A low-power audio amplifier with pulse width modulated power supply rails that track the output signal is presented. Because of the tracking power supply rails, the voltage drop over the power transistors is kept as low as possible and nearly constant, so that power efficiency remains high for low as well as high output level signals. A very simple digital input pulse width modulation scheme provides four power rails to a fully differential class-AB power amplifier. The simplicity of the circuit makes it an attractive solution for low cost portable audio applications, instead of using a more complex pulse width modulated class-D audio amplifier. An efficiency increase of about 10% has been simulated over the same class-AB output stage using fixed DC rails of 3 Volts and 0 Volts, with very little sacrifice in THD. Also presented are results from a 12-Volt, single-ended hardware prototype of the system.
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