Singly and doubly terminated balanced input comb-line filters for current-mode class-S power amplifiers

This paper presents designs for reconstruction filters suitable for a class-S power amplifier operating at a centre frequency of 900 MHz. Previously, we presented analytical comprehensive comparison of doubly and singly terminated filters and proved mathematically that only singly terminated filters owing to their constant input impedance can fulfil the requirements of current-mode switching amplifiers. On the basis of this theoretical background, two balanced input comb-line filters, doubly and singly terminated were designed and experimentally investigated. The singly terminated filter conforms to the expected transfer function and displays a constant input resistance, which is essential for a class-S power amplifier performance.