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'[EE]: Are there any small germanium transistors st'
2002\06\18@055021 by ar=E3es?=

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Hi,

   Does anyone knows about any small signal germanium transistors that are
still in production ???!!

   I need slower vbe than silicon to use in a short circuit protection
system. The 0.6 volts of silicon transistor makes me use a large wire wound
resistor that could have it's value divided by 3 and would also get the
dissipation much lower. For this application the leakage problems should not
make any difference. It is either that or complex op-amps circuits and the
thing has high side drivers wich make it harder to measure current. The
project is very price sensitive so I am not able to use protected FET's.

Best regards,
Alexandre Guimaraes

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2002\06\18@061155 by Mark J. Dulcey

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Alexandre Guimarães wrote:
> Hi,
>
>     Does anyone knows about any small signal germanium transistors that are
> still in production ???!!

I believe that the short answer is no.

The long answer is that SiGe (silicon-germanium combinations) are one of
the very latest things for transistors for extremely high frequencies (up in the microwave range). But they're not what you're looking for.

Electronic Goldmine has some NOS (very old) germanium transistors for $1
each:

http://sales.goldmine-elec.com/prodinfo.asp?prodid=3661

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2002\06\18@091628 by Michael Rigby-Jones

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{Quote hidden}

resistor.  Use a diode to provide one Vbe volt drop on the output of the
sense resistor and reference the base of the transistor to a fraction of the
voltage accross the diode.  This gives the possibility of very fine current
control and reduces the temperature sensitivity of the current limit
(depending on the fraction of the diode drop you sense).  HTH.

Regards

Mike

        Rsense
---+-----\/\/\/\/---+--------+------
  |                |       _|_
   \ e             \       \ /
     >|__________> /       -+-         /| b          \        |
   / c             /        |
  |                |        |
  |                +--------+
  |                         |
  |                         \
  |                         /
  |                         \
  |                         /
---+-------------------------+------



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2002\06\18@092213 by Michael Rigby-Jones

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{Quote hidden}

would obviously go to whatever you are using to control voltage.

Mike

        Rsense
---+-----\/\/\/\/---+--------+------
  |                |       _|_
   \ e             \       \ /
     >|__________> /       -+-         /| b          \        |
   / c             /        |
  |                |        |
  |                +--------+
  |                         |
  |                         \
  v To current limit        /
                            \
                            /
-----------------------------+------



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2002\06\18@134827 by Dwayne Reid

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At 06:49 AM 6/18/02 -0300, =?Windows-1252?Q?Alexandre_Guimar=E3es?= wrote:
>Hi,
>
>     Does anyone knows about any small signal germanium transistors that are
>still in production ???!!
>
>     I need slower vbe than silicon to use in a short circuit protection
>system. The 0.6 volts of silicon transistor makes me use a large wire wound
>resistor that could have it's value divided by 3 and would also get the
>dissipation much lower.

You can bias the base of the transistor so that it is not quite turned
on.  There are a couple of ways of doing this - how temperature stable does
this whole thing need to be?

The easiest way should just involve adding a single resistor.  You probably
have the base connected to your current sense resistor via a 1K or so
protection resistor.  Just add another resistor to the base that pulls the
base towards conduction by 0.4V or so.

The words sound confusing but it is very simple.  It would be easier if you
could describe your circuit.  Or even better - draw your circuit in ASCII
and post it.

dwayne

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2002\06\19@140130 by Peter L. Peres

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I am sure that there is a way to rearrange your circuit so you can use a
PNP transistor with gain instead of Vbe of a NPN (or the other way
around).

Peter

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2002\06\23@001445 by Russell McMahon

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You MAY be able to current sense at lower voltage drops using a long tailed
pair using only two (or possibly three) transistors.

Also something like an LM358 (dual opamp) or LM339 (quad comparator) is
quite cheap - probably around $US0.20 in volume, maybe less.)

Having a transistor partially turned on with external bias would also allow
lower voltage drop across the sense resistor but complicates the circuit
somewhat.


           Russell McMahon



{Original Message removed}

2002\06\24@005055 by ar=E3es?=

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Hi, Russel

> You MAY be able to current sense at lower voltage drops using a long
tailed
> pair using only two (or possibly three) transistors.

   I am not quite sure that I could trust the match between the transistor
to form a differential amplifier and get good results. It might be worth a
try together with a "current mirror" circuit. A little more complex but
might work.

> Also something like an LM358 (dual opamp) or LM339 (quad comparator) is
> quite cheap - probably around $US0.20 in volume, maybe less.)

   I have some problems with that option. The environment I will be working
with has very noisy supply and voltage can go easily up to 80v ! I would
have to power the opamp from a "shunt" power supply referenced to the
positive rail because I have to use high side driver transistor and have not
control on ground return ! Cheap op-amps do not like to work too close to
the supply rails. Simple op-amps would be too much trouble in my opinion.

> Having a transistor partially turned on with external bias would also
allow
> lower voltage drop across the sense resistor but complicates the circuit
> somewhat.

   That is exactly what I am trying to accomplish right now. It should work
but I am still trying to sort out some problems. I decided to use it using
the RDS-ON off the FET together with the shunt resistor and there are some
nasty problems when the FET is off or there is no load. As a good side
effect I should be able to show when there is a short circuit or detect an
open load condition. I should have it working by the end of next week and I
will get the results back here. It is the least complicated solution, even
with the added parts to bias the transistor.

Best regards,
Alexandre Guimaraes

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2002\06\24@023323 by Jinx

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> > You MAY be able to current sense at lower voltage drops using
> a long tailed
> > pair using only two (or possibly three) transistors.
>
>     I am not quite sure that I could trust the match between the
> transistor to form a differential amplifier and get good results

How about a supermatch pair, eg LM394 ?

http://www.national.com/pf/LM/LM394.html

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2002\06\24@025405 by ar=E3es?=
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Hi,


> > > You MAY be able to current sense at lower voltage drops using
> > a long tailed
> > > pair using only two (or possibly three) transistors.
> >
> >     I am not quite sure that I could trust the match between the
> > transistor to form a differential amplifier and get good results
>
> How about a supermatch pair, eg LM394 ?
>
> http://www.national.com/pf/LM/LM394.html

   Those are very expensive parts. It would be cheaper to buy a ready made
current sensor chip !

Best regards,
Alexandre Guimaraes

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