Input protection, (C-B current of transistor)

It's got to be a base current limit. The one spec that I checked didn't have one, though someone might have extrapolated the 200mA Ic limit with the standard

Thanks, I figured something like that. I'm 'allowed' to push as much current through the BC junction as through the BE?

George H.

I'd say the 20mA is just a safe ball park figure, not a precisely calculated limit. My datasheet for the 2N3904 does not give an absolute maximum base current rating. It's probably around

If you connect BE together, it should take the full 200 mA that the datasheet suggests for Ic(continuous); that would be the safe limit of the connection wiring. Base current goes through Rbb, so there might be some non-ideal-diode behavior in using it for a clamp.

The BAV199 series of dual diodes leak at pA levels, several orders of mag below regular signal diodes. BAV199DW has two series pairs in one package.

A nice current limiter is back-to-back LND250 depletion mosfets.

This can stand +-250 volts:

ftp://jjlarkin.lmi.net/22S490B_ch12.pdf

John

Oh, +/- 15V supply rails. not enough reverse bias voltage in the Vbe junction.

George H.

Opps sorry I misunderstood your post. BE together? I'll have to try it. Thanks,

George H.

shure, revers mode. This is well known. The chfe isn't big in this configuration but most of the current will flow through emitter to collector as it should be.

Marte