Pulsed Reset Preamplifiers

Pulsed-Reset Preamplifiers

Pulsed-reset preamplifiers were developed to eliminate the noise contributions of the preamplifier feedback resistor, and to improve the high counting rate capability of the preamplifier. There are two types: Pulsed optical feedback preamplifiers are often employed with Si(Li) detectors for x-ray spectrometry,¹ and transistor-reset preamplifiers are used to achieve high counting rates with germanium detectors.^2,3 In both cases the feedback resistor is replaced with a feedback device that is turned on only for the very short time needed to reset the preamplifier output back to the baseline. The behavior at the output of the preamplifier is illustrated in Fig. 4(a).

With no feedback resistor to remove the charge from the feedback capacitor between detector events, each event steps the preamplifier output up to a higher dc voltage. Eventually, the staircase of pulses approaches the power supply voltage, and the voltage across the feedback capacitor must be reset back to the starting value. A voltage comparator in the preamplifier senses the upper limit of the staircase, and turns on the reset device just long enough to discharge the feedback capacitor back to the starting condition. By this method, the preamplifier output is maintained within its linear operating range, even at high counting rates. The limitation on counting rate with a pulsed-reset preamplifier is the percent dead time caused by the reset. At higher counting rates the reset must happen more frequently. When the percent dead time from resetting becomes too high to tolerate, the upper limit on counting rate has been reached.

Although the preamplifier output looks different from that with resistive-feedback preamplifiers, the function of the amplifier with pulsed-reset preamplifiers is similar. The pulse-shaping amplifier must preserve the amplitude of the steps from the preamplifier, and cause the pulses to return to baseline quickly between the steps. This function is demonstrated in Fig. 4(b) using a semi-Gaussian, pulse-shaping amplifier. Although slightly rounded in shape to improve the signal-to-noise ratio, the amplitudes of the amplifier output pulses are proportional to the step amplitudes from the preamplifier.

One additional characteristic shows up at the amplifier output with a pulsed-reset preamplifier. Each preamplifier reset causes a large, negative polarity, output pulse to be generated. The duration of this reset recovery pulse is determined by the pulse-shaping circuits in the amplifier, the gain of the amplifier, and the voltage swing of the reset. Typically, it lasts two to three times as long as the positive polarity pulses from detector events. During the reset-recovery pulse, data collection must be inhibited to prevent measurement of distorted pulse heights. The inhibit logic signal in Fig. 4(c) is generated by the preamplifier and/or the amplifier, and is used to inhibit data acquisition in the pulse-height analyzer during reset recovery.

With both the resistive-feedback preamplifier and the pulsed-reset preamplifier, the amplifier input must be able to accept the voltage swings of the preamplifier output without causing any distortion of the pulse amplitudes.

¹Ron Jenkins, R.W. Gould, Dale Gedcke, Quantitative X-Ray Spectroscopy, Marcel Dekker Inc, New York, 1981, pp 175177.
²D.A. Landis, C.P. Cork, N.W. Madden, F.S. Goulding, IEEE Trans. Nucl. Sci., NS-29(1), 619 (1982).
³C.L. Britton, T.H. Becker, T.J. Paulus, R.C. Trammell, IEEE Trans. Nucl. Sci., NS-31(1), 455 (1984).

Figure 4. (a) The Output from a Transistor-Reset Preamplifier; (b) the same events after passing through a Semi-Gaussian Pulse-Shaping Amplifier; (c) the Inhibit Signal, which prevents data collection during reset and reset recovery.