A recently introduced structure for the capacitive micromachined ultrasonic transducers (CMUTs) has focused on the applications of the asymmetric mode of vibration and has shown promising results in construction of super-resolution ultrasound images. This paper presents the first implementation and experimental results of a transceiver circuit to interface such CMUT structures. The multiple input/multiple output receiver in this work supports both fundamental and asymmetric modes of operation and includes transimpedance amplifiers and low-power variable-gain stages. These circuit blocks are designed considering the trade-offs between gain, input impedance, noise, linearity and power consumption. The high-voltage transmitter can generate pulse voltages up to 60 V while occupying a considerably small area. The overall circuit is designed and laid out in a 0.35 μm CMOS process and a four-channel transceiver occupies 0.86 × 0.38 mm(2). The prototype chip is characterized in both electrical and mechanical domains. Measurement results show that each receiver channel has a nominal gain of 110 dBΩ with a 3 dB bandwidth of 9 MHz while consuming 1.02 mW from a 3.3 V supply. The receiver is also highly linear, with 1 dB compression point of minimum 1.05 V which is considerably higher than the previously reported designs. The transmitter consumes 98.1 mW from a 30 V supply while generating 1.38 MHz, 30 V pulses. The CMOS-CMUT system is tested in the transmit mode and shows full functionality in air medium.

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In ultrasound imaging, capacitive micromachined ultrasonic transducer (CMUT) technology has been considered as a promising alternative for the conventional piezoelectricbased technology. Its advantages include better image quality, higher operational frequency and ease of fabrication and integration with CMOS read-out circuitry. This paper presents the design of an on-chip adaptive biasing unit which improves the sensitivity of the CMUT elements during the receive mode. This unit consists of a DC-DC converter to generate a range of bias voltage levels and a digital control unit to select the desired voltage. It is shown that by applying a different bias voltage during the transmit and receive modes, the receive sensitivity of the system is increased significantly, resulting in a better SNR and higher image quality.