The performance of gallium-nitride (GaN) discrete transistors has steadily increased, with manufacturers now offering devices with impressive performance in terms of available gain, RF output power, and efficiency. In the white paper, “The Design of a 125W L-band GaN Power Amplifier,” Plextek RFI demonstrates the design of a single-stage, 125-W power amplifier (PA). The PA, which is optimized for a frequency range of 960 to 1,215 MHz, is designed with a commercially available GaN transistor that is housed in a metal-based ceramic package.
The PA is designed using a 32-mil-thick Rogers 4360G2 laminate with 1-oz metallization. By using this relatively thick substrate, the trace widths of the required characteristic impedance are wide enough to handle high RF power and dc current under large-signal drive conditions. The transistor operates with quiescent current of 260 mA at a drain voltage of +50 Vdc.
Large-signal load-pull data demonstrated the device is capable of providing +51 dBm of output power at the 3-dB compression point at 1.1 GHz. The complete PA schematic is presented, with passive components carefully selected to handle high dc voltages and RF power levels alike. Both the gate and drain bias networks utilize printed transmission lines along with shunt surface-mount-technology (SMT) capacitors. Furthermore, the input and output matching networks are lowpass structures that transform the 50-Ω system impedance to the lower source and load impedances needed to achieve optimum power transfer.
The final assembly is mounted onto an aluminum alloy carrier that acts as a heat spreader. This carrier can also be attached to a heatsink. Small-signal S-parameters were measured at package-base temperatures of -40°C, +25°C, and +85°C. These measurements demonstrate a small-signal gain of approximately 19 dB across the frequency range, which varies by less than 2 dB over temperature. Large-signal measurements were performed over the same temperature range. Those results prove the PA can deliver 125 W of output power at the 3-dB compression point at midband. The document also details the PA’s efficiency performance.
Plextek RFI, London Rd., Great Chesterford, Saffron Walden, CB10 1NY, UK; +44 (0) 1799 533200
