8

Circuits Based on Semiconductor Devices

In Section 1.4 we discussed early development of the radio. At first both the transmitter and receiver were based on a spark gap. Then came a coherer as a detector, and an electron tube was invented in 1904. Nowadays semiconductor devices have replaced electron tubes in most applications.

8.1 From Electron Tubes to Semiconductor Devices

Electron tubes came into wide use in radio engineering in the 1910s. They were used as oscillators, modulators, amplifiers, mixers, and detectors. A suitable electron tube was developed for different applications: diode, triode, tetrode, pentode, hexode, heptode, and octode. The names of the different electron tubes are based on the number of electrodes: a diode has only a cathode and an anode; a triode has in addition one grid, which can be used to control the current from the anode to the cathode by a small applied voltage the same way as is done today in the field-effect transistor (see Section 8.2.2); a tetrode has two grids; and so on. Although the transistor was invented in 1948 and a semiconductor diode earlier, electron tubes are still in use. They are used as transmitter tubes in LF, MF, HF, and VHF radio broadcasting stations, and in some military electronics because of their high tolerance for strong electromagnetic pulses.

Currently, in most radio applications we use semiconductor devices for signal generation, amplification, detection, and so on. The advantages

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