Transistor backwards... But it works ?_?

You sure it's not reading them out backwards?

Bipolar transistors are somewhat symmetrical; it's an N-P-N structure so the base-collector and base-emitter junctions are similar. The transistor won't work as well backwards but it's still a transistor. The "reverse" beta isn't zero.

Transistors have reverse beta, current gain with emitter and collector swapped. Reverse bata can be as high as 5 or more, and your circuit only needs about 1.

Some old transistors were symmetrical, no difference between emitter and collector. Some on purpose. Ditto jfets.

I once measured one BC547C(or 548C) from old stock that was pretty good:

Forward biased it still had beta > 800 @ 1nA Ic (yup, no mistake, that hardly more than 1pA base current) and reverse biased it still had circa

I think I still have it, carefully stored, somewhere. Well, somewhere...

The old Ge ones must have been even better- you could get tens of uA collector current with 1pA of base current.

LOL. It was of course checked for leakage :-)

If can I can find it I'll check it and post the results...

NPN is NPN even in reverse. The properties usually differ from the forward case.

In the time before FET's, the reversed 2N3704's were used as analog switches, as the saturation voltage was very low, at the cost of a lousy beta. For more information, look for Ebers- Moll transistor model, which explains the behavior pretty well.

IIRC that was particularly the case with a lot of germanium alloyed transistors.

With a *REALLY* leaky one you could get a few tens of mA with no base current at all.

BFT25 runs reverse beta around 4. C-B diode leakage is 10s of fA, much better than a far inferior PAD1 (shorted jfet) sort of part.

Yup. The classic class-A resistor biasing scheme often sucked leakage current *out of* the base.

;)

Or with the base disconnected.

Cheers

Phil Hobbs

I once put in a 2N3904 emitter follower "backwards" (the EBC pinout is nicely symmetrical). Didn't even notice a difference.

Occasionally, it's intentionally useful. You can't get a >30V input differential any other way. (All the classic 30V bipolar analog chips, such as LM358, LM393, etc., with 30V differential inputs, essentially did this. In fact, their PNP input transistors were composed of two back-to-back collectors -- go figure!)

Tim

Where? I do not see any transistor in "backwards". Swapping emitter and collector functions results in lower beta and lower saturation voltage (which can be useful). Ain't happening in ANY of the circuits on that page.

Some of the old PopTronics circuits had a signal being coupled into the base of a germanium transistor with just a capacitor.

When I was about 10, I had a black-cased ge transistor, 2N107 or some such, and I noted that its collector leakage current was temperature sensitive. I put it in a flashlight reflector and played with the circuit. I got it to swing the meter on my VTVM when I passed my hand through the focal path three feet away. I didn't realize it at the time, but I had played with the supply voltage and load resistance such at to put it just at the edge of thermal runaway, so the gain went way, way up.

Fun. That's sort of how superconducting bolometers work.

Cheers

Phil Hobbs

The beta looks like it goes up, and then changes sign. In the region between V_CEO and V_CES, the base current has the opposite sign but the device still blocks collector current, more or less.

Cheers

Phil Hobbs

I've used that to identify B and E on an unknown pinout. The way round that gives the highest beta, and highest BVceo is the correct one.

So, what happens if you have an NPN with grounded emitter, positive voltage on the collector, and you zener the base negative? What does Ic do? I've wondered about that for decades, and never took 5 minutes to try it.

As with other posters, the parts will work inverted. I see a funtional gain requirement of slightly more than 0.5 for basic function, though the output of the parallel port may be unpredictable, given hardware designer's leeway.

Of greater importance is the limited supply voltage, below the normally-expected Veb breakdown values.

I'm not sure you'd get usefull inverted beta if the EB jn were avalanching.

RL

The CB junction usually has lower forward voltage, too.

Cheers

Phil Hobbs