A JFET-CMOS fast preamplifier for segmented germanium detectors

A JFET-CMOS fast charge-sensitive preamplifier for germanium detectors, able to operate at cryogenic temperatures, has been designed, realized, and characterized. The monolithic part of the circuit is realized in a mature 5 V 0.8 µm Si CMOS technology, which yields better performances than scaled technologies in this case. The input transistor is an external Si JFET, which can be easily replaced if necessary. The charge-to-voltage gain and the fall-time are as well set through an external RC,network. The circuit works in the wide temperature range of -196 to 55 degrees C and is able to drive a terminated coaxial cable with an exceptionally fast and clean transition. Namely, with a detector capacitance of 16 pF and a negative power supply of -3 V it is able to provide a -2.4 V pulse onto a 100 ohm load in less than 13 ns with no ringing. The static power consumption is 8 mW excluding the JFET. The area occupancy of the integrated circuit is as little as 366 x 275 µm². The noise performance with a 16 pF detector capacitance is 110 r.m.s. electrons both at room temperature and at -196 degrees C, at a quasi-Gaussian shaping time of 10 µs. The obtained performance is adequate for gamma-ray spectroscopy and pulse-shape analysis with bulky HPGe segmented detectors.