Power amplifiers of the type used in telecommunication applications are often inefficient. It is not unusual for such amplifiers to have energy efficiencies of well under 50%. This causes problems for the providers of telecommunication services because it is often desirable to provide power amplifiers in remote locations, such as hilltops. Delivering electrical power to such remote locations can be exceedingly expensive. Dealing with excess heat given off by inefficient power amplifiers can also be expensive and inconvenient.
Some designs of power amplifiers are known to be more efficient than others. However, not all types of amplifiers are well adapted to telecommunication or radar applications. Such applications typically require amplification systems which meet specifications relating to linearity, bandwidth and/or adaptability.
Switching mode power amplifiers (also known as class S amplifiers) are known to be capable of high efficiencies. The class S amplifier has an idealized efficiency of 100%. Class S amplifiers have mainly been used in lower frequency applications (typically up to a few kHz). To use a class S amplifier at higher frequencies requires switching devices capable of switching state very fast.
The prior art describes various power amplification systems, some of which promise increased efficiency. These include the systems described in U.S. Pat. Nos. 4,717,984; 5,159,287; and 5,905,407.
The characteristics of an amplification system depend not only upon the design of the amplifier itself but also depend upon the way in which the amplified signal is modulated. The so-called delta-sigma modulator has been studied extensively and has some desirable properties. Specific embodiments of the delta-sigma modulator are described, in U.S. Pat. Nos. 5,446,460 and; 5,714,916. Delta-sigma modulators are analysed and discussed in S. R. Norsworthy et al., Delta Sigma data Converters: Theory Design and Simulation, IEEE Press, New York, 1997; James Cherry and W. Martin Snelgrove, Continuous Time Delta Sigma Modulators for High speed A/D Conversion, Kluwer Academic Publishers, Boston, 2000; and Arun Jamayaraman et al., Linear high-efficiency Microwave Power amplifiers using Band-pass Delta Sigma Modulators, IEEE Microwave and Guided Letters Vol. 2 No. 3 March 1998, each of which is hereby incorporated by reference.
Despite the range of power amplification options provided by the prior art there remains a pressing need for power amplification systems which offer increased efficiency. There is a particular need for such systems which can be practically used for telecommunication applications, especially in remote locations