From: Winfield Hill
Subject: Re: type of NPN transistor for minimul voltage drop C to E
Date: 23 Nov 2002 02:30:16 -0800
Organization: Rowland Institute
References: <6jzD9.165153$MGm1.firstname.lastname@example.org> <email@example.com>
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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)?
> 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
> type parts.
It's very impressive to see a low 40mV Vce(sat) maximum spec of
at 100mA with a base current of only 1mA! (Ic/Ib = 100) for the
ZTX688B TO-92 transistor. The 520mV Vbe at 1mA is also unusual as
you say, that implies a junction 10-20x larger than typical parts.
And helps explain its high gain of 100 (min) at 10A and its 350mV
saturation voltage at 3A with only 20mA drive (Pd = 1W). This is
a BJT that gives TO-92 power MOSFETs a real run for their money!
The reverse-beta numbers are very nice to know, they're not in
the data sheet; Thanks John, for these numbers!