Charge sensitive preamplifier with continuous reset by means of the gate-to-drain current of the JFET integrated on the detector

In this paper we present a charge sensitive preamplifier for a Si drift detector where both the input n-JFET and feedback capacitor are integrated directly on the detector chip. This solution allows to minimize the stray capacitances of the connections and to obtain a capacitive matching between detector and front-end transistor. A continuous discharging mechanism for the leakage current and for the signal charge is obtained by exploiting the gate-to-drain current of the front-end JFET. The suitable value of gate current is reached by means of a "weak" avalanche breakdown mechanism which occurs in a high-field region of the transistor channel. The advantage of this solution is that the discharge is obtained directly by means of the front-end transistor without the need of any additional integrated device. A feedback loop in the charge preamplifier sets the suitable value of drain-gate voltage in order to compensate for variations of the leakage current to be discharged. The results of the experimental characterization of the detector+preamplifier system are presented.