Resistive Feedback Preamplifier

The Resistive-Feedback Preamplifier

Figure 3(a) illustrates the typical output pulse shapes from a resistive-feedback preamplifier. The output for each pulse consists of a rapidly rising step, followed by a slow exponential decay. It is the amplitude of the step that represents the energy of the detected radiation. The exponential decay time constant is normally determined by the feedback resistor in parallel with the feedback capacitor. Decay time constants of 50 µs are prevalent, but longer time constants are encountered on some preamplifiers.

For detectors with very short charge collection times, the rise time of the preamplifier output pulse is controlled by the preamplifier itself, and the rise time is usually in the range from 10 to 100 ns. For detectors with long charge collection times, such as NaI(Tl) detectors, proportional counters, and coaxial germanium detectors, the output rise time of the preamplifier is controlled by the detector charge collection time. The output rise time can range up to 700 ns for large coaxial germanium detectors, and into the microsecond range for positive ion collection with proportional counters. For NaI(Tl) detectors, the scintillator decay time causes a preamplifier output rise time of approximately 500 ns.

In normal operation at ordinary counting rates, the rising step caused by each detector event rides on the exponential decay of a previous event, and the preamplifier output does not get a chance to return to the baseline. Since the amplitude of detector events is usually variable and the time of occurrence is random, the preamplifier output is usually irregular, as shown in Fig. 3(a). As the counting rate increases, the piling up of pulses on the tails of previous pulses increases, and the excursions of the preamplifier output move farther away from the baseline. The power supply voltages eventually limit the excursions, and determine the maximum counting rate that can be tolerated without distortion of the output pulses.

Before amplification, the pulse-shaping amplifier must replace the long decay time of the preamplifier output pulse with a much shorter decay time. Otherwise, the acceptable counting rate would be severely restricted. Figure 3(b) demonstrates this function using the simple example of a single-delay-line, pulse-shaping amplifier. The energy information represented by the amplitudes of the steps from the preamplifier output has been preserved, and the pulses return to baseline before the next pulse arrives. This makes it possible for an analog-to- digital converter (ADC) to determine the correct energy by measuring the pulse amplitude with respect to the baseline. With the shorter pulse widths at the amplifier output, much higher counting rates can be tolerated before pulse pile-up again causes significant distortion in the measurement of the pulse heights above baseline.

Figure 3. Output Pulse Shapes from (a) a Resitive-Feedback Preamplifier, and (b) the Delay-Line Shaping Amplifier Connected to the Preamplifier.