From: "John S. Dyson"
References: <6jzD9.165153$MGm1.email@example.com> <firstname.lastname@example.org>
Subject: Re: type of NPN transistor for minimul voltage drop C to E
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Date: Sat, 23 Nov 2002 11:08:15 -0500
NNTP-Posting-Date: Sat, 23 Nov 2002 11:00:41 EST
"carltons" wrote in message news:email@example.com...
> In article , Winfield Hill
> > John S. Dyson firstname.lastname@example.org wrote...
> > >
> > >> Brill Pappin wrote:
> > >>
> > >>> Any ideas an what type of transistor I should look at to get
> > >>> the minimum voltage drop when on (NPN)?
> A whole lot of other stuff in between with no one asking the question of:
> "How low a voltage?". If the OP needs zero or close to it, then use a
> relay. Is switching speed an issue? Is voltage polarity? What is the
> budget for a device? What resources are available to the OP as far as
> obtaining the parts, etc. ? What is the space available? .... on and on
> and on. Solutions are provided and the problem has not even been defined.
It is a good thing to provide information for someones' toolkit. All too much
of the theory of cool parts like BJTs is disjoint from the technology other
than for the 'elite few' who do chip design or are crazy hobbyists (or eclectic
engineering types like me.)
> BTW, at saturation, the base collector and base emitter are both forward
> biased. Unless this forward bias voltage is the same for both junctions,
> you won't have zero volts from collector to emitter on the bipolar
> switch. Ithink that you would have to have the same doping levels in both
> the emitter and the collector to get this even close and then you would
> have to include the IR losses due to wirebonds, forward current, etc.
> What this has to do with anything in this discussion I will leave to
> others to mangle. I mean, manage.
The relationship between reverse beta and saturation voltage is actually
described in the defining transistor equations. We have the good old
Ebbers-Moll equation, but the more general form actually has more of
the relationships (including saturation voltages.)
When talking about 'real' parts or dynamic situations, then the relationships
aren't so tidy, are they? :-). IMO, the biggest problems with those large
junction, thin base and more symmetrical parts are the huge capacitances (which
can be relatively easily dealt with in many cases), and/or the low breakdown
> Steve WB4CZR
> The reverse-beta numbers are very nice to know, they're not in
> > the data sheet; Thanks John, for these numbers!
> > Thanks,
> > - Win
> Hi Win,
> You're right. The reverse betas are not usually provided, but only a
> manager would hook up a transistor in reverse. ;-)
FYI, I found them in the Spice models. I was designing a low noise
audio amp, and noted the 'odd' characteristics. When reviewing the
parts more carefully, except for medium/high speed/frequency applications,
they are really the ultimate jellybean. Maybe it is more appropriate
to say that when the good old traditional jellybeans aren't quite right,
it is really good to have a few ztx69* or ztx79* parts available, just as
it is good to have some MPSH10 or other RF parts available. There
are certainly places where an ATF54143 is useful, and places where
the ztx689 is useful, but the overlap is zero :-).