Development of front-end circuits for high frequency ultrasound system

This dissertation describes the development of a novel integrated preamplifier with a Sallen-Key Butterworth filter and novel bipolar transistor-based limiter for a high frequency ultrasonic transducer. First, the motivation for the integrated preamplifier results from the fact that the integrated preamplifier may reduce cable loss because the impedance matching with the high frequency transducer can affect the performance of the transducer due to cable loading effect. The design of an integrated circuit consisting of a preamplifier with the Sallen-Key Butterworth filter for a high frequency ultrasonic transducer will be presented. The front-end circuits consisting of integrated preamplifier with Sallen-Key Butterworth filter is usually interfacing with transducer and power amplifier such that the simulations and experiments with transducer and power amplifier is required to consider their effect to obtain proper performances. The simulation and experimental results of the integrated preamplifier and filter chain demonstrate the improved performances of pulse-echo response and wire-phantom imaging with a high frequency thick film (LiNbO3) transducer and thick film (PZT) array element transducer. The integrated preamplifier which had high frequency silicon germanium (SiGe) hetero-junction bipolar transistors (HBT's) was fabricated in an IBM 0.18 μm bipolar complementary metal oxide semiconductor (BiCMOS) process. This implementation represents the first step in the eventual realization of a complete integrated high frequency receiving system.;Second, high-frequency ultrasound imaging systems are not only dramatically affected by the electrical impedance mismatch between the transducer and the electronics but the attenuation caused by passive components of the protection devices and the system. These protection devices are used to protect the front-end circuits from the unexpected voltage signals induced by a transmitter. Therefore, to optimize the performances of the imaging system, there is a need to improve the design of these limiter circuits. These novel limiter circuits were analyzed with the small signal and large signal models of bipolar transistors. And, the improved performances of these limiters such as magnitude frequency responses, total harmonic distortions and total noise figures including preamplifiers were measured and compared with a commercial diode limiter. The capability of the novel limiters was also demonstrated through pulse-echo responses with a high frequency ultrasonic transducer. These new bipolar-transistor-based limiters may have applications for the high-frequency ultrasound imaging where both the bandwidth and signal to noise ratio are crucial.