On Thu, 17 Dec 2009 11:39:35 -0500, "Michael A. Terrell"
wrote:
Jim Thompson wrote:
On Thu, 17 Dec 2009 15:48:49 -0000, "ian field"
wrote:
"Jim Thompson" /Snicker
wrote in message ...
On Thu, 17 Dec 2009 02:27:23 -0800, Robert Baer
wrote:
ian field wrote:
What's the best approach to testing salvaged germanium transistors?
Even the peak atlas transistor analyser reads leakage current as gain
current and the DMMs I have are completely useless at testing germanium
transistors. Somewhere I have a schematic for a transistor tester that
nulls
out the leakage before taking a gain measurement, the leakage is read
off
the calibrated null pot, but I'm wondering whether it might be better to
measure the AC gain?
What I'm thinking of is driving the transistor under test with an
oscillator
with its output clipped by an inverse parallel pair of diodes and
measuring
the rectified output of the TUT to calculate gain.
One particular advantage I'm thinking of, is an amplifier can be added
to
evaluate how much hiss the TUT contributes.
The clever bit would be deciding what biasing circuit to use that would
betray the leakage figure by simple voltage measurement under DC
conditions.
Any comments/suggestions welcome.
TIA.
Once upon a time, a long time ago, RS under their Micronta brand made
a tester that bypassed the DC gain problem by the use of a transformer
for AC feedback, and a pot on the feedback secondary for feeding the
signal to the base.
Pot setting gave an indication of the gain (at the collector current
set by the DC biasing; think that was a common bace bias scheme).
They used an inverse log taper pot to get a more linear gain readout.
Maybe this is of some help.
I can't even remember what the expected gain ranges were for Germanium
;-)
But I thoroughly remember several weeks spent in the classroom (1960)
studying biasing techniques.
Anyone have a curve tracer?
Which variant of common emitter is most susceptible to leakage?
The 2 choices I'm thinking of are the voltage divider base bias and emitter
resistor,
That's actually pretty good if adequate drop is taken across the
emitter resistor. 0.25V or more is best for Silicon... I can't
remember for Germanium ;-)
or the large nfb collector to base resistor with no emitter
resistor.
Or no nfb... just straight to rail... commonly know as "suicide bias".
:-(
The worse the stage is affected by leakage, the better I can check leakage
by measuring the drop on the collector resistor (I think?!).
Sure.
(For analysis, simply add a current labeled ICBO from collector to
base, then calculate what your bias network does to improve that.)
The ARRL handbooks had a transistor tester in a lot of editions, but
the only one I have at hand is the '55 edition. I'll look later today
or tomorrow for a mid '60s edition.
Thanks for the reminder, I may actually have a drawing around here
myself, for such a tester.
...Jim Thompson
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