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Subject: Re: Explosions in electronics.
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
Date: Thu, 3 Oct 2002 19:46:23 -0400
NNTP-Posting-Date: Thu, 03 Oct 2002 19:41:28 EDT
Organization: Golden Triangle On Line Inc.
> That brings up a question. Does anyone use transistors in avalanche mode
Oh yeah, that triggers old memories...
I remember a project in university - building a device to measure the length
of optical fibres by means of measuring the time it takes for the light to
travel and by using one of the first available laser diodes (i guess it was
a LD 60). The laser diode was rated with a max. current of 10A producing 3W
burst output, BUT at a duty cycle of 1:1000! To measure accurately, the
rising time needed to be sufficiently short as an accuracy of 10cm was
anticipated. This roughly translated to 1ns for the 10 amps or 0.1ns/A. At
the time, for these currents the only feasible way seemed to be forcing
transistors into the avalanche mode. To produce a good rectangular pulse of
about 50ns I used 5 meters of coax cable using it as charge storage. To
attain that current, the generator was built out of 4 individual avalanche
transistor stages, where each collector was hooked up with 4x5m coax cable
(16 cables in total) in parallel and the voltage to charge the cables was
around 70-100V. The discharge current then added itself on all emitters and
flowing through the laser diode to the common ground. To gate all 4
avalanche transistors on their base I used another small avalanche circuit
with a small capacitor instead of coax cable. The really fantastic thing
was, that each transistor stage produced about 0.2ns/A and all together made
it to 1ns and the desired 10A current. My professor was amazed by the idea
and I got a straight A. Well there were obvious shortcomings, like power
dissipation and the temperture coefficient of the avalanche effect, but we
got the diode firing pretty good without damaging it. I must also point out,
that this took place in the 1980's in former East Germany, so except for the
laser diode all parts where made in East Germany. For someone who knew that
time and regime will surely appreciate the story. Thankfully we left this
country in 88 with these memories prevailing...
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