A 27.12-MHz kV-scale Power Amplifier with a Tunable Multistage Matching Network for a Compact Ion-beam Accelerator

This paper presents a 27.12-MHz kV-scale power amplifier (PA) with a tunable multistage matching network suitable for driving a variable load capacitance in a compact ion-beam accelerator. In this application, the PA generates a high voltage at a high frequency to accelerate ion beams to high energy levels. Since the PA is designed to provide large gain at a fixed frequency, slight variations in the PA’s load capacitance – for example, due to manufacturing tolerances in the accelerating plate-pairs – can detune the PA and drastically reduce its output voltage. The proposed PA addresses this challenge by employing a multistage matching network with a variable capacitor. The value of the variable capacitor is appropriately adjusted to compensate for variation in load capacitance. Since this variable capacitor is connected at an intermediate stage of the matching network, it can be realized using devices with lower voltage rating, which is much lower than load voltage. Furthermore, the multistage matching network efficiently provides a large current step-up, improving the PA’s thermal performance by reducing the current processed by its inverter transistors. A comprehensive methodology which maximizes PA’s compensation range under certain loss constraints is also proposed. A 27.12-MHz prototype PA is designed, built and tested to validate the concept and design methodology.