Low transimpedance-fluctuation design for 10-GHz Si-bipolar preamplifier in 10-Gb/s optical transmission systems

摘要

Design considerations concerning Si preamplifiers for 10-Gb/s optical transmission systems are discussed. A preamplifier with 300-Ω transimpedance was designed by focusing on attaining low transimpedance fluctuation in the frequency response despite bias variation caused by the photo current. To ensure good design accuracy, we optimized the current density of the transistor and the open-loop voltage gain using measured results to obtain the desired bandwidth. We also developed a low-loss pad structure that has U-grooves to improve the bandwidth. The U-grooves increase the pad parasitic resistance and reduce signal-loss from pad to Si substrate. A preamplifier IC fabricated using a Si bipolar technology with f_T of 35-GHz provided a bandwidth of 10.2 GHz, transimpedance fluctuation within 0.5 dB, an input dynamic range of up to 1.6 mA_(p-p), and a low averaged input noise current density of 11.5 pA/Hz~(1/2).