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Capitalize on Class-D driver benefits

Posted: 01 Mar 2008     Print Version  Bookmark and Share

Keywords:Class-D audio amplification  MOSFETs  headphone amplifiers 

Class-D audio amplification circuits have been available for decades, offering high efficiency and small size compared to the more prevalent linear Class-AB topology. The voltage across the output device in a Class-AB amplifier equals the difference between the speaker and rail voltages and changes according to the audio signal. The power loss is, therefore, this voltage times the output current. Typical efficiencies are 30 percent. As a result, Class-AB output stages usually require heat sinks and possibly a fan, particularly at power levels exceeding 50W.

By contrast, the output devices� switching and conduction losses dominate in a Class-D power amplifier. These transistors�typically power MOSFETs operating in saturation�routinely deliver efficiencies in excess of 90 percent. A passive LC filter smoothes the power stage�s rail-to-rail output to recover the audio signal. Class-D headphone amplifiers help preserve battery life and small size in many portable systems. High-power applications use Class-D topologies to reduce size, weight and cost�notably by reducing or eliminating heat sink mass.

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