Noise in charge amplifiers-A gm/ID approach. http://dx.doi.org/10.1109/TNS.2012.2208270

Abstract

Charge amplifiers represent the standard solution to amplify signals from capacitive detectors in high energy physics experiments. In a typical front-end, the noise due to the charge amplifier, and particularly from its input transistor, limits the achievable resolution. The classic approach to attenuate noise effects in MOSFET charge amplifiers is to use the maximum available power, to match capacitances at the input node, and to use a minimum-length input device, but these conclusions, reached by analysis based on simple noise models, lead to sub-optimal results. In this work, a new approach on noise analysis for charge amplifiers based on an extension of the gm/ID methodology is presented. This method combines circuit equations and results from SPICE simulations, both valid for all operation regions and including all noise sources. The method, which allows to find the optimal operation point of the charge amplifier input device for maximum resolution, shows that the minimum device length is not necessarily the optimal, that flicker noise is responsible for non-monotonically decreasing noise for high currents, and that capacitance matching is the optimal solution only when flicker noise is dominant.