Extending the Bandwidth of the Doherty Power Amplifier

Doktorsavhandling, 2014

The Doherty power amplifier (DPA) is one of the most popular power amplifier (PA) architectures used to obtain high efficiency for modern communication signals having high peak-to-average power ratios. Its often narrowband performance does, however, make it difficult for the DPA to meet the increasing demands of frequency agility in modern wireless communication systems. This thesis examines the theoretical and practical bandwidth limitations of the DPA and presents new methods to overcome them. A new type of output network topology that serves to reduce the influence of device output parasitics and to overcome manufacturing limitations, thereby extending the bandwidth, is proposed. The utility of the network is demonstrated by implementation in a gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) DPA. Measurements show that by doing so, a power added efficiency (PAE) higher than 30% at 9 dB output power back-off (OPBO) is obtained across a 6.7–7.8 GHz frequency range. To overcome the inherent bandwidth limitations imposed by the impedance inverter, a modified DPA is proposed. A comprehensive theoretical analysis is presented which shows that the modified DPA has a significantly larger bandwidth compared to the standard DPA, as well as reconfigurable efficiency. The theoretical findings are validated by the design and characterization of two demonstrator circuits. The first circuit, having dual RF-inputs and using baredie GaN devices, has a drain efficiency higher than 48 % at both full output power and at 6 dB OPBO across a 1.5–2.4 GHz frequency range. The second circuit is a single RF-input GaN MMIC DPA that delivers a PAE higher than 30% at 9 dB OPBO from 5.8 to 8.8 GHz, constituting a fractional bandwidth of 41%. To achieve bandwidths greater than one octave, a linear multi-harmonic analysis method based on a Doherty-outphasing continuum is proposed. By using this method a dual RF-input amplifier is designed. Measurements show that the amplifier provides more than 45% PAE at 6 dB OPBO over a 1.0–3.0 GHz frequency range, corresponding to a 100% fractional bandwidth. In summary, the results presented in this thesis shows that the DPA no longer needs be considered as a necessarily narrowband amplifier. This thesis is therefore an important contribution in the pursuit of high efficiency and frequency agile power amplifiers targeting the needs in future mobile communication systems.