Overview

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...