A Class D amplifier output stage with low THD and high PSRR

This paper presents a class D amplifier output stage with low total harmonic distortion (THD) and high power supply rejection ratio (PSRR). The class D output stage reduces the non-linearities and supply noise by means of a second-order negative feedback loop embodying a single stage second-order integrator and a Schmitt trigger comparator. Unlike conventional feedback, the reference input signal of the feedback loop is a digital pulse width modulated signal. The feedback loop compensates for any external errors or non-linearities in the output PWM signal by modulating the pulse width of the output signal. Based on simulation using AMS 0.35 mum CMOS process, our proposed closed-loop output stage can achieve a PSRR of .90 dB at 1 kHz and a THD well below 0.05% up to 10 kHz. This shows that negative feedback can effectively be employed to improve the PSRR and THD performance of a class D output stage with digital PWM input.