Two-Port Network Theory-Based Design Method for Broadband Class J Doherty Amplifiers

This paper proposes a new methodology for designing wideband Class J Doherty power amplifiers (DPAs). The paper begins by presenting a network analysis that leads to a set of design equations to govern the synthesis of the combiner network parameters that satisfy Class J load requirements. The combiner network produces a complex-to-complex Doherty load modulation capable of avoiding potential transistor voltage clipping due to a mismatch between the fundamental and harmonic impedances. Consequently, this method improves the high power AM-AM characteristic and reduces the DPA's peak power variation versus frequency. A wideband Class J DPA was designed as a proof-of-concept demonstrator to operate from 2.7 GHz to 4.3 GHz. Under continuous wave stimuli, over the entire band, the measured AM-AM distortion in the Doherty region of the fabricated DPA was found to be lower than 1.2 dB with a relatively constant output power of 38.9 ± 0.3 dBm at saturation. Moreover, good drain efficiencies of about 42% and 54% were recorded at 6-dB output back-off and saturation powers, respectively. In addition, the linearizability of the fabricated DPA was confirmed under both intra- and inter-band carrier-aggregated signal stimuli. In fact, measurements using an 80 MHz inter-band carrier-aggregated signal revealed that the proposed DPA could deliver an adjacent channel leakage ratio of better than -48 dBc after digital pre-distortion with a good average drain efficiency of 45% - 49% over the entire band of interest.