For the Component Abuse Challenge our hacker [Tim Williams] observes that N-P-N reads the same way forwards and backwards, so… what happens if we reverse bias one? (Note: this remark about N-P-N reading the same forward and backward is a lighthearted joke; in fact the level of doping in the emitter and collector is different so those Ns are not fungible and will exhibit different properties and have different characteristics.)
In the margin you can see how the question was originally posed by Bob Pease back in March 18, 1996.
In his article [Tim] mentions that some transistors are specifically designed to operate when reverse biased, which [Tim] calls “inverted mode”, whereas most transistors are not designed to work in this fashion and that’s the sort of abuse that could damage the component and lead it to malfunction.
But what is Vout? [Tim] reports that he measured approximately -0.4 volts using his high-impedance meter. We tried this experiment in the lab ourselves but we were not able to duplicate [Tim]’s result; however there is a long list of potential reasons for such an outcome. If you do this experiment yourself we would love to hear about your results in the comments section!
If you’re still learning about transistors you might like to check out our five part series on transistors as amplifiers, starting here: Won’t Somebody, Please, Think Of The Transistors!
Thanks to [Tim] for his submission, we wish him the best of luck in the competition!
I did not have that on my Hackaday bingo card.
lol
Reverse biased NPNs are good for draining capacitors.
Symmetric BJTs have current gain of maybe 10 to 25. To get better gain, real ones are doped asymmetrically, with the collector-base depletion region being larger than the base-emitter depletion region.
To get as much current through the transistor as possible, the CE diode’s forward voltage is set about 200mV lower than the BE diode’s forward voltage at maximum current. That value is called Vce.sat: the voltage between the collector and emitter when the collector is saturated (passing as much current as it can). Therefore you can never get a BJT’s collector all the way to 0V.
Running in the reverse transistor region changes that behavior. The current gain is terrible, but the CE diode will suck current out of the BE diode as fast as it leaks in. A side effect is that Vce.sat gets pretty close to 0V.
Jim Williams used that trick in Linear AN-13 to drain the integrating capacitor of a 10-bit ADC within about 1mV of GND.
Thanks. Found over here: https://www.analog.com/media/en/technical-documentation/application-notes/an13f.pdf