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'[OT] Schottky transistors; why are there not'
1998\06\09@184738 by Morgan Olsson

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< curious, maybe someone here on the list have a clue >
I think most of you found the same as me:
Sometimes we would like to have a transistor with extra low Vbe.

We can sometimed use a germanium transistor, but they are rather expensive,
do not come in small packages, and do not like heat, etc...

There are disctete schottky diodes with advantages like low forward voltage
and high speed.

There are schottky transistors in some logic chips: LSTTL etc.

But why don«t we see discrete schottky transistors??  I believe they would
be very fast, have low Vbe etc.

/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  spam_OUTmrtTakeThisOuTspaminame.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\06\09@203307 by Mike Keitz

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On Tue, 9 Jun 1998 21:48:05 +0200 Morgan Olsson <.....mrtKILLspamspam@spam@INAME.COM> writes:
>< curious, maybe someone here on the list have a clue >

>There are schottky transistors in some logic chips: LSTTL etc.
>
>But why don=B4t we see discrete schottky transistors??

There is no such thing as a "schottky transistor."  The "schottky
transistor" used in LSTTL chips is a combination of a regular transistor
and a schottky diode.  The diode is connected to draw current away from
the base of the transistor as the collector voltage approaches the
emitter voltage.  For an NPN transistor, the cathode of the diode is
connected to the collector and the anode to the base.  This regulates the
drive to the transistor so it doesn't go into full saturation.  Since the
diode conducts with less than Vbe, it conducts when the collector voltage
reaches about 0.3V.  Any excessive drive current flows through the diode,
preventing the base voltage from rising, and the transistor doesn't
saturate.  A fully saturated transistor takes a relatively long time to
turn off, so preventing saturation speeds up the operation of a logic
circuit.

The properties of a "schottky transistor" are similar to those of an
ordinary one except as saturation is approached.  The base-emitter
voltage required to turn it on is the same.  It would be kind of an
interesting device to have in discrete form, though you could build one
yourself from a transistor and a Schottky diode.  Note that schottky
diodes don't like heat either, becoming even more leaky than usual.

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1998\06\09@204522 by Stuart Broad

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You can considerably speed up a silicon transitor and lower it's Vbe by
connecting a schottky diode between the emitter and base.

Regards
Stuart
{Original Message removed}

1998\06\10@151730 by Pedro Barrios

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To make a Schottky transistor just connect a schottky diode
between Base and collector of any BJT. For an NPN connect
Anode to Base and Catode to Collector. For PNP do the opposite.

Regards,

Pedro

At 09:48 PM 6/9/98 +0200, you wrote:
{Quote hidden}

1998\06\10@173857 by paulb
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Morgan Olsson wrote:

> But why don4t we see discrete schottky transistors??  I believe they
> would be very fast, have low Vbe etc.

 To summarise the other answers; Schottky transistors have the same Vbe
as a normal (Silicon) device.

 If however it was in fact Vcesat that you were thinking of, you're in
for a nasty shock as by the same token, Schottky-clamped transistors
have a *higher* Vcesat.

 Cheers,
       Paul B.

1998\06\11@151732 by Josef Hanzal

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Try to swap colector and emiter. You will end up with lower Uce (and also
lower h21e).

Josef

======================================================================
Electronical devices for chemical laboratory, custom electonics design
----------------------------------------------------------------------
Snail Instruments                     Josef Hanzal
Vojanova 615                          phone/fax: +420-311-24433
266 01 Beroun                         e-mail: .....euroclassKILLspamspam.....pha.pvtnet.cz
Czech Republic                        URL: http://www.vitrum.cz/snail/
======================================================================

1998\06\12@063703 by Morgan Olsson

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At 20:33 1998-06-11 +0200, Josef Hanzal wrote:
>Try to swap colector and emiter. You will end up with lower Uce (and also
>lower h21e).

Do you mean i get lower Ucesat when i swap c and e?
Can i really do that?
Lower hFE i know, and probably lower Ucemax and other sideeffects...

I once repaired a ignition system for car, and found they have swapped c
and e.
I thought that was a design error as it worked OK when i replaced it with a
new device which i reconnected to the normal way.   Probably i did not
understand the smart thing... or the designer did not understand problem
with Ucemax or something...

PS  Thank you everybody answering to my schottky question   :)  DS
/Morgan

/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  EraseMEmrtspam_OUTspamTakeThisOuTiname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\06\14@221243 by Josef Hanzal

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>>Try to swap colector and emiter. You will end up with lower Uce (and also
>>lower h21e).
>
>Do you mean i get lower Ucesat when i swap c and e?
Yes, I meant Ucesat.

>Can i really do that?

I remember a very old article (it was at the time, when we were klimbing in
trees here in Czech - at least what concerns electronic) about digital
voltmeter, made from discrete components, roughly eqivalent of ICL7106, but
driving these red-orange vakuum digit displays. At that time there was
nothing close to FETs available to the poor engineers, so they used this
trick for switching analog signals. I do not have hands-on experience with
this, but you try out yourself.

>Lower hFE i know, and probably lower Ucemax and other sideeffects...
>
>I once repaired a ignition system for car, and found they have swapped c
>and e.
>I thought that was a design error as it worked OK when i replaced it with a
>new device which i reconnected to the normal way.   Probably i did not
>understand the smart thing... or the designer did not understand problem
>with Ucemax or something...
======================================================================
Electronical devices for chemical laboratory, custom electonics design
----------------------------------------------------------------------
Snail Instruments                     Josef Hanzal
Vojanova 615                          phone/fax: +420-311-24433
266 01 Beroun                         e-mail: euroclassspamspam_OUTpha.pvtnet.cz
Czech Republic                        URL: http://www.vitrum.cz/snail/
======================================================================

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