Design of a low-power proof-of-concept multi-stage amplifier test stand to model and implement outphasing control for the LANSCE 805 MHz solid-state high-power RF amplifier

Los Alamos Neutron Science Center (LANSCE) has a project to investigate the feasibility for a replacement radio-frequency (RF) amplifier that is not reliant on vacuum electron tubes, has a similar footprint, and equivalent RF functionality. Gallium Nitride (GaN) on Silicon Carbide (SiC) high electron mobility transistors (HEMT) will be used in combined configuration. To maintain existing operational capabilities with these GaN amplifiers, the low-level control system needs to be modified for maximum transistor lifetime. The HEMT operates in a saturated condition, with a constant amplitude drive signal to avoid the high-power dissipation of linear operation with reduced drive. This leaves the phase of the RF inputs as a control mechanism, utilizing outphasing for amplitude modulation of the multistage amplifier. The GaN amplifiers also require a bias sequencing/protection board that is being designed and tested separately. To test and verify the control system, a low power test rack using commercial wideband RF components was built. This model system includes drive control, four 10 W amplifier stages, a final combination chassis, and accelerator timing system. The information from this test rack will be used to learn how to efficiently control a multistage high-power GaN amplifier to fit the requirements of the LANSCE linear accelerator.