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Class-D Audio Amplifiers: System Considerations

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Class-D (switch mode) amplifiers are principally used in applications where high efficiency is a primary consideration to solve thermal budget problems or to extend run time while operating from batteries. In this paper the class-D amplifier topology is reviewed, an analysis of the factors that leads to an efficient power topology is developed with regard to amplifier losses. Example calculations and system measurements are reported. The output demodulation low-pass filter is analyzed with simple resistive loads and effective (electrical analog) loudspeaker loads. The performance of the filter with the loudspeaker as the load shows excessive peaking at the band edge. The peaking is modified with a Zobel network. Passive components are suggested that offer the performance, size and cost to meet the output filter constraints. A simplited output filter (reduced components) is proposed which offers comparable and, in some cases, better performance than the classical output filter. Finally, removing the output filter completely and connecting the output of the class-D amplifier directly to the speaker is discussed. In summary, the class-D amplifying system offers excellent thermal performance and battery life extension at low values of THD.

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