2n3906 transistors from Tayda

[Rob Strand]

if you want to lower the gain to be about 1/3rd, just swap the collector and emitter,

The reverse gain is usually only 1 to 10.   (You can also get problem with the BE junction conducting if the voltage exceeds about 10V.)

There's plenty of circuits which would happily work with hFE = 400.

[PRR]

Very OLD Ge transistors were sometimes symmetrical. Same gain both ways. (They did not have enough control to optimize the doping.)

[iainpunk]

if you want to lower the gain to be about 1/3rd, just swap the collector and emitter,

The reverse gain is usually only 1 to 10.   (You can also get problem with the BE junction conducting if the voltage exceeds about 10V.)

that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction

cheers, Iain

[antonis]

I can ensure you, Iain, for a bunch of damaged BC317s conversely fitted in 15V CE (grounded E) amp circuit..

[Rob Strand]

that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction

Interesting.  I've measured the values in test set-ups quite few times and I always get low values.  That's for small signal transistors all the way up to large TO-3 packages.

I only *use* transistors in reverse at low voltages, for example as controlled resistors.   The other motivation for measuring reverse beta is to come-up with spice models (many spice models have dubious reverse parameters and don't work correctly in reverse).

I'd have to check out what's happening.   It's possible BE leakage, promoted by breakdown is helping.

[iainpunk]

that's not what my experience with reverse BJT's is, a BC547 with an Hfe 318 in reverse measured Hfe 93, and a BD139 with gain of 85 measured 32 in reverse. that is in circuit, not in a DMM or component tester.
yes, reverse breakdown is a risk, but not so prevalent in 9V circuits, and enough datasheets list the max reverse BE voltage rating that you can account for that with series diodes/LEDs to lower the potential voltage on that junction

Interesting.  I've measured the values in test set-ups quite few times and I always get low values.  That's for small signal transistors all the way up to large TO-3 packages.

I only *use* transistors in reverse at low voltages, for example as controlled resistors.   The other motivation for measuring reverse beta is to come-up with spice models (many spice models have dubious reverse parameters and don't work correctly in reverse).

I'd have to check out what's happening.   It's possible BE leakage, promoted by breakdown is helping.

now im starting to doubt my measurements, it could have been a fluke, we measured the gain of the BD139 in an amplifier circuit based on the currents through the pins, and the BC547 was at a practicum where we had to measure Hfe of a transistor, among other things.

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)

cheers, Iain

[Axldeziak]

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)

You just saved me a ton of time and worry on a completely unrelated project where a BF245 was suggested as a sub but was reverse from the original schematic. Thanks!

Little things like this are why I read this place everyday.

[iainpunk]

a good transistor to use in reverse is the BF245, since the gain is roughly the same both ways (its a JFET)

You just saved me a ton of time and worry on a completely unrelated project where a BF245 was suggested as a sub but was reverse from the original schematic. Thanks!

Little things like this are why I read this place everyday.

not gonna lie, this made my laugh.
there are a lot of JFETs where the source and drain are interchangeable, the BF245 is my personal favourite because it sounds very good in an SRPP style circuit.

cheers, Iain

[Yazoo]

After all that, it turns out I owe Tayda an apology. While I did get an order from Tayda recently, the 2N3906s I bought for the same project came from an eBay seller, not Tayda.

So, grovel grovel, sorry Tayda.