Design of a low-current shunt-feedback transimpedance amplifier with inherent loop-stability

In this paper we propose a new architecture for enhancing the performance of a transimpedance amplifier used for low-currents, and in particular, that used in biosensing. It is usually the first block in biomedical acquisition systems for converting a current in the nanoampere and picoampere range into a proportional voltage, with an amplitude suitable for further processing. There exist two main amplifier topologies for achieving this, current-mode and shunt-feedback mode. This paper introduces a shunt-feedback amplifier that embodies current-mode operation and thereby offers the advantages of both existing schemes. A conventional shunt-feedback amplifier has a number of stages and requires compensation components to achieve stability of the feedback loop. The exemplary circuit described is inherently stable because a high gain is effectively achieved in one stage that has a dominant pole controlling the frequency response. Exhibiting complementary symmetry, the configuration has an input port that is very close to earth potential. This enables the configuration to handle bidirectional input signals such are as met with in electrochemical ampero-metric biosensors. For the 0.35 µm process adopted and ± 3.3 V rail supplies, the power dissipation is 330 µW. With a transimpedance gain of 120 dBΩ the incremental input and output resistances are less than 2 Ω and the − 3 dB bandwidth for non-optical input currents is 8.2 MHz. The input referred noise current is 3.5 pA/√Hz.