Filterless class D amplifiers: power-efficiency and power dissipation

Filterless Class D amplifiers are based on the tri-state modulation architecture. In this study, the authors show that it is possible to employ the conventional Class D amplifier (that requires an LC output filter) as a filterless amplifier (hence not requiring the LC filter) if certain circuit conditions are satisfied - specifically a sufficiently high carrier frequency and a loudspeaker with sufficient inductance. As the high-frequency (beyond the audio range) components of the tri-state modulation and of the conventional modulation are dissipated in the loudspeaker, the rating of the loudspeaker would need to accommodate not only the audio signal power but also the power, P-sw, due to the attenuation of the high-frequency components. By means of double-Fourier series analysis, the authors derive expressions for P-sw and for power-efficiency of filterless Class D amplifiers based on the tri-state modulation and on the conventional modulation. These derived expressions provide meaningful insight into the design and application of these amplifiers, including practical means to improve their power-efficiency and the associated implications thereof. To determine the dissipation of the high-frequency components in the loudspeaker, the authors extend the well established loudspeaker model in the audio range to a high-frequency model, and the required added power rating of the loudspeaker is determined. The authors show, somewhat serendipitously, that if certain conditions are satisfied, the power-efficiency of the tri-state and the conventional filterless amplifiers are nearly comparable. The analyses herein are verified by experimental measurements.