MOSFET diode as a feedback reset element on charge amplifiers

A MOSFET, connected as a diode with gate and drain short circuited and biased in saturation mode, is proposed as a substitute for the high-value feedback resistor to provide dc feedback and signal reset in integrated charge amplifiers with BJT input transistors. This paper describes the MOSFET design rules to set the desired value of the discharge time constant, to minimize its noise contribution, and to operate it in a low voltage circuit. The behavior of the MOSFET as an almost fixed value resistor is evidenced by the single time-constant discharge of the signal at the output of a prototype preamplifier for fast (<50 ns) shaping, whose experimental noise performance is also reported.