A versatile low-noise wide-range charge-sensitive preamplifier for HPGe detectors

A fast low-noise hybrid charge-sensitive preamplifier for HPGe detectors, has been designed for the new array detector 'GALILEO' under construction at INFN Legnaro National Laboratory (Italy). The preamplifier includes an over-threshold fast-reset circuitry that greatly reduces the dead time due to preamplifier and/or ADC saturation. A 1-bit digital output signal is provided in LVDS or CMOS format, whose width yields an indirect precise measurement of the large over-threshold signals. Combining the under- and over-threshold working modes an unprecedented spectroscopic dynamic range is obtained of 10^5, from 0.1 fC to 10 pC, i.e. from ∼2 keV to 180 MeV of equivalent energy in germanium. A fast analogue signal is optionally provided as a trigger for ancillary detectors. The preamplifier is optimized to feature low noise, fast rise time and wide amplitude range when connected to anode detector electrodes. It includes a fast differential buffer for low-noise signal transmission to a remote ADC, with a preset common-mode DC component, that can be adjusted to match the ADC requirement. The rise time, as measured at the test bench, is as fast as 7.5 ns, with a detector capacitance of 23 pF and with a 5m 50ohm twisted-pair cable connecting the preamplifier to the ADC. With its input transistor working at 150K the preamplifier features an intrinsic noise of ∼90 electrons r.m.s., i.e. ∼600eV fwhm in germanium, and a noise slope of 8eV/pF.