A Tunable Impedance Wideband Low Noise Amplifier

低雜訊放大器為超音波生醫儀器中關鍵性之零件，其作用為可放大從組織中反射之微小信號且不放大雜訊，超音波探頭與低雜訊放大器之阻抗匹配為相當重要之一環，其功用為在所需頻帶內提供穩定之增益。現今之低雜訊放大器之輸入阻抗為了與探頭之阻抗一致普遍設計為一固定值，若能利用不同的偏壓電流產生不同之輸入阻抗設計適用於各種不同超音波探頭之低雜訊放大器，意味著可以節省可觀之成本。 於是本論文提出一可調式輸入阻抗低雜訊放大器電路原型分別使用異質接面電晶體與0.35 μm CMOS製程設計。在量測結果中顯示於100 kHz~ 30 MHz之操作頻率下，可調整輸入阻抗為30歐姆至90歐姆之任意值，在不同輸入阻抗情況下之增益平均皆有13 ~14 dB，且雜訊指數為7.33 dB。Low noise amplifier ( LNA ) is a critical component for ultrasound medical equipment, since it amplifies tiny echo signals reflected from tissues without adding much noise. Wideband input impedance match with the loading of ultrasound transducers is therefore important for LNAs to provide constant gain over a wide frequency range. State-of-the-art LNA for general applications is designed such that the impedance is matched with a specific impedance. To extend the capability of LNA to different transducers, which would lead to significantly lower cost, a wideband tunable input impedance of the amplifier is designed by varying bias current of the transistor, which is used to provide the system gain. Two prototypes amplifier are constructed to validate the proposed architecture with discrete HBT transistors and 0.35 μm CMOS process, respectively. And it has shown the small gain variation with a wide tuning range of input impedance from 30 to 90 Ohm. The measured noise figure is 7.33 dB with an average gain of 13.5 dB over the range of input impedance