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The linear amplifier can be based on solid state or vacuum tube technology and can be defined as a device whose output is proportional to its input, but is capable of delivering more power into a load. The linear amplifiers find a general use in amateur radio and it usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts. While talking of the linear amplifiers today, the most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tubes or valves and can provide between 10 to 20 times RF power amplification. This can be explained with the help of an example, 100 watts input from a transmitter will be amplified to 2000 watts (2 kW) output to the antenna using the valve tubes. Other than the valve tube amps, the solid state linear amplifiers are more commonly in the 500 watt range and can be driven by as little as 25 watts to be precise, although AM radio broadcast transmitters of up to 50 kW are now solid state. In the case of International long, medium and shortwave broadcasting between 500 kW up to 2MW large vacuum valves or klystrons are still used, in such a case usually several 500 kW transmitters in parallel are used to complete the task. Amateur radio transceivers for output between 100 and 150 watts an amplifier is needed to reach 2000 or 400 watts and in most of the cases it does. Mostly the large valved linear amplifies are based on old radio broadcast techniques and usually 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.
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