RE: Some G08 FAQ updates . . .

    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
or most negative for NMOS devices.) 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.....
 
    OB Vector: So anyone planning on bidding on that Zektor on eBay?
Anyone know who's selling it?

  _____

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 10:17:12 2004