Physics and Engineering of Radiation Detection

466 Chapter 8. Signal Processing

Cd

Vout

Rs

Rf

Vs

Rs

Ra

Vout

Q +

−

i

(b)

(a)

Figure 8.1.1: (a) Principle design of a volt-

age sensitive preamplifier connected to a

source, such as a radiation detector output.

The input impedanceRaof the preamplifier

is kept very large so that it draws minimal

current from the source. (b) A simplified

but realistic voltage sensitive preamplifier

with feedback resistorRf. Cdis the com-

bined detector and stray capacitance andRs

is the combined impedance.

This simplistic picture does not include a feature found in all real detectors, that
they possess some capacitance as well. In fact the potential differenceVsactually
appears across the combined capacitance of the detector and other components such
as cable. This potential is approximately given by

Vs≈

Q

Cd

, (8.1.3)

whereQis the charge collected by the readout electrode andCdis thecombined
detector and stray capacitance. Hence we can conclude that the voltage at the
output of a realistic amplifier, to a good approximation, is given by

Vout≈A

Q

Cd

. (8.1.4)

The reader should readily realize the problem with this configuration, i.e., the
output voltage depends on the detector capacitance. For a perfectly linear operation,
therefore, any change in detector capacitance must be accounted for in the conversion
constants through recalibration. This, of course, is not practical for systems, such
as semiconductor detectors, whose capacitance may change during operation due to
small fluctuations in temperature. For other systems, such as ionization chambers,
this type of amplifier can be effectively used.
An advantage of voltage sensitive amplifiers is that they provide low impedance
output signal, which is ideal for signal transportation at large distances. But alone
this advantage can not justify its preference over other types of preamplifiers. A
realistic but simplified voltage sensitive preamplifier is shown in Fig.8.1.1(b).
Up until now we have not included the detector’s instantaneous current in the
discussion. We know that the charges produced in the active volume of the detector
by incident radiation actually constitute a current, which can be measured. In