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Oral History – George Ludwig
(Continued)
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That really was my learning for
transistor circuit design. I have to confess that my
design work of most of the transistor circuits was a combination of intuition and experimentation,
rather than from a theoretical background.
I think that at that time in the development of
transistors there was quite a bit of this type of design work – wouldn’t you say?
Absolutely! That is the way most of
it was done that I came into contact with – even the people at the Naval
Research Lab were doing that.
So, I built a binary
scaler circuit with those surface barrier transistors. It worked OK, but my biggest problem was in getting the power down,
because the circuits that were being
published used relatively high power. I had to go through
a long series of experiments involving many, many tests and many hours with a temperature chamber,
varying resistor and capacitor
values to arrive at a circuit. I
didn’t have a requirement
for a very high counting rate, but it had to operate
down at very low power, so we started increasing the collector resistors up
toward a megohm and more, and we got
them to work pretty well. Then, as we got the junction germanium transistors, I switched over to those, and eventually the silicon transistors came
out and I switched to them.
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Oral History – George
Ludwig
(Continued)
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We used a combination of silicon and germanium transistors in the
final design. We strongly preferred the silicon, because of their
temperature stability, but there were several circuits for which the
germanium worked better because of their
lower base-emitter junction voltage. Silicon transistors required about 0.5 volt base-emitter junction voltage and we were operating off of
a 2.8 volt power supply. So a
0.5 volt base-emitter junction voltage was a pretty large fraction of the
supply voltage. The germanium
transistors, on the other hand, operated at about a 0.2 volt base-emitter junction voltage, and they worked better for
very low voltage circuits.
The silicon transistor types you
selected for the scaler circuit, the TI types 2N335 and 905 – was there any specific reason
you chose these types? Did you try others?
I tried a whole bunch of different transistors. For the silicon transistors, I was in touch with the people at TI on a fairly regulär basis, and as they would have some new ones
coming out, I would get some samples to try them out. I think I settled
on that one as one that was available in reasonable
quantities, and that had desirable characteristics. For later satellites I
switched to newer ones as they appeared.
Go
To Ludwig Oral History, Page 10
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