Bandwidth extension of transimpedance amplifier by implementing source/emitter follower topology

Abstract

This thesis proposes a new bandwidth extension technique and its MMIC transimpedance amplifier (TIA) implementation for 2.5 Gbps and 10 Gbps applications, using commercial 0.5 um metal semiconductor field effect transistors (MESFETs) and InGaP/GaAs hetero junction bipolar transistors (HBTs). The bandwidth extension technique using a source follower or an emitter follower topology is devised and demonstrated. In order to reduce the source impedance of the amplifier, the source follower or emitter follower stage is adopted prior to the common source or common emitter amplifier, which extends effectively the bandwidth of the amplifiers. A 2.5 Gbps application transmipendance amplifier is designed based on low cost a 0.5 um MESFETs offering cutoff frequencies, $f_T$, of 17.5 GHz, maximum oscillation frequency, $f_{max}$, of 30.8 GHz, and transconductance, $g_m$, of 190 mS/mm. Proposed method replaces the on-chip inductors or capacitors for inductive-peaking or capacitive-peaking with active devices. The measurement result of the source follower topology TIA permits an improvement of 1.7 GHz compared to a common source structure : while a traditional common source TIA shows a 2.35 GHz bandwidth with 59 dB $\omega$ gain, the proposed source follower TIA reveals 4.05 GHz bandwidth without sacrificing its transimpedance gain. It is equivalent to an improvement of 72% bandwidth. For 10 Gbps applications, emitter follower topology TIA is designed with InGaP/GaAs HBT. Its $f_T$ is 59 GHz, and $f_{max}$ is 49 GHz. The proposed emitter follower TIA is simulated to have a 11.35 GHz bandwidth with 57.6 dB$\omega$ gain, while a traditional counterpart, the common emitter TIA shows a 9 GHz bandwidth with 57.6 dB $\omega$ gain. But, the fabricated TIA chip does not show bandwidth extension effect. In order to analyze, the differences between the simulation and measurement, we evaluate the fabricated transistors in comparison with the modeled characteristics. The major reason...