Extending the dynamic range of a charge-preamplifier far beyond its saturation limit : a 0.35 μm CMOS preamplifier for germanium detectors

The dynamic range of integrated charge-sensitive preamplifiers for germanium detectors is typically limited to ~5 MeV, owing to the intrinsically low available voltage swing of scaled CMOS technologies and to the very high sensitivity needed to achieve energy resolutions of ~0.1% @ 2 MeV. We propose a circuit technique that allows for high-resolution energy measurements, compatible with gamma-spectroscopy standards, also in the case of large input signals yielding a deep saturation of the charge preamplifier. The preamplifier has been designed and simulated in a 5V 0.35 μ m CMOS technology. The idea relies on the fact that the physical information, i.e. the charge released by the germanium crystal, is not destroyed by the saturation of the charge sensing stage. The exceeding charge, that cannot be stored on the feedback capacitance, is temporarily stored on the other capacitances connected at the input node, namely the detector capacitance and the input JFET capacitance. The CMOS circuit comprises a fast-reset device connected at the input node of the circuit and performing the fast de-saturation of the charge sensing stage. A first-order linear relation exists between the input charge and the reset time. By estimating the input charge through the direct measure of the reset time, a substantial increase of the energy measurement range is achieved.