Bipolar transistors in reverse

Low beta, low saturation voltage. Gummel-Poon models capture that well. They don't seem to do as well when you reverse bias the emitter- base junctionto the point of breakdown.

The semicondutor industry models apparently do better, but they aren't easily accessible.

LTSpice includes the VBIC model - pull out the VBIC parameters for a saturating switch like the 2N2906 and you might solve some of my problems.

Bill Sloman, Nijmegen

That's an old trick to get a very small saturation voltage and tiny beta.

In the time before FET's (late 1960's) we used 2N3904's in reverse in R/2R ladder switches in a DAC.

One of the problems with using "regular" transistors in the inverted connection is that the applied voltage between "collector" (really the emitter) and base is limited to about 7V, the Vbeo rating of the transistor. Texas instruments used to make a line of PNP transistors that were symmetrical in that the forward and inverse betas were equal, and the Vbeo and Vceo ratings were equal. They were designed for chopper applications. I'm not sure that they are still available, since FETs are far superior in this application.

I once found some 2SC2878's in some junk. Vcbo = 50V, Vebo = 25V, Ic =

Tim

-- Deep Friar: a very philosophical monk. Website:

One of the problems with using "regular" transistors in the inverted connection is that the applied voltage between "collector" (really the emitter) and base is limited to about 7V, the Vbeo rating of the transistor. Texas instruments used to make a line of PNP transistors that were symmetrical in that the forward and inverse betas were equal, and the Vbeo and Vceo ratings were equal. They were designed for chopper applications. I'm not sure that they are still available, since FETs are far superior in this application.

Hmm, Datasheet Archive lists the 2SC3327 as a cross to the 2SC2878. Data looks pretty close, 25Vebo and such. A few others have moderate Vebo, like

Tim

-- Deep Friar: a very philosophical monk. Website:

MPSA17 from Motorola was specified for 15V emitter-base voltage, and MPSA16 for 12V. Breakdown is higher than the specification, of course. I think these types were intended for chopper duty in reverse connection.

Yes, inverted, good memory. Now totally gone, sadly.

Wasn't there a _symmetric_ NPN device called the INCH, for chopper use? ...Jim Thompson

Hmmm. No sign in my records.

There is a PNP chopper transistor still for sale. It's Motorola's old mmbt404a, with Vebo = 25V, and it's offered by LRC and by Planeta (in Russia). I dunno where to (easily) buy it!

But I have observed that many discrete PNPs have high Vebo, and lots (if not all?) PNP BJTs in ICs seem to as well. As for their inverted-mode gains, well that's another matter.

Integrated PNP's are usually laterals, so can be made completely symmetric, but have lousy betas and fT's :-( ...Jim Thompson

It took less than tens seconds to find:

At 85 cents each.

Cool! Kind of expensive, and you never know when stock runs out. Better than nothing though.

Tim

-- Deep Friar: a very philosophical monk. Website: