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From: Tim Williams on 24 Aug 2007 11:49
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message
news:spstc35732r25euh7ql1tbbmt92pjp2h6j(a)4ax.com...
> A 2N3904 is a fast epitaxial transistor, and in my limited experience,
> epitaxials tend to not make decent avalanche pulses.

Yeah, but like I said, I tested these in a different environment and got a
hearty 90V dropping to a couple V in under 4ns. (I was watching on the
solderless breadboard with the 10x probe and 6" ground wire, so don't expect
any remarkable figures.)

All the 2N3904's I have are from the same batch, or at least the same date
(Fairchild logo, "2N3904", "331" = 2003, 31st week?). I think I have some
in my assorted NPN bin though.

> The best pulsers
> are older diffused parts with fairly low Ft's, things more in the
> 2N2000's. I think 2N2369 is a candidate, but even then you may have to
> select different parts, maybe from different manufacturers, to get a
> good one.

I've been here before: you may remember this, which I believe I posted here.
http://webpages.charter.net/dawill/Images/Pulse_1.jpg
This was with a PH2369 from around +100V, which I supplied by an ad hoc
inverter consisting of whatever inductor was handy, a transistor and signal
generator.

> The Zetex avalanche transistors are dependable and make huge pulses,
> essentially shorting out like an SCR when they fire, but are rather
> slow, in the 2 ns rise sort of range.

Interesting, the waveform I observed from the '3904 was a steep fall for
most of the voltage, followed by a flat region about 1us long, followed by
an ~RC rise to ~Vcbo. It really did look like a fast SCR. It occasionally
had short misfires of about 10V in height once reaching Vcbo.

More information on the project: I wend back down and played around with it
some more. My best guess is the sloppy 8ns pulse is actually the sync amp
being amplified (or more likely, followed, given where the emitter is) by
the avalanche transistor, which isn't avalanching, even from +150V. It's
acting more like a zener, obviously with some negative resistance (varying
the voltage up and down shows a hysteretic offset to the output voltage),
but not enough to break over seriously. But get this, it works perfectly
*with* a pulse line. So I get <2ns rising and falling edges (and a stepped
tail, but that's my not-quite-50-ohm attenuator) when I have a cable more
than about 3' long, and it also syncs just fine. But no matter what
capacitance I put on the hot side (up to 15pF), it doesn't work alone!

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms


From: James Arthur on 24 Aug 2007 12:55
On Aug 24, 8:17 am, James Arthur <dagmargoodb...(a)yahoo.com> wrote:

> I tested various transistors in avalanche some time back--I'll check
> my notes on the 2n3904.

Couldn't find what I wanted, but did see that I used a PN2222A
successfully, and the 2n2369a--the old standby--as well.

I never did use the avalanche stuff. I found I could make 500pS edges
with logic gates into ordinary RF transistors, which was a lot more
convenient.

HTH,
James Arthur

From: Martin Griffith on 25 Aug 2007 04:17
On Sat, 25 Aug 2007 00:41:33 -0700, in sci.electronics.design Robert
Baer <robertbaer(a)copper.net> wrote:

>http://webpages.charter.net/dawill/Images/PulseGen1.jpg
http://webpages.charter.net/dawill/Images/PulseGen2.jpg



Martin
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