The primary function of a preamplifier is to extract the signal from the detector without significantly degrading the intrinsic signal-to-noise ratio. Therefore, the preamplifier is located as close as possible to the detector, and the input circuits are designed to match the characteristics of the detector. Different pulse processing techniques are typically employed, depending on whether the arrival time or the amplitude (energy) of the detected event must be measured. Pulse shaping for either application is normally implemented in a subsequent module. This module can be located at some distance from the preamplifier, provided that the signal fidelity is not degraded due to the length of the interconnecting coaxial cable.

Several types of detectors produce moderately large signals at their outputs, and this relaxes the restrictions on the noise contribution from the preamplifier. Detectors that typically fall in this category are: photodiodes operating with intense light pulses, photomultiplier tubes (PMT), scintillation detectors (scintillator mounted on a PMT), microchannel plate PMTs, microchannel plates, channeltrons, and electron multipliers. For such detectors, a wideband amplifier with a low input impedance can be used directly at the detector output to generate short, fast-rising pulses for timing or counting purposes. For pulse-amplitude (or energy) spectroscopy, a relatively inexpensive preamplifier, such as the Model 113 or the Model 142IH, can be used to integrate the charge in the pulse at the detector output.

Detectors with much better resolution are frequently used for energy spectroscopy with x rays, gamma rays, and charged particles. Typical detectors in this category are: Si(Li) (planar), germanium (coaxial, LO-AX™, and planar), silicon charged-particle detectors, and gas proportional counters. Because such detectors produce very small output signals, it is essential that the input stage of the preamplifier contribute little noise. The requirement for low noise and stable sensitivity with these detectors is met by using a charge-sensitive preamplifier with an FET (Field-Effect Transistor) input stage. For silicon charged-particle detectors and proportional counters the entire preamplifier usually is operated at room temperature. However, the excellent resolution of the cooled germanium and Si(Li) detectors necessitates lowering the temperature of the FET input stage of the preamplifier to reduce the noise. Operation at a temperature near 120° K is accomplished by mounting the FET near the detector inside the cryostat. Specifications for these cooled preamplifiers are incorporated with the relevant detector in the ORTEC Detector section. The signal at the output of the charge-sensitive preamplifier can be used for either timing or energy spectroscopy.

ORTEC manufactures a preamplifier to fit your detector, your application, and your budget. The applications information and selection guides will help you to choose the optimum preamplifier for your task.