Rich Grise wrote:
On Mon, 24 Apr 2006 18:55:44 -0400, John Popelish wrote:
John Fields wrote:
John Popelish wrote:
I think I have made circuits that saturated emitter followers.
I just drove the base till the base to collector junction was forward
LOL, you didn't saturate it as much as you turned it into two
forward-biased diodes, I think!
I call a transistor that has a forward biased collector-base junction
and a forward biased emitter-base junction, saturated. How do you
define a saturated transistor?
Whow, dude, that didn't take much thought! If Vbe is .7 and Vcesat is .3,
then duh! (it's kewl, to me, because I've never looked at it that way.)
But that's not like forward-biasing a diode at .4V and having it conduct.
For some reason, I'm having an image of a transistor receiving a wedgie.
My point is that a transistor has no idea whether you are thinking of
it as a common emitter inverter or a common collector follower. It
just responds to the biased we apply to it. An emitter follower that
has a forward biased collector-base junction is every bit as saturated
as a common emitter amplifier with its collector pulled so far down
that its collector-base junction becomes forward biased. From inside
the transistor, both situations are exactly the same.
Once the collector base junction becomes forward biased, it does not
necessarily imply that there is diode current across that junction.
It just means that the collector bias is no longer efficient at
sweeping out charge that diffuses out of the base layer. So the
current gain falls dramatically, and the collector to emitter path
becomes a variable resistor (that is governed by ohm's law), instead
of acting more like a current regulator.