Re: Some G08 FAQ updates . . .

And I thought I was the only one that went dangerously off topic! OK,
I'll bite.

Joseph J. Welser wrote:

> Actually, because a bipolar transistor is a somewhat symmetrical
> device, all bipolar transistors can be reversed (i.e. emitter and
> collector can be reversed) to some extent.
>
> I carefully used the words "somewhat symmetrical" because even
> though the emitter and collector are both P-type material (in a PNP
> transistor like the MPS-U60) they are doped differently, resulting in
> a "reverse beta" which is different (lower) than the normal "forward
> beta."
>
> If the reverse beta is high enough, the circuit may still work
> properly with the emitter and collector of a bipolar transistor
> reversed. Therefore, if the confusion about what terminal was
> connected to the case of the transistor resulted from hooking it up
> with the case connected where the emitter should be connected, it is
> very possible that things actually worked fine.

You CAN reverse the collector and emitter, but will have the following
problems:

    1) Gain sucks

    2) Voltage limited to emitter-base breakdown voltage, so now
instead of a 200 volt part (for example) you now have a 6 volt part

    3) Really usable only at very low power levels (see below)

> However, other than being academically interesting, I can't
> really think of too many useful reasons to hook up a BJT backwards.
>

Only one really good reason to use it this way (has actually been
done). Like I said, only good for low power and low voltage, but
saturation voltage is actually better running reversed. At my Real Job
many years ago a circuit used this configuration because they needed the
low saturation voltage in a low-level switch.

> Also, any MOS device is perfectly symmetrical, and the source and
> drain can always be reversed. Just be careful that the bulk
> (substrate) remains connected to the source if that is the way it was
> originally configured (and there is a separate connection for the bulk.)

Uh, no, not this time. On a JFET you can do it because it's a gate to
channel function, but MOSFETs reference gate to source (because of the
internal source to substrate connection). And in the case of a power
FET there is a parasitic diode from source to drain. Reversing polarity
will turn on the diode. No harm done since the diode can handle the
full current of the transistor.

>
> l8r,
>
> Joe

While we have the can of worms open, can anyone tell me the forward
voltage drop of an 1N825 6.2 volt zener diode?

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Received on Tue Feb 24 04:05:19 2004