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Doherty type Power Amplifier (DPA) design is one of the most practical efficiency enhancement methods that provide moderate linearity. Asymmetrical device usage and employment of bias adaptation are among the most commonly used Doherty architectures in recent applications. In this paper, the efficiency performances of bias adapted DPA and asymmetrical DPA are compared based on the efficiency expression that is derived in terms of the conduction angle. The efficiency of bias adapted DPA is analyzed in terms of conduction angle of the peaking device; various bias waveforms are proposed and their effects on enhanced efficiency performance are demonstrated. This paper also facilitates an approach to determine the required relative periphery of the peaking amplifier in order to have a fully load modulated asymmetrical DPA. Both DPA structures are designed and implemented at the output power of 50 dBm with nearly 60% drain efficiencies in 6 dB load modulation region. The measurements verify the better efficiency characteristics of the bias adapted DPA and asymmetric DPA in comparison to the conventional DPA. For the first time in the literature, as a fair comparison, the performances of asymmetrical DPA and bias adapted DPA are compared on the same platform and their advantages as well as drawbacks are demonstrated using measurement results.

Abstract-In this paper, an octave bandwidth Doherty power amplifier (DPA) using a novel combiner is presented. The fundamental bandwidth limitation of the load modulation concept of a conventional Doherty structure is solved based on the proposed combination method. For verification, an octave bandwidth asymmetric Doherty architecture is implemented by using gallium-nitride (GaN) HEMT Cree CGH40010 and CGH40025 devices in the carrier and peaking amplifiers, respectively. The carrier and peaking amplifiers are designed to achieve optimal operation with 25 Ω load and source impedances. The reduced load and source impedances simplify the matching circuits for broadband operation. Key building blocks, including the proposed combiner, carrier and peaking amplifiers as well as the 50/25 Ω input power divider, are outlined. The measurement results represent higher than 37% and 52% drain efficiencies in 6 dB load modulation region across the frequency range from 0.85 to 1.85 GHz and 0.90 to 1.60 GHz, respectively. The implemented Doherty amplifier represents acceptable linearity across the whole operation frequency range. In two-tone signal characterization, the implemented DPA performs with a drain efficiency of 55% and an inter-modulation distortion (IMD) of −30 dBc at an average output power of 41.2 dBm at the center operation frequency of 1.35 GHz. In order to observe wideband signal characterization, a single carrier wideband code-division multiple access (W-CDMA) signal with a peak-to-average power ratio (PAPR) of 6.5 dB is applied and a drain efficiency of 51% with an adjacent-channel leakage ratio (ACLR) of −31 dBc is achieved at an average output power of 38.4 dBm.