A 1.7-GHz GaN MMIC Doherty power amplifier using an adaptive bias circuit with a quadrature coupler

A 2-stage 1.7-GHz Doherty power amplifier (PA) with an adaptive bias (ADB) circuit was designed for small-cell base station applications. An uneven power cell size ratio was used to provide a large back-off efficiency for high peak-to-average power ratio (PAPR) signals. To simplify the power divider and inter-stage matching network, a lumped Lange-type quadrature coupler was used. The leakage signal from the isolated port of the coupler was used as an input signal for the ADB circuit of the peaking amplifier. The PA was implemented in a 0.25-μm GaN on a SiC MMIC process. For a 1.7-GHz continuous wave, the PA has a 45.8% power added efficiency (PAE) at the output power of 38.6-dBm and a 31.2% PAE at 7-dB back-off power. For an LTE downlink signal, adjacent channel leakage ratio (ACLR) of -25.9-dBc was obtained at the average output power of 31.4-dBm without a digital pre-distortion (DPD).