A low-noise charge amplifier with fast rise time and active discharge mechanism

A new hybrid, low-noise, wide-bandwidth charge amplifier, using a new active discharge mechanism, has been designed and built primarily for gamma-ray tracking detectors, where a fast rise time is required. In order to realize the discharge of the feedback capacitance, a noninverting low-gain (A) stage has been inserted between the output of a conventional charge preamplifier and the high-value feedback resistance. This technique allows to make the discharge time constant shorter (by factor A), while keeping the feedback resistor as the feedback capacitance unchanged. Special care has been devoted realizing the hybrid layout to reduce the stray capacitance of the feedback resistor and particular attention has been dedicated to assure a good signal-to-noise ratio and a fast rise time. A low-power consumption allows the use of this charge-sensitive preamplifier in vacuum chamber without cooling system. The noise level is equivalent to less than 880-eV full width half maximum for an input capacitance of 22 pF, using 3-mus shaping time. The noise grows with a slope of less than 10 eV/pF in the range of 20-60 pF input capacitance. This suggests that the preamplifier can be operated at room temperature maintaining a good noise usually reachable with a cooled setup.