From: Robert Baer
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Subject: Re: type of NPN transistor for minimul voltage drop C to E
References: <6jzD9.165153$MGm1.email@example.com> <firstname.lastname@example.org>
Date: Sat, 23 Nov 2002 08:44:16 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 00:44:16 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
"John S. Dyson" wrote:
> "Spehro Pefhany" wrote in message news:email@example.com...
> > On Fri, 22 Nov 2002 23:44:02 GMT, the renowned "Brill Pappin"
> > wrote:
> > >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
> type parts.
To bad i did not read your missive first before responding to Spehro
So i dug out my Digikey, and see no maximum Hfe listes for the NPNs,
but the PNPs are severely limited (Hfe max of 800), with only *one*
exception: the ZTX718.
I haven't looked at that aspect of transistors since the 70's and was
so used to crappy reverse betas.
Thanks for mentioning these modern types.