Compact low-loss wideband-matched RF filters for class-S power amplifier systems

This paper describes the requirements for, and successful implementations of, a reconstruction filter suitable for a class-S power amplifier system. Beside tailoring the pass-band and the stop-band, mode and impedance matching to the push-pull switch-mode amplifier, driven by a delta-sigma modulator, over a wide bandwidth are crucial to sustain a high system efficiency. Two designs of balanced-input comb-line filters are presented for operating frequencies of 450 MHz and 900 MHz. Measurements revealed an insertion loss of less than 1 dB, fractional bandwidths of 6% and 500% for the pass-band and stop-band, respectively, as well as broadband impedance matching to the final amplifier stage.