Impact of 3rd Harmonic Reflection on CW PA Efficiency With 4:1 Combiner and LPF

The discussion addresses efficiency degradation in high-power continuous wave (CW) push-pull power amplifiers (PAs) used for industrial applications, operating approximately 3 dB into compression to meet target efficiency. A 4:1 combiner with integrated low pass filter (LPF) is employed to achieve conducted emission specifications of -50 dBc. However, third harmonic reflections from the combiner and LPF are reflected back into the PAs, causing an approximate 8% efficiency reduction. One proposed solution is the insertion of an LC parallel resonant circuit tuned to the third harmonic frequency between the amplifier and the cable, which isolates the PA from reflected third harmonic energy by presenting high impedance at that frequency. Adjusting phase length via cable has shown improvement but is currently not feasible. Additional attempts include using absorptive filters at the PA outputs before an in-phase combiner with low pass characteristics. Theoretical maximum efficiencies are noted as 25% for typical configurations and up to 50% for transformer or inductively coupled configurations, highlighting the importance of harmonic management to optimize efficiency and reduce power losses.
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