Improvement of the “CC2” charge sensitive preamplifier for the GERDA phase II experiment

Phase I of the GERDA experiment, aimed at investigating neutrino-less double beta decay of ⁷⁶Ge is active in Italy at the INFN LNGS (Gran Sasso Laboratory of the National Institute for Nuclear Physics) since one year. Germanium detectors are operated immerged in liquid Argon, with front-end electronics based on the "CC2", a three channels, low noise, cryogenic charge sensitive preamplifier, designed to cope with the peculiar characteristics of the GERDA experiment (radio-purity, power consumption, connection to the outside of the cryostat by means of long and resistive cables, etc.). The new proposed preamplifier "CC3" is derived from the general scheme of the previous circuit, with a few effective changes that mainly allow the physical separation of the preamplifier into two sections: i) the very-front-end, comprising the input JFET and the feedback elements that needs to be positioned very close to the detectors and ii) the rest of the circuit, that will be up 1.5 meters away from the detectors. While keeping the same electrical properties of the previous circuit in terms of noise, bandwidth, power consumption, etc., the new configuration will decrease the overall radioactivity contribution of the preamplifiers and thus fit the more stringent requirements of the phase II experiment. Prototype printed circuit boards have been designed and manufactured; the first experimental tests are in agreement with the simulations and show satisfactory results.