Current mirror reset for low-power BiCMOS charge amplifier

We present a circuit solution to provide DC feedback and signal reset in charge amplifiers that overcomes the difficulty to integrate high value resistors in VLSI technology. The feedback resistor is substituted by a MOSFET current conveyor that re-direct to the input node the current already available at the output follower. The lower noise of this “active resistor” with respect to a physical resistor of equal value makes possible a first shaping within the preamplifier. The circuit has been implemented for a fast shaping time system (20 ns peaking time) using a BJT as input transistor for best noise performance. The circuit has been powered with single supply as low as 1.6 V with a total power consumption down to 220 μW/ch and has shown a measured noise of 660 electrons rms, in accordance with the theoretical expectation.