Amateur Radio
Most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tubes (valves) and can provide 10 to 20 times RF power amplification. For example, a transmitter driving the input with 100 watts will be amplified to 2000 watts (2 kW) output to the antenna. Solid state linear amplifiers are more commonly in the 500 watt range and can be driven by as little as 25 watts. However, AM radio broadcast transmitters of up to 50 kW are now solid state. Large vacuum tubes are still used for international long, medium, and shortwave broadcast transmitters between 500 kW up to 2 MW.
As most amateur radio transceivers can output between 100 and 150 watts, an amplifier is needed to reach higher power levels. Large vacuum-tube linear amplifiers are based on old radio broadcast techniques and generally rely on a pair of large vacuum tubes supplied by a very high voltage power supply to convert large amounts of electrical energy into radio frequency energy. Linear amplifiers need to operate with class A or class AB biasing, which makes them relatively inefficient. While class C has far higher efficiency, a class-C amplifier is not linear, and is only suitable for the amplification of constant envelope signals. Such signals include FM, FSK, MFSK, and CW (morse code).
Read more about this topic: Linear Amplifier
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