Circuit Configurations

The most common application of vacuum tubes is in amplifiers, especially in radio and television transmitters at power levels of more than 1 kW. Some high-fidelity audio systems also employ vacuum tubes. In recent years, tubes have gained favor with some popular music bands. Some musicians insist that “tube amps” provide richer sound than amplifiers using power transistors. There are two basic vacuum-tube amplifier circuit arrangements: the grounded-cathode configuration and the grounded-grid configuration.

Grounded Cathode

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Simplified schematic diagram of a grounded-cathode RF amplifier circuit using a triode tube.
Above figure is a simplified schematic diagram of a grounded-cathode circuit using a triode tube. This circuit is the basis for many tube-type RF power amplifiers and audio amplifiers. The input impedance is moderate, and the output impedance is high. Impedance matching between the amplifier and the load can be obtained by tapping a coil in the output circuit, or by using a transformer.

Grounded Grid

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Simplified schematic diagram of a grounded-grid RF amplifier circuit using a triode tube.
Above figure shows a basic grounded-grid RF amplifier circuit. The input impedance is low, and the output impedance is high. The output impedance is matched by the same means as with the grounded-cathode arrangement. The grounded-grid configuration requires more driving (input) power than the grounded-cathode scheme. A grounded-cathode amplifier might produce 1 kW of RF output for 10-W input, but a grounded-grid amplifier needs 50 W to 100 W of drive to produce 1 kW of RF output. A grounded-grid amplifier has a significant advantage, however: it is less likely to break into unwanted oscillation than a grounded-cathode circuit.

Plate Voltage

The plate voltages (+600 V dc) in the circuits of Figs. topmost and just top are given as examples. The amplifiers shown could produce 75- to 150-W signal output provided they receive sufficient drive and are properly biased. An amplifier rated at 1-kW output would require a plate voltage of +2 kV dc to +5 kV dc. In high-power radio and TV broadcast transmitters producing in excess of 50-kW RF output, even higher dc plate voltages are used.