Today... Build Germanium Transistor Projects
with Silicon Transistors!
Return with us now to the halcyon days of germanium!
Germanium transistor projects intrigued experimenters from the early days in the 1950s
until the early 1970s. In the Seventies, the superior silicon devices began to take over the hobby
market. As silicon transistors took over, hobbyists began to attempt germanium-to-silicon
conversions. Often, the germanium circuits didn't cooperate. Several difficulties occurred.
Fortunately, each one has a solution. While the solution doesn't always work, it usually does.
Conversion Problems
Three problems. In converting from germanium to silicon devices, you must solve three
main problems...
- Current gain. The rule of thumb is that silicon devices develop ten times
the current gain of germanium devices. (This statement is only true of small-signal
devices.)
- Leakage. Germanium devices often leak enough that they can bias
themselves. Silicon devices hardly leak at all. If the germanium circuit doesn't have
bias resistors, you must add them. Otherwise, your silicon transistor won't work.
- Bias voltage. For Class-A operation, germanium devices require a no-signal
bias of 0.3 volt. The equivalent Class-A bias for silicon devices is 0.7 volt.
Conversion solutions. For small-signal NPN devices, substitute a 2N2222A silicon
transistor. For small-signal PNP devices, substitute a 2N3906 or 2N2907 silicon transistor. For
power devices, you're on your own. A 2N3055 might work in some circuits. The table below offers
solutions for typical problems...
Conversion Process
|
Symptom
|
Problem
|
Solution
|
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Transistor distorts, or doesn't come on.
|
Improper bias for silicon.
|
- If no base resistor, use base resistor = 400X collector resistor.
- If one base resistor: Change to 400X collector resistor.
- If voltage divider bias: Increase value of resistor to common until resistor drops 0.7
VDC. (Double or triple resistor value.)
|
|
Circuit blocks.
|
Gain of silicon transistor is way too high.
|
In one-resistor bias circuits, multiply base resistor by about 20. (Base resistor is collector
resistor times 400.)
|
|
Circuit distorts.
|
Gain of silicon transistor is slightly high.
|
- Remove emitter bypass capacitor.
- If none, add emitter resistor. Resistor value should be 20% of collector resistor value.
- After adding emitter resistor, adjust base resistors as necessary. (2-resistor circuits:
Increase BE resistor to drop 0.7 VDC. 1-resistor circuits: No change, or
decrease by one standard value until circuit operates properly.)
|
More conversion problems & solutions. See... CK722 and 2N107 Replacements
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