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Push-pull power amplifiers (PAs) operating at HF through to GHz frequencies typically employ a transmission line balun structure. This letter demonstrates that their performance characteristics in high efficiency PA applications are critically different from conventional transformers. For the first time, the effect of the even-mode open-circuit termination on the output waveforms is measured. A novel mathematical formulation is proposed to describe the time-domain waveforms by defining the harmonic impedance environment in terms of odd and even mode excitation rather than tuned harmonics. Experimental verification using harmonic load-pull measurements showed good agreement with the theoretical waveforms generated from factorized waveform expressions.
Date of Publication: May 2012