A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio. Other types of linear amplifier are used in audio and laboratory equipment.
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An RF linear amplifier can be based on either solid-state or vacuum-tube technology. For an amplifier to be considered linear, the amplified output must be a faithful reproduction of the input. The basic linear amplifier has limitations that are functions of its load impedance, supply voltage, input bias current, and limitations of power output capabilities.[1]
There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy. Their use in RF applications are listed briefly below:
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.
The legal power limit for licensed amateur operators vary from country to country but in the United States it is legal to transmit up to 1.5 kW peak envelope power output, while in the UK the limit is 400 watts PEP.
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, and CW (morse code).
The first large amplifier used in the United States for public domestic radio broadcasting was in operation between 1934 and 1939 at WLW in Cincinnati. It was an experimental amplifier and was driven by the radio station's regular 50 kW transmitter. Not a linear amplifier, it operated in class C with high-level plate modulation. The amplifier required a dedicated 33 kV electrical substation and a large pond complete with fountains for cooling. It operated with a power input of about 750 kW (plus another 400 kW of audio for the modulator) and its output was 500 kW.
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