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Using transistors in reverse

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nsled:
What is the negative to flipping a transistor to change its properties?

The only place I've seen it is here

 

at the 4:50 mark where collector and emitter are flipped but it's more a case I've "I've got this transistor and I'm damn well going to use it" in an expedient sense.

Has anyone felt a need, desire or mistakenly flipped a transistor? what generally is the result?

TheUnnamedNewbie:
When you flip a 4-terminal MOSFET or 3-terminal JFET, you usually get the exact same device (in the case of a planar mosfet the source and drain are identical). BJTs usually have a 'reverse beta' in datasheets to illustrate the 'reverse operation' performance. It just becomes a shittier transistor, I don't know of any good reason why you would do it? But old-timers probably have loads of good stories where it just happened to work out they needed that transistor performance!

Keith956:
The negative is that bipolar devices in general are not designed for reverse operation.

It's a bit like fitting the seat in your car facing backwards and driving in reverse.  Sure you can do it but it makes no sense.

Kleinstein:
With a BJT the reverse gain is usually quite a bit lower lower, though the number is not very common in the datasheets.
The leakage goes up and the maximum voltage before breakdown it also lower (often only some 5-10 V).
There is mainly one positive effect: the CE saturation votlage is usually lower in reverse. This was used when lower saturation is needed. However more available MOSFETs are even better in that respect - so the trick is no longer needed.

In the circuit is the lower gain that in this circuit may be a slight advantage - though higher values resistors are also available. Some RF transistors also have relatively low gain to start with.

Whales:
It's possible to use mosfets backwards too; however you need to keep Vds very low (so the body diode doesn't conduct).  They can still amplify :)

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