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Class S Power Amplifiers and Modulators

Class S Power Amplifiers and Modulators

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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.

Chapter Contents:

  • 6.1 Class S Power Amplifier
  • 6.2 Class S Modulator
  • 6.3 Practical Considerations
  • 6.4 Notes
  • 6.5 References

Inspec keywords: modulators; power amplifiers

Other keywords: envelope elimination; phase modulator; modulator efficiency; collector AM; radio transmitters; RF output power; DSB suppressed carrier AM signal; high efficiency unit; band limited RF signal; EER system; vestigial sideband AM signal; RF power amplifier; switching mode PA; audio frequency signal; class S power amplifiers; series coupled collector modulation; single sideband AM signal; low frequency signal; simultaneous amplitude; RF amplifier; transformer coupled collector modulation; high power double sideband; high power AM signal; TCCM system; collector amplitude modulation; supply collector voltage; high efficiency amplification

Subjects: Modulators, demodulators, discriminators and mixers; Amplifiers

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