The AGATA charge-sensitive preamplifiers with built-in active-reset device and pulser

New charge-sensitive preamplifiers for the highly segmented HPGe detectors of AGATA have been designed and built, as developed to read out simultaneously the segment- and the core-electrode signals. A new fast-reset circuitry is used to reduce the dead time brought about by a background of energetic particles in the 10 to 100 MeV range that could hit the detectors in nuclear-physics experiments with radioactive beams. The input field effect transistor and the feedback resistor are placed in the detector cryostat and cooled to ~120 K to minimize the electronic noise. The FET is biased at an unusually low drain voltage (2 V) because a power budget of only 20 mW per FET is allowed for thermal-dissipation constraints in the cryostat. A differential output driver is used to transmit the analog signals through 5-m twisted-pair LVDS cables to an external flash-ADC module. A precise programmable pulser has been developed and installed on a side of the "core" preamplifier board, to be used to inject a calibration pulse onto the core electrode and thereafter to all segments through the detector bulk capacitance.