A Class-D Amplifier with Digital PWM and Digital Loop-Filter using a Mixed-Signal Feedback Loop

A digital class-D audio amplifier is presented that is based on digital pulse-width modulation (PWM) and a combination of digital and analog feedback. Unlike other recent implementations for low-power applications this amplifier directly accepts digital input as the digital-to-analog conversion is an inherent feature of the presented topology.The feedback topology uses a hybrid scheme with digital feedback to improve the performance of the PWM and analog-feedback to mitigate analog imperfections and to improve power supply rejection. Because of the proposed feedback scheme, the requirements on the analog-to-digital converter (ADC) in the feedback loop are greatly relaxed allowing the use of a continuous-time ∆Σ-modulator with low power consumption.The class-D amplifier was realized in a standard 180 nm CMOS technologies and drives 1.2 W into an 8 Ω load and achieves a total harmonic distortion plus noise (THD+N) of −96 dB, a signal-to-noise ratio (SNR) of 99.9 dB having an efficiency of 91 %.