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 higher-grade silicon devices began to take over the
hobby market. Hobbyists adapted, by attempting germanium-to-silicon conversions. Often, the
germanium circuits didn't cooperate. Several difficulties occurred. Fortunately, each one had a
solution. While the solution didn't always work, it usually did. This page describes the
Four Problems. In converting from germanium to silicon devices, you must solve four
- 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
- Leakage. Germanium devices often leak enough that they can bias
themselves. Silicon devices hardly leak at all.
- 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.
- Silicon device shape might differ from the germanium
device shape. The device pins might be in a different order.
- Current gain. In some circuits, more gain will be helpful. For example, the revised
circuit might be more sensitive than the original circuit. But sometimes, limiting the
gain (adding an emitter resistor, etc.) will be necessary.
- Leakage. Bias by leakage won't be possible with normal silicon devices. If the
germanium circuit doesn't have bias resistors, you must add them. Otherwise, your silicon
transistor won't work.
- Bias voltage. Correct bias is imperative. The main point of this page is rebiasing
your transistor base-emitter junction. The correct bias will allow the device to
- Silicon device shape. Package profiles have changed over the years. Expect
the silicon device to be a different shape than the legacy germanium part. We're
looking for reasonably similar behavior, not form. Often, the device pins will be in
a different order. With persistence and ingenuity, you'll be able to fit the new
device in the cabinet. You might have to shape the pins to fit the mounting holes
in the PC board.
Universal Replacement Parts
For universal replacement parts, see our page Replacement Transistors,
Capacitors & Diodes.
The table below offers help with typical conversion
Solution (silicon circuit)