High-power filters for switched-mode power amplifier systems

This paper presents the design and experimental characterisation of a coaxial comb-line band-pass filter for a switched-mode power amplifier (class-S), for an operating frequency of 450 MHz. The class-S amplifier architecture is laid out for very efficient amplification and high-power operation. Both requirements present challenges for the design of the reconstruction filter terminating the final stage. According to measurement results, an initially developed filter provided a bandwidth of 26 MHz, an insertion loss of 0.55 dB, and a second pass-band only at 2.8 GHz. The maximum input power handled by the filter amounted to 54 W, staying below the requirement. This power level was limited by electrical breakdown. Therefore, a high-power adaptation was devised and realised in an improved design subsequently. The resulting filter not only provided an even lower insertion loss of 0.37 dB but also handled more than 300 W input power.