Overview

RF power amplifiers use a wide range of active devices, such as vacuum tubes and transistors (usually BJTs or MOSFETs). Vacuum tubes, still widely used in high-power RF amplifiers, have the following important advantages [1, 2]:

• Tubes are available in a wide range of frequencies (up to several GHz) and power ratings (up to several megawatts).

• Their parameters are almost constant over a wide frequency range.

• They exhibit high power gain (up to 15 20 dB). This is especially true for tetrodes in a common-cathode circuit.

• Tubes exhibit high efficiency when operated in Class C or F1.

• They have low harmonic content when operated in Class A or Class AB push-pull circuits.

• They exhibit low nonlinear distortion when used in plate amplitude modulation circuits.

Modern ceramic tubes also offer high reliability, long service life, and low power consumption for the filament (heater) circuit. Vacuum tube cathodes lose their emission capability gradually and exhibit a slow degradation in performance that allows for timely replacement. New techniques used in vacuum-tube radio transmitters, such as pulse step modulation or dynamic carrier control, allow these transmitters to achieve remarkable performance.

Bipolar and MOS transistors are used increasingly in high-power RF amplifiers for powers up to more than tens of kilowatts. They display important advantages over the vacuum tubes, including higher reliability and longer life, small size and weight, lower cost, easier cooling, and low input and output impedances (reducing parasitics).

Today s technologies only allow for powers of 300 to 600...