Supply-Inverted Bipolar Pulser and Tx/Rx Switch for CMUTs Above the Process Limit for High Pressure Pulse Generation

Abstract

A combined supply-inverted bipolar pulser and a Tx/Rx switch is proposed to drive capacitive micromachined ultrasonic transducers (CMUTs). The supply-inverted bipolar pulser adopts a bootstrap circuit combined with stacked transistors, which guarantees high voltage (HV) operation above the process limit without lowering device reliability. This circuit generates an output signal with a peak-to-peak voltage that is almost twice the supply level. It generates a bipolar pulse with only positive supply voltages. The Tx/Rx switch adopts a diode-bridge structure with the protection scheme dedicated to this proposed pulser. A proof-of-concept ASIC prototype has been implemented in 0.18-m HV CMOS/DMOS technology with 60 V devices. Measurement results show that the proposed pulser can safely generate a bipolar pulse of-34.6 to 45 V, from a single 45 V supply voltage. The Tx/Rx switch blocks the HV bipolar pulse, resulting in less than 1.6 V at the input of the receiver. Acoustic measurements are performed connecting the pulser to CMUTs with 2 pF capacitance and 8 MHz center frequency. The variation of acoustic output pressures for different pulse shapes were simulated with the large signal CMUT model and compared with the experimental results for transmit pressure optimization. A potential implementation of the methods using MEMS fabrication methods is also described. Index Terms-Pulser, capacitive micromachined ultrasound transducer (CMUT), medical ultrasound imaging, Tx/Rx switch, ASIC, large signal model. I. INTRODUCTION apacitive micromachined ultrasound transducer (CMUT) has been shown to be a viable alternative to conventional piezoelectric sensors and transducers in ultrasound diagnostic Manuscript

A compact bipolar high‐voltage pulser with a novel transistor gate–source protection scheme for ultrasound imaging applications

“…5,14 In bipolar HV pulsers, however, a T/R switch typically requires several HV devices so as to provide bidirectional isolation between the output and ground. [15][16][17] To address this issue, the T/R switch can be embedded inside the RTZ stage of the pulser using a lowvoltage (LV) MOS transistor. By doing so, a separate T/R switch is no longer required, and since HV MOS transistors typically occupy a very larger area than LV MOS transistors, removing several HV MOS transistors can result in a significant reduction in the total die area.…”

“…In unipolar HV pulsers, the T/R switch may be implemented using a single HV MOS transistor 5,14 . In bipolar HV pulsers, however, a T/R switch typically requires several HV devices so as to provide bidirectional isolation between the output and ground 15–17 . To address this issue, the T/R switch can be embedded inside the RTZ stage of the pulser using a low‐voltage (LV) MOS transistor.…”