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6 Theory of Operation
6.1 Functional Description
Charge sensitive preamplifiers are used when radiation is detected as a series of pulses, resulting in brief
bursts of current flowing into or out of the preamplifier input. Depending on the type of detector, this burst
of current may be very brief (<1 ns) or as long as a few seconds. For an idealized detection current pulse
taking the form of a delta function, the detected charge (time integral of the input current) will ideally take
the form of a step function.
The output waveform of an actual charge sensitive preamplifier will of course have a non-zero rise time:
for the CSP10...13 this figure is approximately 7 ns. Furthermore, capacitance at the preamplifier input
(i.e. detector capacitance) will further slow the rise time at a rate of 0.4 ns / pF. Keep in mind the output
rise time will also be limited by the speed of the detector. For example, the detection current pulse from a
Cs(Tl) / photodiode scintillation detector has a duration of approximately a couple s, so the expected rise
time of the charge sensitive preamplifier output will be at least that long.
The output waveform of the CSP10...13 using a capacitively-coupled fast square wave pulser at the input
is shown below to the left. At long time domains, the output decays due to the discharge of the feedback
capacitor through the feedback resistor, with an RC time constant of 140 µs. This decay of the output
waveform is also shown below, to the right.
6.2 Detailed Circuit Description
Fig. 6.1 shows a simplified equivalent circuit diagram of the hybrid amplifier module used in the
CSPA10...13, which is a two stage amplifier. The first stage is high gain, and the second stage is low gain
with an emphasis on supplying sufficient output current to drive a terminated coaxial cable. Rf (100 MΩ)
and Cf (1.4 pF) are the feedback resistor and capacitor respectively
(tdecay= 140µs). The feedback values for the other models are:
Rf =10 MΩ and Cf =15 pF, tdecay= 150 µs (CSPA11),
Rf =680 kΩ and Cf =15 pF, tdecay= 50 µs (CSPA12),
Rf =68 kΩ and Cf =150 pF, tdecay= 50 µs (CSPA13).
The CSPA10...13 preamplifier module consists of:
hybrid charge sensitive preamplifier
input DC block capacitor
test input with 50 ohm termination
1 pF test capacitor
high voltage path with filter circuit and
power supply circuitry
ComTec GmbH
F
Theory of Operation
15
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