Automatic resonant frequency matching of power amplifier by transformer loop sampling

A new automatic resonant frequency matching technique based on voltage detection by sampling the output transformer loop of the radio-frequency power amplifier (PA) in ultrasonic medical applications is proposed. The proposed automatic resonant frequency matching circuitry is composed of a single-loop coil added to a PA output transformer, voltage divider, diode detector and micro-controller unit for frequency adjustment. Using the proposed circuitry, the resonant frequency shift due to output load variations can be effectively tracked and compensated to provide optimised power transfer to a nonlinear load. For verification, a prototype ultrasonic medical application using a piezoelectric transducer is implemented and examined with silicone oil to check the resonance.