Leading electronic technology expert, Mitsubishi Electric, has announced the development of a 28 GHz High-Efficiency GaN Doherty 5G Power Amplifier (PA) using National Instruments’ NI AWR software (Which has not been acquired by Cadence). The software helps design engineers dramatically reduce development time and cost for components, circuits, systems and subsystems employed in wireless, high-speed wired, broadband, aerospace and defense, and electro-optical applications.
The Ka-band is a popular frequency for millimeter-wave (mmWave) applications for 5G. High efficiency and high output power amplifiers (PAs) are required to reduce power consumption and increase data transmission distance. However, although gallium nitride (GaN) is widely used for wideband applications, most Ka-band GaN amplifiers have only 10% efficiency at backoff output power due to their Class-AB single-ended configuration.
Mitsubishi Electric engineers were challenged to meet the requirements for a PA with high output power and high efficiency at back-off output power by designing a 28 GHz PA monolithic microwave integrated circuit (MMIC) using a high-efficiency Doherty architecture. The PA MMIC was to be fabricated using a 0.15 µm GaN process technology with a 50 µm silicon carbide (SiC) substrate (Figure 1).
The design goals for the GaN Doherty PA MMIC were 28 GHz operation, 3 W saturated output power, and power-added efficiency (PAE) over 20% at 6-8 dB back-off output power. The designers chose NI AWR Design Environment platform for this exacting design and believe that the resulting Doherty PA device is the state of the art in 5G amplifier devices for the Ka-band.
The parameters of the Doherty output combiner, which consisted of microstrip lines and metal-insulator-metal (MIM) capacitors on a 50-µm thickness SiC substrate, were calculated using Microwave Office circuit design software. Electromagnetic (EM) simulations were performed on the Doherty output combiner using the AXIEM planar EM simulator. The layout and EM simulation are shown in Figure 2.
Figure 3 below provides the measured and simulated large-signal characteristics of the fabricated GaN Doherty PA MMIC. The fabricated 2-stage Doherty PA using a GaN high electron mobility transistor (HEMT) achieved a measured saturation output power of 35.6 dBm (3.6 W) and peak PAE of 26%. PAE of 23% and 20% was obtained at 6 dB and 8 dB back-off, respectively.
Because the 5G market is developing rapidly, early delivery of amplifiers to market is critical. According to the Mitsubishi design team, the straightforward, integrated NI AWR software environment enabled fast development of the GaN PA MMIC while meeting all requirements. The AXIEM simulator reduced analysis time by 50%. In addition, ready access to a design kit for the Mitsubishi Electric GaN process that enabled the engineers to optimize the development environment.
The speed of the circuit and EM simulation was so fast that the AXIEM simulator was able to do the required simulations very quickly and provided high accuracy results. The designers at Mitsubishi recommend NI AWR software and plan to use it from now on because of its beneficial development environment and simulation speed and accuracy.
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