Amplifier manufacturers have long compared the advantages and disadvantages of solid state power amplifiers, SSPA, and traveling wave tube amplifiers, TWTA. Every few years there is typically a technology advance that gives a slight advantage to one technology over the other. For SSPAs, the advantage often follows an advance in Gallium Arsenide (GaAs) device technology. A higher power transistor enables the amplifier designer to increase the output power capability of an SSPA. On the other hand, solid state linearizer or predistortion circuits are now frequently used with TWTAs to improve distortion and make the TWTAs power transfer curve more closely approximate that of the SSPA. In recent years depressed collector techniques have improved the efficiency of TWTAs. Despite efficiency increases in SSPA and TWTA technology, both still suffer from relatively low overall efficiency. This requires both technologies to employ high-power, power supplies to operate their respective microwave devices. Anytime low efficiency circuits consuming high power levels are encountered, thermal design becomes a major concern. In fact, as the reliability of both tubes and transistors improves, it is noted that amplifier system failures are often due to power supply and thermally related problems.
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