An auto-zero charge preamplifier with "cold" discharge mechanism for ionising radiation sensors

A circuit technique is presented which eliminates the offset error in the output voltage of low-noise charge preamplifiers for ionising radiation sensors. Offset cancellation is obtained actively through a negative feedback path whose “error” variable is the offset itself. As a result the output voltage offset is automatically eliminated with no trimming, irrespective of the bias point of the input transistor and the dark current of the sensor. Using the proposed technique in a low-noise charge preamplifier for germanium detectors we obtained an output voltage offset < 1 mV against an original offset of -291 mV. The output offset of these preamplifiers is typically in the range of -500 mV to +100 mV, as mainly dictated by the bias voltage of the gate of the input FET (Field Effect Transistor). The noise of the preamplifier is unaffected by the offset cancellation circuit. In addition, an optional resistor permits reduction of the the fall time of the preamplifier by a factor of 1 to 10, while leaving unchanged the noise and the sensitivity of the charge preamplifier. This helps reduce the pileup at high count rates.