A fast VLSI preamplifier for segmented HPGe γ-ray detectors

Abstract

We developed and realized a fast integrated chargesensitive preamplifier for segmented High Purity Germanium (HPGe) detectors, able to directly drive long 50 Ω coaxial cables. The circuit is realized in a 5V 0.8µm CMOS technology. It includes an external input FET, mod. BF862, which can be easily replaced if necessary. The charge-to-voltage gain and the fall time are set by an external RC feedback network, and can be changed in a broad range of values. The preamplifier works both at room and at cryogenic temperatures. The area occupancy of the chip is 366×275µm² and the static power consumption is 8mW excluding the FET. The preamplifier is optimized for hole signals, i.e. negative output voltage swings. With a 100Ω output load it is able to provide an output swing of -2.4V, against a negative power supply of -3V. The transition time for such a large pulse is 12.8ns. The noise performance with a 15pF detector capacitance is 110 r.m.s. electrons both at 300°K and at 77°K, at a quasi-Gaussian shaping time of 10µs. The obtained performance is adequate for gamma-ray spectroscopy and pulse-shape analysis with HPGe segmented detectors.