Efficiency Enhancement of GaN MMIC Power Amplifiers via Distributed Power Combining

Given the significant role that broadband microwave amplifiers play in a range of applications such as 5G and B5G communication systems, radar systems, electronic warfare, and test equipment, there has been an increasing focus among designers to develop high-power amplifiers that possess wide bandwidth and high efficiency. To this end, Gallium Nitride (GaN) transistors, distinguished by their superior electron mobility and wide band gap properties, offer compelling benefits including heightened power density, elevated efficiency, and expanded operational bandwidth. The primary motivation for utilizing a distributed design in high-voltage 0000power amplifier construction, especially with high-voltage GaN transistors, stems from the need to overcome bandwidth constraints imposed by the Bode-Fano criterion on impedance matching. In the distributed structure, the input and output of the transistors are distributed on the gate and drain lines respectively. Using this approach the input and output parasitic capacitance of the transistors are absorbed into the gate and drain transmission line resulting in bandwidth increase. In this research work, we focus on the drawbacks of the conventional distributed structure and propose new modified structures to improve the performance of this structure like power added efficiency, gain, and frequency bandwidth. The first design introduces an enhanced design for a 4-cell Non-Uniform Distributed Power Amplifier (NDPA). The amplifier is divided into two distinct sections: the initial pair of cells forms the first section, while the subsequent pair makes the second. Power is divided between these sections, and their output currents are combined via a tapered drain line. At the input of the second section, a Distributed Input Matching Network (DIMN) is employed to align the phase of the currents from both sections, facilitating constructive addition of cell currents along the drain line. By splitting power in this manner, cells closer to the load are maintained at comparable drive levels to the first two cells across the frequency band. This approach effectively addresses the typical gate line attenuation issue in standard NDPAs, which often leads to a decrease in Power Added Efficiency (PAE) with rising frequency. Additionally, the design allows for a distinct tapering method for cell sizing, aimed at enhancing NDPA performance. The amplifier was fabricated using a commercial 0.12μm GaN-HEMT technology. Notably, this amplifier demonstrates a relatively consistent PAE at the upper spectrum of its frequency range. Across a bandwidth of 3.5 GHz to 15 GHz, the amplifier consistently demonstrates over 10 dB of small signal gain, 3 to 4 W of saturated output power, and a PAE of 28 % to 45 %.
In the second research, the focus was on the analysis and design of a Doherty-like three-way distributed efficient amplifier. For the first time, a series of analytical formulas were developed for a three-way distributed efficient power amplifier (DEPA) design that streamlines the process, allowing for precise calculations of key parameters. This formula involves several variables and gives a more efficient way to develop high-performance amplifiers. A methodology for designing this type of amplifier was introduced, and following this method, a three-way distributed efficient amplifier was designed, simulated, and fabricated. The accuracy and efficacy of the proposed formulas were examined by both simulation and measurement results. The constructed DEPA exhibited impressive performance, with a saturated output power of 35–36 dBm, a 6-dB back-off drain efficiency (DE) ranging from 25% to 35%, and a saturated DE of 40% to 48% across the 9–11 GHz frequency spectrum.