Re: Some G08 FAQ updates . . .

Joseph J. Welser wrote:

> RE: BJTs, I agree with everything you said (forgot about the
> breakdown voltage aspect of reversing C and E, though.) In my
> previous post, beta = base-to-collector current gain, so when I said
> "bad-beta" that's essentially the same as saying "gain sucks" (what
> you said.)
>
> RE: MOSFETs. What you said is true if the bulk is always tied to
> the source, which is not always the case. The device does not care
> which terminal is the source or drain except for the fact that the
> source might be tied to the substrate (the substrate also might be
> tied to the most positive or negative voltage present in the system --
> most positive for PMOS devices

in effect the source

> or most negative for NMOS devices.)

Still the source

Do what you want but keep in mind the gate source maximum voltage spec
will be the limit to voltage handling. Still don't think it will work
on the average power MOSFET, know for a fact it will work on small
signal JFETs.

> How could there be a parasitic diode between Drain and Source in a
> power MOSFET when both D and S are the same type of material? All
> MOSFETs have a parasitic BJT present from D-B-S -- oh maybe that's
> what you mean, if B and S are always tied together, than that BJT is
> essentially a diode. But that occurs in every MOS device. Anyways, I
> guess we are way off topic.....

Time to look at a data sheet. ALL the IR HEXFETs (pretty much the
industry standard) have source to drain diodes. There are even times it
can be a problem (see this application note
<http://www.irf.com/technical-info/an936/an-936p8.htm>).

>
> OB Vector: So anyone planning on bidding on that Zektor on eBay?
> Anyone know who's selling it?

Not me on either question.

>
> ------------------------------------------------------------------------
> From: owner-vectorlist@vectorlist.org
> [mailto:owner-vectorlist@vectorlist.org] On Behalf Of Rodger Boots
> Sent: Tuesday, February 24, 2004 3:05 AM
> To: vectorlist@vectorlist.org
> Subject: Re: VECTOR: 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 15:04:48 2004