From: "John S. Dyson"
References: <6jzD9.165153$MGm1.firstname.lastname@example.org> <email@example.com>
Subject: Re: type of NPN transistor for minimul voltage drop C to E
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 22 Nov 2002 20:07:29 -0500
NNTP-Posting-Date: Fri, 22 Nov 2002 19:59:55 EST
"Spehro Pefhany" wrote in message news:firstname.lastname@example.org...
> On Fri, 22 Nov 2002 23:44:02 GMT, the renowned "Brill Pappin"
> >Any ideas an what type of transistor I should look at to get the minimum
> >voltage drop when on (NPN)?
> >A typical diode (1N4001 you can expect about .6V (rule of thumb) a
> >transistor will be about the same (give or take) is there a particular type
> >that will have significantly less voltage drop?
> No, a saturated silicon BJT (transistor) will be < 100mV from
> collector to emitter, in most cases, for a small signal transistor.
Given the same BJT geometry, you want a transistor with the
highest beta in the direction opposite that you plan to use (for
lowest offset voltage.)
So, if you use a normal transistor, in a non-inverted circuit, and
need the high beta, then you also need a transistor with a high
reverse beta. The high forward beta will keep your base
current drive minimal, but the high reverse beta will tend to keep
the voltage drop across the part minimized.
Normal transistors tend to have a BF in the range of 20-2000,
while the BR tends to be in the range of 2-10. In cases where
you can deal with high base current drive, but need to absolutely
minimize the voltage drop, then you might try connecting the
transistor in reverse. The bad news is that some of the breakdown
voltages will tend to be much less.
If you only need a slow transistor, with high gain and moderately
high current, then the zetex line of parts (ztx688-ztx696 for
NPN or ztx788-ztx796 for PNP.) The forward beta on these parts
tends to the 400-2000 range, and the reverse beta can be as high
as a couple of hundred. Additionally, the junctions are large, with
high Is, so the associated drops at 1ma (for example) are very low.
If you can stand a low breakdown voltage, a ztx688 or ztx689 can
do some things that are uncommon WRT normal 2n3906 or 2n2222