Germanium diodes

Germanium diodes are a type that has been in use since the early days of radio and although they have been largely superceded by silicon signal diodes and integrated circuits, they may still be found as signal diodes in older radio circuits, where they are used in such applications as radio signal detectors (demodulators) where the voltages involved in demodulating AM radio signals are likely to be small, less than 1Vpp (0.5Vpk). Germanium diodes will conduct at a forward voltage of only about 0.15V, but a silicon diode will not start to conduct until a forward voltage (positive on the anode) of 0.6V is reached.

The demodulator that recovers the audio signal from the amplitude modulated (AM) radio waves can use a germanium diode as a rectifier to remove the unwanted half of the AM signal. It works in a similar way to a rectifier diode in a simple power supply; it passes one half of the incoming radio signal and blocks the other half. The main differences being that the frequency of operation is much higher, also the voltages used in a demodulator are tiny by comparison to a mains (line) power supply.