This work presents a low-noise amplifier (LNA) for miniature 3D ultrasound probes. Time gain compensation (TGC) is required to provide continuously variable gain and compensate for the attenuated echo signal, resulting in decreased output dynamic range (DR). As TGC is embedded in the LNA, a power-hungry LNA is no longer needed to handle the full dynamic range of attenuated echo signals. Compared to the prior art where TGC is applied after the LNA, this structure reduces die area and power consumption greatly.

The LNA with built-in TGC functionality is comprised of a transimpedance amplifier (TIA) with an exponentially increasing feedback resistive network. Since a transducer with a relatively high impedance is targeted, a TIA is utilized to interface with the transducer and sense the signal current. TGC is implemented in a continuous fashion by tunable resistors so as to alleviate imaging artifacts associated with gain-switching moments. The resistive feedback network is achieved by triode transistors with exponentially decreasing gate voltages. Three parallel branches of triode transistors are varied simultaneously to obtain a 40dB gain range. Each branch consists of two back-to-back triodes to mitigate non-linearity related to the body effect.


The variable-gain loop amplifier employing a current-reuse topology enables constant closed-loop bandwidth in an energy-efficient way. The first stage is a fixed-gain stage with dynamic biasing to save power at the lowest gain setting. The next two stages are variable-gain stages with variable resistive loads. The load resistor is implemented in the same fashion as the TIA's feedback resistor to achieve intrinsic gain matching. The last stage is a buffer to provide low output impedance for stability.

The LNA has been designed in 0.18 $\mu m$ CMOS technology and occupies an estimated die area of 0.0339 $mm^2$. The effective gain range is 40 dB with $\pm 1$ dB gain error. The LNA's noise floor at the highest gain is below 1.15 $pA/\sqrt{Hz}$ and its harmonic distortion is better than -40 dB. During 100 $\mu$s receive period, the total power consumption is 6 mW from a $\pm0.9$ V supply. The LNA featuring a small area and high power efficiency is a promising circuit for miniature 3D ultrasound probes.

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.

In transit-time ultrasonic flow measurement systems, the system's ability to operate reciprocally in the absence of flow is a highly desirable property. If the reciprocity is lacking in the system, any time delay that appears between the upstream and downstream signals at zero-flow conditions will result in false flow measurement. This phenomenon is known as the zero-flow error.
This thesis presents a system in which the reciprocity property has been ensured by matching the electrical impedances of the front-end electronic circuits, namely: the output impedance of the transmitter Z$_\text{out}$ and the input impedance of the receiver Z$_\text{in}$. To achieve this, a circuit has been developed that can be used as both a transmitter and a receiver. This is the main feature that distinguishes the proposed design from those presented in previous work. The transmitter-receiver circuit has been implemented using a three-stage operational amplifier in unity gain feedback configuration. The class AB output stage of the amplifier is equipped with an additional function being used in receive mode for sensing and amplification of the signal. The simulation result obtained by the cross-correlation method yields a zero-flow error value of 30 fs, which is at least by three orders of magnitude smaller than the results achieved in prior work. The input and output impedances are equal to Z$_\text{in}$=73.2$\angle$80.7$^ \text {o}$ m$\Omega$ and Z$_\text{out}$=79.6$\angle$77.3$^ \text {o}$ m$\Omega$ respectively. A small mismatch remaining between the impedances prevents perfect reciprocity to be established in the system. A prototype IC has been taped out in TSMC 0.18 µm BCD Gen2 technology.

In this design, we propose a new method to process the large static signal in the received signal of Doppler ultrasound brain imaging. Through a reproduction-compensation method, a medium resolution ADC and a DAC are used to replace the high-resolution ADC used in the past for the dynamic Doppler signal observation.

Capacitive micromachined ultrasonic transducer (CMUT) technology has been considered as a promising alternative for the conventional piezo electric-based technology in ultrasound imaging systems. Its potential advantages include better image quality, higher operational frequency, and ease of fabrication and
integration with CMOS read-out circuitry. However, CMUTs usually need high-voltage DC bias in order to transmit and receive acoustic waves. This work presents the design of a DC-DC converter to bias a CMUT for ultrasound imaging systems. Two different cases have been investigated: biasing at 63 V from an input of 5 V using a fully-integrated converter realized in a 180nm BCD technology that can handle up to 65V; and biasing at 120 V by employing minimal off-chip components. The proposed work explores the transient properties of a conventional on-chip switched capacitor DC-DC converter: the Dickson charge pump. A MATLAB model of the Dickson charge pump is developed to understand the relation between dynamic efficiency and the charge pump capacitor. Prior research works suggest that the dynamic efficiency of the Dickson charge pump is limited for a particular number of stages. However, the results obtained from the MATLAB model suggests that the dynamic efficiency can be improved by making the charge pump capacitor smaller. Various circuit simulations have been done to understand the results obtained from the MATLAB model. When the charge pump capacitor becomes very small, the parasitics start dominating and affect the overall efficiency of the Dickson charge pump. An optimization strategy is discussed to find the optimum number of stages and charge pump capacitor value to maximize the dynamic efficiency and minimize the circuit area. Based on the results obtained from this study, a DC-DC converter is designed to bias the CMUT at 63V, which consumes an estimated circuit area of 0.6586mm2 and has a simulated efficiency of 0.752.
In addition, a new design idea that incorporates two DC-DC converters to bias the CMUT beyond the process voltage limitations is also discussed in this thesis. This hybrid converter involves the optimized DC DC converter designed earlier to generate 63V from an input of 5V, a control unit circuit, and some off-chip
components. The hybrid converter designed consumes an estimated circuit area of 0.842mm2 and has a simulated efficiency of 0.577

This work presents a type of low noise amplifiers (LNA) that are used for ultrasound imaging systems. To account for the attenuating nature of ultrasound echo signals, a so-called time gain compensation (TGC) circuit is required. By increasing the gain over time, the output dynamic range is decreased, while the switching artifacts are suppressed by the continuous gain control.
The proposed work combines the LNA structure with TGC functionality. This is done, such that the first component in the ultrasound receive chain does not need to be able to handle the full dynamic range of the input signal. The goal is to reduce die area and power consumption costs compared to systems that utilize separate LNAs and TGCs. The combined TGC-LNA consists of a transimpedance amplifier (TIA) with an exponentially-varying feedback resistance. As the feedback network and feed-forward path are separately designed, the design of the TGC functionality and low noise functionality can for a large part be independently designed. The TGC functionality is implemented by an exponentially-varying feedback resistance. This is achieved by means of implementing triode transistors as voltage-controlled resistors. Three branches with differently sized triode devices are required to obtain the full gain range of 40 dB. A two-stage telescopic amplifier realizes the loop amplifier. The bias current of the first stage is a linear function of the total feedback resistance to create a constant unity-gain bandwidth in a power efficient method. Realized in 0.180 μm BCDMOS technology, the combined TGC-LNA amplifies the signals from a Capacitive Micromachined Ultrasound Transducer (CMUT) with a center frequency of 7.5 MHz. The total achieved gain range is 40 dB where the gain varies during a receive period of 100 μs. During the receive period the total harmonic distortion remains below -44 dB and the noise floor is 1.12 pA/√(Hz) at the highest gain setting. Drawing 5.5 mW from a 1.8 V supply and requiring approximately 0.01 mm² die area, the proposed TGC-LNA provides a new method for reducing power consumption and area costs for miniature ultrasound applications.