Several detector types, such as photomultiplier tubes and microchannel plates, generate a moderately large and fast-rising output signal through a high output impedance. Pulse processing for timing or counting with these detectors can be rather simple. A properly-terminated 50-Ω coaxial cable is attached to the detector output, so that the current pulse from the detector develops the desired voltage pulse across the 50-Ω load presented by the cable. For scintillators mounted on 14-stage photomultiplier tubes, this voltage signal is usually large enough to drive the input of a fast discriminator without further amplification. For single-photon counting, 10-stage photomultiplier tubes, or microchannel plate PMTs, additional amplification is needed between the detector and the discriminator, and this is the function of the current-sensitive preamplifier.

The 50-Ω input impedance of the current-sensitive preamplifier provides proper termination of the 50-Ω coaxial cable, and converts the current pulse from the detector to a voltage pulse. If the rise time of the preamplifier is negligible compared to the detector rise time, and the voltage gain of the preamplifier is A, the amplitude of the voltage pulse at the preamplifier output will be

V_out = 50 I_in A          (1)

where I_in is the amplitude of the current pulse from the detector. For counting applications this signal can be fed to a fast discriminator, whose output is recorded by a counter/timer.

For timing applications the dominant limitation on timing resolution with photomultiplier tubes and microchannel plates is fluctuation in the transit times of the electrons as they cascade through the detector. This causes a jitter in the arrival time of the pulse at the detector output. However, if the detector signals are small enough to require a current-sensitive preamplifier, the effect of preamplifier input noise on time resolution must also be considered.

The noise added to the signal by the preamplifier causes an uncertainty or jitter in the time at which the pulse crosses the threshold of the timing discriminator. The result is a degradation of the time resolution. Therefore, it is important to choose a current-sensitive preamplifier whose rise time is similar to the rise time of the pulse at the detector output.¹ A preamplifier rise time that is much faster than the detector rise time does not improve the signal rise time. But, it does contribute extra noise, because of the unnecessarily wide bandwidth. This excess noise will increase the timing jitter. Choosing a preamplifier rise time that is much slower than the detector rise time reduces the preamplifier noise contribution, but not enough to overcome the degradation in pulse rise time and amplitude. Consequently, the timing jitter becomes worse.

Although the optimum choice depends on the rise time and amplitude of the detector signal, as well as the characteristics of the preamplifier input stage, a good guideline is to choose a preamplifier rise time that is within a factor of 2 of the detector rise time (faster or slower). Rise times for photomultiplier tubes range from 1.5 to 10 ns, making the Models VT120, 9301, and 9305 Preamplifiers appropriate for consideration. The Model 9306 1-GHz Preamplifier is the optimum choice for timing with the 150-ps rise times encountered with microchannel plate PMTs.

Most fast preamplifiers with gains in excess of 10 must employ ac-coupling between internal amplifying stages to achieve fast rise times and to eliminate dc drift with temperature. This is an excellent solution if the average spacing between pulses is greater than 100 times the individual pulse width. But, when the average spacing between pulses becomes comparable to the pulse width, the ac-coupling causes the baseline between pulses to shift so that the preamplifier output signal circumscribes as much area above ground potential as it does below ground. This effect distorts the amplitude measurement in subsequent modules. The Model 9305 Preamplifier offers a solution to this problem in cases where lower gain (A=10) and a slightly slower rise time (3 ns) is acceptable. The Model 9305 is dc-coupled and exhibits excellent dc stability. By operating a photomultiplier tube with the cathode at high voltage, the anode can be dc-coupled to the input of the Model 9305 Preamplifier. This scheme eliminates the baseline shift at high counting rates, and permits operation at much higher counting rates.

Most current-sensitive preamplifiers designed for timing applications have ac-coupled time constants in the range of a few hundred nanoseonds. The model 9326 overcomes that limitation by offering a low-frequency roll-off at an exceptionally low 10 kHz.

¹S. Cova, M. Ghioni, and F. Zappa, Rev. Sci. Instrum. 62 (11), Nov. 1991, pp.2596-2601.