Analysis and design of PWM-in-PWM-out Class D Amplifiers

Conventional analog Pulse-Width-Modulation (PWM) Class D Amplifiers (CDAs) require analog input signals that are typically provided by a Digital-to-Analog Converter (DAC). A PWM-in PWM-out CDA, on the contrary, is able to receive digital PWM signals as its input signals, hence not requiring said DAC as the `interface´ to digital signal processing circuits. In this paper, we analytically investigate the distortion and the noise-immunity (qualified by Power Supply Rejection Ratio, PSRR) of a PWM-in PWM-out CDA. Interestingly, we find that, contrary to conventional CDAs, the PWM-in PWM-out CDA features zero intrinsic distortions, and its PSRR is largely independent of the gain of the embodied integrator and is instead dependent on the switching frequency. The derived analytical expressions are afterwards verified on the basis of simulations of a practical PWM-in PWM-out CDA. In addition, the comparison between the said CDA and a conventional CDA demonstrates that the PWM-in PWM-out CDA is superior both in terms of Total-Harmonic-Distortion and PSRR.