Total Harmonics Distortion (THD) Reduction Techniques in Class D Amplifiers

Abstract

Class D amplifiers find widespread application in audio devices for driving load speakers, primarily due to their remarkable efficiency. Nonetheless, this enhanced efficiency often comes at the expense of reduced linearity. Hence, techniques for reducing Total Harmonic Distortion (THD) are important in the context of class D amplifiers.

The analysis of the distortion mechanisms is first presented. Specifically, emphasis is placed on the distortion generated within the power stage, encompassing aspects such as deadtime distortion and rising and falling time distortion. Both of them are found to be related to the input signal. Subsequently, the compensation technique is applied to the conventional class D amplifier to reproduce and cancel the error. The idea of the compensation approach involves modifying the amplitude of the triangular waveform based on the input signal. A 12 dB THD improvement is achieved in the concept verification section, which is conducted in LTspice.

The negative feedback serves as another technique to achieve THD reduction. A straightforward two-step design methodology is presented to avoid design iterations in the concept design phase. The phantom zero technique is applied when doing the frequency compensation of the feedback loop. The validation of the concept is performed through the use of SLICAP, while the circuit implementation and simulations are carried out within Cadence. Remarkably, this technique results in an impressive -111.8 dB THD reduction, achieved when the output power equals 1 W.