Germanium transistor replacements?

Mosaic

Well-Known Member

• Apr 8, 2012

• #1

Hey all,

I was looking around for a low Vbe transistor to use as a switch. Germaniums had a low Vbe but are hard to come by now. I'd rather not have to use an OPA comparator, any suggestions?

 

L

ljcox

Well-Known Member

• Apr 8, 2012

• #2

Why does it need to be low Vbe?

We may be able to help you more if you tell us what you're wanting to do.

eg. post your circuit.

 

ronsimpson

Well-Known Member

Most Helpful Member

• Apr 8, 2012

• #3

You could bias the base so it is 1/2 on. (bias at .3 volts)
OR
Bias the emitter at -.3 volts.

If you try to bias at 0.3 volts, don't just use 0.3 volts but use 1/2 of a diode drip so it will be temperature stable.

 

Mosaic

Well-Known Member

• Apr 9, 2012

• #4

Well, it's a bit of an experiment, but I want to use a transistor to act as a current limit for a P Fet. The P FET Rds on is around .17 ohm, I'd like to limit the current to around 2 amps without adding more series resistance. So a .34 Vbe is too low for silicon transistor switching.

 

unclejed613

Well-Known Member

Most Helpful Member

• Apr 9, 2012

• #5

you're kind of doing this backwards, since reducing the current through the FET (by reducing the gate drive) INCREASES Vds. if your current limiter is not the FET, then it might work. you can use an op amp to sense Vds across the FET.

 

Mosaic

Well-Known Member

• Apr 9, 2012

• #6

Hmm, I guess I didn't think that one thru...

 

unclejed613

Well-Known Member

Most Helpful Member

• Apr 9, 2012

• #7

you could, however feed the FET with a 2A constant current source... it would be easier to help if i had more detail about what you're trying to do.