched transmission line, that is to say, for example, a 50 ohm ... The ratio of forward voltage to reverse ... how line impedance mismatch applies to "multidirectional DC and AC". ...
    (SuSE)
  • Re: Transmission Line Reflections
    ... the current would reflect back and change phase. ... However, if one examines the equations for current and capacitive voltage, then it falls out of the math. ... So the sum of the forward and reverse current wave is zero at the end, and the only way that can be is for the forward and reverse waves to be equal in magnitude and out of phase. ...
    (rec.radio.amateur.antenna)