Ok, I'm tired of hunting old mesa transistors on Ebay for pulse generators. I also want to get my loop area down to shorten the pulse. Anyone have a SMT part that is available at Digikey or Mouser that avalanches well? Say 3$ a unit max? Only other requirement is a packaging that a 40 year old pair of eyes can handle.
I know about the $14 to 20$ Zetex/Diodes Inc parts.
Thanks,
Steve
What kind of pulse do you want to make? There are many ways to make fast pulses these days. Avalanche is sort of hit-or-miss, involving selecting transistors usually.
Modern epitaxial transistors don't avalanche well. Older diffused-junction types sometimes do. The Zetexes are made in Russia, presumably on an old fab line.
The old standard was 2n2369. I've also used 2n2222, I think it was. I tested a bunch of varieties. I'd have to dig through old notes.
I got pretty good yields, but today you might as well use ordinary logic. It's faster, cheaper, easier, and makes better pulses. Unless you're trying to make amps, that is.
2N2369 is gold-doped and fast out of saturation, but I don't remember it being used in avalanche mode.
...Jim Thompson
I've seen the BFG541 avalanche nicely, albeit at a somewhat lower voltage than a 2n2369. It's faster too.
Jeroen Belleman
We have a little informal contest here on campus for fastest hand made pulser under 20$. Right now I'm loosing to the EE Dept's tech by 50 pS. He had time during the summer to hand select devices. Only avalanche and SRD is allowed.
Why Else? Convenience....
I use pulsers like this in a ultrafast laser lab for testing photodetectors. They drive 1.3 micron VECSELs with a little bias network. Since the budget has to came out of my own pocket, I like the simplicity. I can build them dead bug.
While no where near as fast as a femtosecond laser, its good enough to see if the PD is still good, if the termination is correct etc.
Steve
How fast is he getting? How much voltage?
The fastest commercially available SRD that I know of is 30 or maybe
35 ps. They are a little hard to drive, slamming a bunch of current into them before the stored charge is depleted.Tunnel diodes, ca 1965 or so, had 25 ps risetimes.
There are ECL/CML gates with risetimes in the 12-15 ps range, way out of the $20 budget.
Fast stuff is fun.
I did a little laser driver recently...
and I can get 160 ps light pulses, if I tease the laser just right. We've seen 50 ps light from a fancier driver and the right laser.
At higher current drives, we tend to see this:
where we think the tail is inherent to the laser.
But only if you get the 100-200 mA ones--they have like 200 pF of junction capacitance, so the 1 mA ones are depressingly slow.
Cheers
Phil Hobbs
I've always found 2N3904s work well, surprising for an epitaxial. Fast and RF transistors tend to work as well. Jim Williams was fond of the
2N2369, as suggested; I've avalanched a, I think it was 2SA1016 (if I have that wrong, it's a nearby number, with specs something like 20V, 100mA, fT > 1GHz).Tim
-- Deep Friar: a very philosophical monk. Website:
I posted a Ltspice using LT1720 (fast comparator) that can generate a
15nS pulse, variable up to 250nS. It uses an inductor on the (+) input as the time constant (500uh). Input generator set to match what a Cmos version of the 555 timer can do on the raise and full time to trigger an interval.I wonder how a HEMT would do for you?
Jamie
? 2012?10?8???? UTC
-7??1:42:46?Jeroen???
May I know the BFG541's avalanche switch's resistor parameters you are usin g? Charging resistor and base resistor value?
Thanks
It' been a while... Try 10k from base to GND and a 100k charging resistor.
Jeroen Belleman
Have something to add? Share your thoughts — no account required.
Ask the community — no account required