RF power amplifiers operate at very high frequencies but have relatively narrow bandwidths. They typically employ frequency-selective networks as load circuits. RF power amplifiers can be classified into three operating modes based on the conduction angle of current: Class A, Class B, and Class C. Class A amplifiers have a conduction angle of 360°, making them suitable for small-signal low-power amplification. Class B amplifiers have a conduction angle of 180°, while Class C amplifiers have a conduction angle less than 180°. Both Class B and Class C are suitable for high-power operation, with Class C offering the highest output power and efficiency among the three modes. Most RF power amplifiers operate in Class C, but the current waveform distortion in Class C amplifiers is too severe, limiting their use to resonant power amplification with tuning circuits as loads. Due to the filtering capability of tuning circuits, the loop current and voltage remain close to sinusoidal waveforms, resulting in minimal distortion.
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