This thesis discusses the architecture, circuit implementation and simulation results of an amplitude-tunable bipolar high voltage (HV) pulser for ultrasound imaging. This design is capable of providing a relatively high-resolution amplitude modulation and allows bipolar pulsing under a unipolar supply with a small die size. The pulser design starts from the driving circuit of the HV output stage. A driving circuit based on a current mirror structure is implemented to drive the ultrasound transducer element with different slewing currents. These currents levels are generated by6-bit current DACs controlled by a calibration loop. This calibration loop senses the pulse output voltage through a capacitive divider. The resulting attenuated voltage is compared with a reference voltage by a low-voltage (LV) dynamic comparator. Based on the comparison result, the input code of the current DACsare adjusted by a successive approximation register (SAR). Several techniques are proposed to flexibly change the pulse amplitude and reduce the die size. First, the pulser is driven in a current mode which allows the transistors at the output stage to operate in the saturation region, so that the currents through the transducer elements always follow the input codes. Second, a separate calibration phase is applied at the beginning of the transmitting and receiving cycles, which will generate flexible and accurate input currents for the corresponding transmission phase. The pulser has been implemented in TSMC180nm HV BCD technology. The simulation results show that the prototype is able to transmit HV pulses with a4-bit amplitude modulation with 0.5V inaccuracy. The transmission power consumes less than 0.2mW per channel when driving a transducer with a capacitance of18.29pFat a pulse repetition frequency (PRF) of4kHz. The estimated die size of the pulse is around 0.04mm2.

Miniature ultrasound probes increasingly employ ASIC that drive the transducer elements with HV pulses and process the received echo signals. Due to size constraints, the HV pulsers tend to be relatively simple circuits, in which HV MOS transistors pull the transducer element to a HV supply and back to ground. An important disadvantage of existing designs is that the rising and falling edges of the generated pulses are not well matched, which leads to second-harmonic components in the generated pulse. This makes these design unsuited for harmonic imaging. Moreover, the pulse amplitude typically cannot be controlled, making it difficult to apply apodization techniques that are often desired to reduce sidelobes. This work presents an ASIC design that addresses these disadvantages. The content discussed in this thesis includes the circuit architecture, circuit implementation and analysis of simulation results for an integrated HV pulser with improved HD2 and programmable amplitude enabling apodization. The design consists of two parts: a transmit driver and the HV pulser. The transmit driver, composed of a negative feedback loop with a replica pulser, a level shifter and other components, can generate a lower HD2 by auto-calibrating the pull-up and pull-down time. It can be shared by multiple HV pulsers that drive the transducer elements. A combination of duty-cycle modulation and slew-rate modulation is proposed for pulse-amplitude control. Finally, an experiment aimed to explore the effective acoustic power of transducer under high and low impedance drive modes is proposed. These drive modes are first investigate by simulating the transducer driven by an ideal current and an ideal voltage source. Then, an experimental circuit is proposed in which the transducer is connected to an OpAmp in different ways to achieve similar impedance conditions as in the ideal situation to verify whether there is a large gap in the acoustic power.