Photomultiplier tubes, electron multipliers, microchannel plates, and microchannel plate PMTs produce moderately large output signals with very fast rise times. Therefore, the most cost-effective preamplifier for pulse-amplitude measurements or energy spectroscopy with these detectors is the parasitic-capacitance preamplifier illustrated in Fig. 2.

Parasitic-capacitance preamplifiers have a high input impedance (~5 MΩ). Hence, the current pulse generated by the detector is integrated on the combined parasitic capacitance present at the detector output and the preamplifier input. This combined capacitance is typically 10 to 50 pF. The resulting signal is a voltage pulse having an amplitude proportional to the total charge in the detector pulse, and a rise time equal to the duration of the detector current pulse. A resistor connected in parallel with the input capacitance causes an exponential decay of the pulse with a time constant ~50 µs. An amplifier having a high input impedance and unity gain is included as a buffer to drive the low impedance of a coaxial cable at the output. The 93-Ω resistor in series with the output absorbs reflected pulses in long cables by terminating the cable in its characteristic impedance.

Parasitic-capacitance preamplifiers are not used with semi-conductor detectors because the gain of this type of preamplifier is sensitive to small changes in the parasitic capacitance. For partially-depleted semiconductor detectors the detector capacitance varies with the bias voltage applied to the detector diode. In addition, small movements of the interconnecting cable can change the input capacitance by a few tenths of a pF. The gain changes caused by these effects are significant for semiconductor detectors, which have energy resolutions better than 1%. However, parasitic-capacitance preamplifiers, such as the ORTEC Model 113, provide more than adequate performance with photomultiplier tubes, microchannel plate PMTs, or scintillation detectors, and are highly recommended for those applications.