NPN operated in reverse question

The degradation seems to stop getting worse after a bit, lots of people build noise sources with this configuration, and I think they find them reasonably stable.

I doubt, with a 100K pull-up, that there will be enough degradation to fret about.

Without actually analyzing your circuit my question would be what effects will the low inverse beta have on the current-limit accuracy? ...Jim Thompson

Zetek makes high inverse beta transistors; well above an otherwise typical 0.01; look for high e-b breakdown. OOPS! now-a-daze, nobody specs actual breakdown; everybody seems to lie and say 8V when it can range from 6V to 20V depending on the manufacturer and 2N.

How about a good PNP?

I don't think a PNP will do what I want. But, rather than run the NPN inverted, I could just us it normally. That would work fine, but then I don't get gate protection.

I measured a Fairchild BCX70 NPN as having inverse beta of 16 and b-e zener voltage 7.9.

Thanks for the info, John. I measured the e-b zener voltage of a 2N4400 about the same as yours. I didn't measure the beta as I am willing to accept a beta down to about 1. More than that would be pleasant.

you don't get it for free, you can add a zener from collector to base.

Has anyone seen quantitative data as regards beta degradation vs integrated b-e zener current?

I guess it would be worse for small devices.

I did, for pulsed zaps, capacitive discharge. Beta falls exponentially, rapid at first then levels off and settles at about 65%-80% of initial beta.

I used only small jelly drop devices like 2n3904, bc847, 2n3702 etc e.g. beta 540 becomes 390. I also tried some 1960s GE 2N2925s I had and the effect was similar but beta was lower, initial beta 80 fell to 62. After a few million zaps of 0.47uF charged to 30V via 330 ohm.

piglet

How quickly do you need the FET to switch? There's a fair amount of gate charge involved.

Sylvia.

That's interesting. Losing half one's beta wouldn't be a disaster. But long-term DC zenering might be different.

A million zaps of .47uF x 30V is 15 coulombs, which is a fair chunk of charge.

What kind of NMOS circuit has 25V in the source circuit??? Do you bother to tell what the drain is connected to? And the NPN collector to GND? And why do you need gain on that clamp??? It's impossible to understand anything from your description.

I understand it.

No switching. It will be a current sink operating in the linear region.

Finaly some one makes sense..

we've dealt with a circuit like in a lab HI-POT unit that works fine in the lab, dies after about 6 monts of use in production using that type of current limiting and suppression.. Since it was a repeating case we decided to turn the replacement tranny around and employ the zener...

The gain on the tranny drops to the point where the current limiting no longer works and causes the mosfet to go into fall conduction which then cooks a couple of other components..

jamie