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In this paper, a design optimization approach for gallium-nitride (GaN)-based Doherty power amplifiers (PAs) is proposed to enhance linearizability and maximize the power-added efficiency (PAE) for multicarrier wideband code division multiple access (WCDMA) applications. This is based on the use of an input offset line at the peaking amplifier path to compensate, at a given backoff level, for the bias and power-dependant phase variation through the carrier and peaking paths. At a 6-dB output power backoff (OPBO), measurement results of the unsymmetrical unlinearized Doherty amplifier using a four-carrier WCDMA signal achieved results of close to 50% for the PAE and about -30 dBc for the adjacent channel leakage ratio. Linearization of the designed Doherty PA using a baseband digital predistortion technique led to quasi-perfect cancellation of spectrum regrowth. At an OPBO equal to the input signal's peak-to-average power ratio, -63- and -53-dBc adjacent channel power ratios were obtained when the Doherty amplifier was driven by single- and four-carrier WCDMA signals, respectively. To the best of the authors' knowledge, this represents the best reported results for PAE and linearity for GaN-based Doherty PAs linearized over 20 MHz of instantaneous bandwidth.