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 of the semiconductor devices over the electron tubes are their small size, low weight, low supply voltage, and long lifetime.
Circuits based on semiconductor devices may be either active or passive. A circuit is said to be active if it generates RF power from dc power (oscillator) or it amplifies an RF signal (amplifier). In a broader sense, we can consider some other circuits as active: A mixer, frequency multiplier, detector, modulator, and demodulator are all active because they convert power from one frequency to another. However, not all circuits containing semiconductor devices, that is, diodes and transistors, are active. They are also used to realize switches, attenuators, phase shifters, and loads needed in impedance matching (e.g., active matching in MMICs). Figure 8.1 presents standardized drawing symbols of some circuits based on semiconductor components.
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