Ultra-low-noise large-bandwidth transimpedance amplifier

Equivalent input current noise and bandwidth are the most relevant parameters qualifying a low-noise transimpedance amplifier. In the conventional topology consisting of an operational amplifier in a shunt-shunt configuration, the equivalent input noise decreases as the feedback resistor (R-F), which also sets the gain, increases. Unfortunately, as R-F increases above a few M Omega as it is required for obtaining high sensitivity, the bandwidth of the system is set by the parasitic capacitance of R-F and reduces as R-F increases. In this paper, we propose a new topology that allows overcoming this limitation by employing a large-bandwidth voltage amplifier together with a proper modified feedback network for compensating the effect of the parasitic capacitance of the feedback resistance. We experimentally demonstrate, on a prototype circuit, that the proposed approach allows to obtain a bandwidth in excess of 100kHz and an equivalent input noise of about 4fA/root Hz, corresponding to the current noise of the 1G Omega resistor that is part of the feedback network. The new approach allows obtaining larger bandwidth with respect to those obtained in previously proposed configurations with comparable background noise. Copyright (c) 2014 John Wiley & Sons, Ltd.