This chapter discusses several types of RF power amplifiers (PAs), most often called Class A, AB, B, and C. Class C amplifiers, in turn, are usually divided into three categories: a) current-source (or underdriven) Class C PAs, b) saturated (or overdriven) Class C PAs, and c) mixed-mode Class C PAs. With the exception of mixed-mode Class C PAs, which behave somewhat differently, the other circuits have the following common features: they have the same basic collector circuit schematic; the circuits are all driven with sinusoidal (or approximately sinusoidal) waveforms; and the active device behaves, at least for a certain portion of the RF cycle, as a controlled-current source.

It is desirable to obtain high RF power amplifier efficiency in many practical applications. At least one, if not all, of the following requirements is important: low power consumption (especially for battery-operated equipment), low temperature rise of the components, high reliability, small size, and light weight. Although the parasitic energy losses in, for example, inductors and capacitors are sometimes important, the major power loss in power amplifiers is usually due to power dissipated in the transistor(s). This was true for the Class A, B, and C PAs (their theoretical collector efficiency was calculated in Sections 2.1 through 2.3).

The Class S amplifier [1-9] is a switching-mode, high-efficiency (ideally, 100 percent) unit used for amplification of low frequency signals - most often audio frequency (AF) signals. High-efficiency amplification of AF signals is especially important for radio transmitters using collector amplitude modulation (AM). As was discussed in Chapters 2, 3 and 4, collector amplitude modulation of Class C or switching-mode PAs is the most efficient way to generate high power double-sideband (DSB) AM signals (full carrier). Collector AM may be also used to generate DSB suppressed-carrier (SC) AM signals, vestigial-sideband (VSB) AM signals, or single-sideband (SSB) AM signals, employing the envelope elimination and restoration (EER) technique discussed in [10-14]. Note that any band-limited RF signal can be regarded as a simultaneous amplitude and phase modulator of a carrier and be amplified with high efficiency using an EER system. The main reason for requiring very high efficiency from the modulator (essentially, an AF amplifier) is that the AF power, which must be supplied to the RF amplifier, has the same magnitude order as the RF output power. Consequently, the overall efficiency of the transmitter is significantly affected by the modulator efficiency. There are two basic solutions for collector amplitude modulation: transformer-coupled collector modulation (TCCM) and series-coupled collector modulation (SCCM) [1-3, 15-20]. The traditional method of obtaining high-power AM signals (employed by vacuum tube amplifiers) uses a transformer coupled AF PA (usually, Class B operated) to change the supply collector voltage of an RF power amplifier (saturated Class C or Class F1 operated). The block diagram of the TCCM system is presented.