MOSFET diode as a feedback reset element on charge amplifiers

A MOSFET, connected as a diode with gate and drain short circuited and bias ed in saturation mode, is proposed as a substitute for the high-value feedb ack resistor to provide de feedback and signal reset in integrated charge a mplifiers with BJT input transistors. This paper describes the MOSFET desig n rules to set the desired value of the discharge time constant, to minimiz e its noise contribution, and to operate it in a low voltage circuit, The b ehavior 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 performanc e is also reported.