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.

--

Tauno Voipio

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 =

300mA, forward hFE 200~1200, reverse hFE 150 typical. Good luck finding more, though.

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

12-15V, and super hFE, like 2SC3622 and 3836, but no reverse hFE listed.

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.

--
 Thanks,
    - Win

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

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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I love to cook with wine.     Sometimes I even put it in the food.

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.

--
 Thanks,
    - Win

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

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

It took less than tens seconds to find:

At 85 cents each.

--
Greed is the root of all eBay.

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

Tim

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