Theory of Operation- Detailed Circuit Description
ComTec GmbH
6-1
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 CsI(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
µ
s. This
decay of the output waveform is also shown below, to the right.
6.2. Detailed Circuit Description
Figure 2 shows a simplified equivalent circuit diagram of the hybrid amplifier module used in the
CSP10...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 M
Ω
) and Cf (1.4 pF) are the feedback resistor and capacitor respectively (tdecay=
140
µ
s). The feedback values for the other models are:
Rf =10 M
Ω
and Cf =15 pF, tdecay= 150
µ
s (CSP11),
Rf =680 k
Ω
and Cf =
7
5 pF, tdecay= 50
µ
s (CSP12),
Rf =68 k
Ω
and Cf =
7
50 pF, tdecay= 50
µ
s (CSP13).
The CSP10...13 preamplifier module consists of:
•
hybrid charge sensitive preamplifier
•
input DC block capacitor
•
test input with 50 ohm termination
•
1 pF test capacitor
