Singly terminated reconstruction filters for current-mode class-S power amplifiers

This paper presents the design of a reconstruction filter for a class-S power amplifier. Previously, a few doubly terminated filters for a switched-mode amplifier system were presented, but measurement results showed significant ripples up to 3 dB in output power. This effect can be explained by the current mode final stage acting like a current source with infinite internal impedance, thus demanding a singly terminated filter. This paper presents a comprehensive comparison of the doubly and singly terminated filters in terms of a symbolic algorithm and, moreover, proves mathematically that only singly terminated filters owing to their constant input impedance can fulfil the requirements of current-mode switching amplifiers.