A Novel Technique for Harmonic Cancellation in Class-D Amplifiers

Due to their high efficiency, class-D amplifiers have become attractive to the audio-frequency, power amplifier industry. However, they also exhibit high THD due to the harmonics associated with a high frequency modulation signal. The simplest form of counteracting this distortion is the implementation of a LC, low pass filter. Unfortunately, higher order filtering is required for acceptable performance. Simply adding additional LC filters results in a more expensive and less compact design. This paper presents a technique in which the order of the low pass filter is effectively increased through the use of active compensation. The capacitor current is sampled by an ideal current controlled voltage source (CCVS). The output of the CCVS is further amplified and differentiated by a CFA (current feedback amplifier) circuit. The output of the CFA is then fed back to the output of the class-D amplifier for harmonic cancellation. Analytical realizations are presented and verified by simulations in LTSPICE.