Power Supply Noise in Analog Audio Class D Amplifiers

A potential drawback of class D amplifiers (CDAs) is their relatively poor tolerance to power supply noise. It has been recently established by means of a linear model that the manifested nonlinearities due to the supply noise are the fundamental noise frequency component and second-order intermodulation components, qualified by the power supply rejection ratio (PSRR) and intermodulation distortion (IMD), respectively. In this paper, multidimensional Fourier series analysis is applied to open-loop, single-, and double-feedback pulsewidth modulation CDAs, and expressions for PSRR, IMD, and fold-back distortion (FBD) are derived. This analysis takes into account the nonlinear pulse modulation process (unaccounted for in the linear model), and all components of PSRR and IMD nonlinearities are hence modeled. It is shown that the first-harmonic PSRR and third-order IMD components, which are usually ignored in CDAs and linear amplifiers, are significant and that, under certain conditions, the third-order IMD components can be higher than the second-order IMD components. It is also shown that the loop gain should be high for high PSRR and low IMD. Furthermore, significant FBD can arise due to the intermodulation between the supply noise, the input signal, and the carrier, and interestingly, the FBD in closed-loop CDAs is more serious than that in open-loop CDAs. The analyses herein are verified by computer simulations and on the basis of measurements on a prototype CDA IC and on other hardware realizations.