Analysis and Design of a High Voltage Integrated Class-B Amplifier for Ultra-Sound Transducers

Pulsers are usually adopted in ultra-sound applications due to their high efficiency. On the other hand, linear amplifiers would enable apodization profiles with high resolution, beams with low harmonic content and instantaneous changes in transmit energy between pulses. Their use is rather limited when relying on a discrete technology approach, due to high manufacturing costs and space occupation. In view of an increased level of integration, the availability of a BCD technology, capable of watts level driving at high frequency, encourages investigation of linear amplifiers, the topic of this work. Advantages in terms of reliability, space and cost would derive. The proposed amplifier, closed in a feedback loop, uses a high gm low-voltage transconductor driving a high voltage trans-impedance stage operating in class-B. Device parameters vary with signal amplitude making circuit analysis involved. Techniques to analyze large signal frequency response, distortion and stability are proposed in this paper leading to useful design insights. The amplifier has been realized in a BCD6-SOI technology and, from experimental results, it shows the following performances: 90Vpk-pk output swing with better than 60% power efficiency, 720 MHz GBW, and HD2 lower than -35 dB when driving a load made of a 100 Ω resistor shunted with a 150 pF capacitor, emulating the ultrasound transducer. Quiescent power dissipation is 37 mW only.