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Old 13th May 2022, 11:31 am   #7
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Germanium transistor replacement

There are quite wide spreads in transistor parameters, so in a lot of respects you can get away with murder. But the most dependable parameter is Vbe. Roughly assumed to be 0.7v for Silicon, and close to half that for Germanium.

Carefully designed circuitry takes into account fairly large variations in gain from device to device, but Vbe is taken as almost gospel.

Where things get difficult is in circuitry with either very low supply voltages, or with very crude bias arrangements. You need to look at the circuitry around each transistor being replaced to spot these cases and then consider bias resistor value changes. Most often, you can do the job by changing one or two resistor values, but you do need to check.

Silicon still has a significant sensitivity to temperature, but it's much tamer than Germanium, so substitution is often an improvement. In general temperature compensation with germanium transistors was a problem, and often not done, so you aren't likely to run into trouble with too little temperature sensitivity.

The current gains of some Germanium power transistors were somewhat better than many silicon types, so care is needed here.

David
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