Qorvo Designs 1-8 GHz GaN Distributed PA MMIC With Novel Trifilar Transformer Utilizing AWR Software
The Design Challenge
Many modern microwave electronic systems specify amplifiers with high output power, wide bandwidth, and high efficiency. Until recently, most wideband high-power amplifier (PA) solutions have relied on vacuum electronics-based technologies. Recent work, however, shows steady progress in realizing high-power, high-frequency, wideband amplifiers utilizing gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) technology that operates from near DC up to 7 GHz.
Qorvo designers were challenged to meet the requirements for an amplifier with high output power, wide bandwidth, and high efficiency by designing a 1-8 GHz PA MMIC fabricated with a 0.15 µm gallium nitride (GaN) process technology. The process featured a 100 µm thick silicon carbide (SiC) substrate and compact transistor layouts with individual source grounding vias (ISVs). The design utilized a non-uniform distributed power amplifier (NDPA) topology with a novel trifilar connected output transformer.
Design goals for the PA MMIC were: 1-8 GHz bandwidth, > 25 dB small signal gain, 10 W saturated output power, and power-added efficiency (PAE) exceeding 30 percent. A small-signal gain goal in excess of 25 dB required at least two amplification stages. To meet the bandwidth requirement, the NDPA topology was adopted and to increase output power, a novel monolithic trifilar coupled-line transformer design (patent pending) was used..
The Qorvo design team chose AWR Design Environment, specifically Microwave Office circuit design software, for this complex task. The two-stage amplifier demonstrated 9.3-13.1W of output power over a 1-8GHz bandwidth with greater than 29 percent associated PAE. Due to physical limitations regarding the location of the low impedance side tap, the microstrip implementation produced a ratio closer to 2:1. Electromagnetic (EM) simulations performed on the transformer using AXIEM 3D planar EM simulator within Microwave Office suggested that an 8:1 bandwidth could be supported with this approach.
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