BJT Base emitter cutoff

This sounds very confused. I've crossposted it to s.e.b, which is the right place for it. I think it'd help if you drew the circuit out, including full details of what youre feeding in and what load you're using.

NT

The transistor can be modelled as having an intrinsic emitter resistor re =

Those cut-off currents in the datasheet are collector-emitter leakage current and base-emitter reverse leakage when the transistor is on the off state.

And those numbers are as useless of the proverbial appendages on the boar hog.

In Spice modeling terms, leakage degradation effects from the ideal logarithmic bipolar behavior are characterized by IS, ISC, ISE, NF, NE and NC. ...Jim Thompson

Transistors do stop acting like transistors below some minimum current level. If you really want your circuit to work properly down into the nanoamps or picoamps, you might want to look at the BFT25A, or even better, the BFG25AW. They're very fast, small geometry devices whose f_T and beta hold up much better than most others' at low I_C.

Cheers

Phil Hobbs

Or use CMOS... it likes COLD ;-) ...Jim Thompson

Sometimes it's the collector current you care about, though, e.g. in some kinds of photodiode front ends, or when low power analogue is the order of the day. There are lots of things you can do with bipolars that are hard with MOSFETs, at least if you don't have thousands of them to play with. ;)

Cheers

Phil Hobbs

Got any idea of betas at low currents?

I know that their leakages are absurdly low.

John

The AW is close to 75 from Ic of 1 uA to about 1 or 2 mA.

Click on data sheet:

Watch out for line wrap.

The AW is close to 75 from Ic of 1 uA to about 1 or 2 mA. ^^^^^^ BFG25AW

Click on data sheet at:

Watch out for line wrap.

They hold up way lower than that. I need to make a bunch of really boring 1/f noise measurements tomorrow, so I'll try measuring a few betas if I get motivated. That'll give me a chance to use my lovely $5 Keithley 410 micro-microammeter. ;)

Cheers

Phil Hobbs

Fantastic! The data sheet curve didn't go below 10^-3 mA.

Your $5 meter is a great bargain, needless to say.

John

I paid $25 shipping, though. ;0

Cheers

Phil Hobbs

ent

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That still counts.

I made $0.50 the one time I gambled. We celebrated with $6 hotdogs.

-- Cheers, James Arthur

Dangnabit, I paid $150 for this one:

ftp://jjlarkin.lmi.net/Keithley_1gig.JPG

John

No, it means that the BJT is working _exactly_ as intended, i.e., when there's no base current, there's very little emitter or collector current. That's why it's called "cutoff."

Hope This Helps! Rich