Fast low-impedance output stage for CMOS charge preamplifiers able to work at cryogenic temperatures

The quality of the output stage is key to the performance of low-noise CMOS preamplifiers for semiconductor detectors, especially when the preamplifier signal is to be transmitted to a remote receiver. This work deals with two implementations of an output-stage circuit architecture particularly suited for low-noise integrated preamplifiers of X and gamma-ray detector signals. The aim is to conjugate a few important features: low output impedance, ability to work at room and cryogenic temperatures, ability to drive a terminated coaxial cable, low power consumption, large voltage swing with a 100 ohm load. In particular we were interested to a large negative voltage swing. The standard solution, i.e. a source-follower stage realized with a p-MOSFET, is not adequate because its threshold and overdrive voltage would severely limit the negative output voltage swing. The proposed circuit structure, inspired to the White follower, builds around a first n-MOSFET configured as source follower, a second n-MOSFET acting as driver for the load current and a negative-feedback loop which stabilizes the working current of the first MOSFET. As a result both a low output impedance and a large negative voltage swing are obtained. Using a prototype of such output stage in a JFET-CMOS charge-sensitive preamplifier for germanium detectors we obtained a large negative voltage swing, a signal rise time of ~13 ns with a resistive load of 100 ohm, and a negligible additional noise.