Bipolar transistors in reverse

[E]

Got just two nice lab psu (100W and weight only 13 kg) off Ebay.
So to put them in some use I measured how well some bipolars work
with collector and emitter swapped.
http://koti.mbnet.fi/hsahko/trans/

-ek

 

[Tauno Voipio]

Got just two nice lab psu (100W and weight only 13 kg) off Ebay.
So to put them in some use I measured how well some bipolars work
with collector and emitter swapped.
http://koti.mbnet.fi/hsahko/trans/

-ek

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.

 

[Tim Williams]

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: http://webpages.charter.net/dawill/tmoranwms

Got just two nice lab psu (100W and weight only 13 kg) off Ebay.
So to put them in some use I measured how well some bipolars work
with collector and emitter swapped.http://koti.mbnet.fi/hsahko/trans/

-ek

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.

 

[Tim Williams]

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

 

[Winfield Hill]

whit3rd wrote...

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.

 

[Winfield Hill]

Jim Thompson wrote...

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

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.

 

[Tim Williams]

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

Tim