PSRR: The Real Story about Closed- and Open-Loop Class-D Amplifiers

As open- and closed-loop Class-D amplifiers increasingly become the preferred choice of consumer-audio electronics designers, a different way of looking at the effects of power-supply ripple is needed to adequately capture the performance of the amplifier. Today's audio designers are increasingly focused on reducing system cost, shrinking form factors and delivering high-quality audio, all of which require high supply-noise rejection architectures. Unfortunately, the power-supply rejection ratio (PSRR) measurement does not accurately capture the performance of Class-D bridge-tied load (BTL) amplifiers.

In this article, we discuss the traditional PSRR specification and measurement technique, and explain why it fails to adequately capture the supply-rejection capability of the amplifier. We then describe an alternate way to look at the effects of power supply ripple on the amplifier's audio performance.