We derive Fourier series expressions for modeling analog closed-loop PWM class D amplifiers. Based on our derived expressions, we are able to analyze the mechanisms of several nonlinearities, specifically the power supply noise and fold-back distortions. We investigate the influence of the finite loop gain on these nonlinearities, and we show that those nonlinearities can be effectively suppressed by the higher loop gain. We verify our analysis by computer simulations and also on the basis of experimental measurements. The derived expressions are practically useful as they provide insights to a designer for PWM class D amplifier designs