Physics and Engineering of Radiation Detection

468 Chapter 8. Signal Processing

Cd

Vout

Rf

Ra

Vout

is Rs

Rs

Q +

−

i

(a)

(b)

Figure 8.1.2: (a) Working principle of a cur-

rent sensitive preamplifier connected to a

current source, which could be a radiation

detector. A current sensitive amplifier is ac-

tually a current to voltage converter. In

order for it to be able to measure instan-

taneous current, its input impedance must

be very small as compared to the detec-

tor’s output impedance. (b) A simplified

but realistic current sensitive preamplifier

with feedback resistorRf. Cdis the com-

bined detector and stray capacitance andRs

is the combined impedance.

Conversion of instantaneous current into voltage requires that the detector capac-
itanceCddischarges much quicker than the charge collection time of the detector.
This implies that we must have

τdRsCd.

Hereτdis the charge collection time and we have neglected the amplifier resistance
RasinceRaRs.

8.1.C ChargeSensitivePreamplifier..................

The dependence of voltage sensitive preamplifier on the input capacitance is a se-
rious problem for many detection systems. In fact this very problem prompted the
detector designers to develop the so calledcharge sensitivepreamplifiers. In this
device, instead of directly amplifying the voltage or converting the current to volt-
age, the charge accumulated on the detector capacitance isintegratedon another
capacitor. The resulting potential on that capacitor is then directly proportional
to the original charge on the detector. Since it essentially accumulates the charge
on detector capacitance on another capacitor, a charge sensitive preamplifier acts
as an integrator of charge. Fig.8.1.3 shows the principle of working of this device.
The feedback capacitorCfaccumulates the charge on combined detector and stray
capacitanceCd. The output voltage is then simply given by

Vout ∝

Qf

Cf

or Vout ∝

Qd

Cf

, (8.1.10)

whereQdis the charge accumulated onCd, which is proportional to the charge
Qfintegrated on the feedback capacitor. The condition thatQf≈Qdcan only