Physics and Engineering of Radiation Detection

8.1. Preamplification 471

Vout

Rf

Cf

Rb

Cc

+

−

Detector

HV

Vout

Rf

Cf

+

−

HV

Detector

(b)

(a)

Figure 8.1.5: Schematics of (a) an AC cou-

pled and (b) a DC coupled resistive feedback

charge sensitive preamplifiers. The charge ac-

cumulated on the feedback capacitorCfde-

cays through the feedback resistorRfwith a

time constant ofRfCf.

100% charge transfer efficiency would require the detector capacitance to be much
larger than the input capacitance, that is forηin≈1wemusthave

Cd  Cin
or Cd  (A+1)Cf. (8.1.17)
Up until now we have just considered how the feedback capacitor charges. For
any subsequent measurement it must be discharged orresetto allow the next charge
accumulation. Two methods are commonly used for this purpose: resistive feedback
and pulsed reset.

C.1 ResistiveFeedbackMechanism

In a resistive feedback charge sensitive preamplifier, a feedback resistor is provided
so that the charge integrated on the feedback capacitor could decay with a prede-
fined time constant (see Fig.8.1.5). The result is a sharply rising pulse that decays
exponentially with a time constantRfCf.
The time variation of the output pulse from anidealresistive feedback charge
sensitive preamplifier is given by

vout(t)=

Qf

Cf

e−t/RfCf

=

ηinQt

Cf

e−t/RfCf. (8.1.18)