Design criteria for the optimization of hybrid charge-sensitive preamplifiers for high resolution gamma-ray spectroscopy

Design criteria for the optimization of hybrid charge-sensitive preamplifiers for germanium high resolution gamma-ray detectors are presented. In particular we studied the optimization of the Transimpedance Amplifier (T.A.) following the input JFET and realized with Bipolar Junction Transistors. Our aim was to highlight and adjust the key parameters of the T.A. in order to maximize the loop gain of the charge-sensing stage while maintaining at the same time an adequate low noise. We analytically studied the expression of the open-loop gain, focusing our attention on the upper limit that can be reached according to the physical parameters of the chosen devices. We analyzed the role played by the Miller effect acting on the input JFET gate-drain capacitance, which reduces the loop gain of the charge-sensing stage, and we proposed a solution to address this issue. We studied the optimization of the folded cascode BJT bias current in order to minimize the T.A. noise. Analytical study is reported and simulation results for typical structures are shown.