A Novel Technique to Suppress Harmonic Distortion in Class-D Amplifiers

Class-D amplifiers have become very popular for audio-frequency power amplifiers. Their efficiency makes them attractive for high power or battery operated applications. The disadvantages of class-D amplifiers are almost all related to the undesirable high frequency harmonics caused by the modulation process. Passive LC filters are typically used to reduce these high frequency distortions. Higher order types of these filters require many bulky and expensive inductors. In this paper, the effective order for the class-D output restoration filter is effectively increased by adding a feedforward network to reduce the unwanted high frequency harmonics in the amplifier´s load. A class-AB current source in the feedforward path counteracts the high frequency carrier and its associated sidebands. Initially, an analytical solution is developed and then implemented with discrete component models in SPICE. A frequency domain analysis is presented for the feedforward compensation technique as well as optimization by parametric variation. SPICE simulations verify the performance of the system by demonstrating the reduction of total harmonic distortion