Performances of operational amplifiers in front-end electronics for nuclear radiation detectors

Operational amplifiers in charge sensitive configuration are an attractive solution for front-end electronics in cost-sensitive radiation detection systems. The noise performances of several JFET input operational amplifiers were tested in a charge-sensitive configuration. Some rather old devices reached noise performances low enough for detection of X and gamma rays in applications where extreme energy resolution is not required as, for instance, dosimetry and ambient monitoring applications. On the contrary the devices with the best specification for noise and speed showed an excess noise when used in a charge sensitive configuration with the low feedback capacitor required by this application. By matching the impedances seen at both input of the amplifier, the noise is reduced near to the theoretical value. The origin of this fact seems to be a non white equivalent current noise generator, whose origin is in the first stage bias circuit. Datasheet parameters are not sufficient, at least in some cases, for computing the actual noise of a charge preamplifier built with an operational amplifier.