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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 (10 to 13 dB). 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.
Large vacuum-tube linear amplifiers generally rely on one or more 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,
Class-C amplifiers are still more efficient. They can be about 75% efficient with a conduction range of about 120°, but they are very nonlinear. They can only be used for non-AM modes, such as FM, CW, or RTTY. The semiconductor or vacuum tube conducts through less than half the RF cycle. The increase in efficiency can allow a given vacuum tube to deliver more RF power than it could in class A or AB. For instance two 4CX250B tetrodes operating at 144 MHz can deliver 400 watts in class A, but when biased into class C they can deliver 1000 watts without fear of overheating. Even more grid current will be needed.
Although class-A power amplifiers (PA) are best in terms of linearity, their efficiency is rather poor as compared with other amplification classes such as “AB”, “C” and Doherty amplifiers. However, higher efficiency leads to higher nonlinearity and PA output will be distorted, often to extent that fails the system performance requirements. Therefore, class-AB power amplifiers or other variations are used with some suitable form of linearization schemes such as feedback, feedforward or analog or digital predistortion (DPD). In DPD power amplifier systems, the transfer characteristics of the amplifier are modeled by sampling the output of the PA and the inverse characteristics are calculated in a DSP processor. The digital baseband signal is multiplied by the inverse of PA nonlinear transfer characteristics, up-converted to RF frequencies and is applied to the PA input. With careful design of PA response, the DPD engines can correct the PA output distortion and achieve higher efficiencies,