Physics and Engineering of Radiation Detection

8.1. Preamplification 465

response time,


power consumption,

dynamic range,


pulse pile-up, and


common mode.

Broadly speaking the preamplifiers used in radiation detection systems can be
divided into the following categories.

Voltage Sensitive Preamplifier


Current Sensitive Preamplifier


Charge Sensitive Preamplifier

8.1.A VoltageSensitivePreamplifier..................

Voltage sensitive preamplifier is the most basic type of preamplifier that can be used
in radiation detection systems. Its function is to simply amplify the potential at its
input stage by some gain factor defined by its components. Fig.8.1.1(a) shows the
principle of working of such an instrument. Here we have represented the detector
as a voltage source. For such a circuit the voltage at the input stage of the amplifier
Vais related to the signal voltageVsthrough the relation

Va=

Ra

Rs+Ra

Vs, (8.1.1)

whereRrepresents resistance with subscriptsafor amplifier input andsfor signal
respectively. For such an amplifier to work properly it is necessary that it does not
draw any current from the source, since any current drawn by it would decrease the
potential drop acrossRs. This would require its input resistance (more accurately,
impedance^1 ) to be infinite, something that is not practical and can only be achieved
up to a good approximation. In fact, looking at the above relation it can be inferred
that forVato approachVsthe input impedance of the preamplifier should be very
largeas compared tothe source impedance, that is

RaRs =⇒ Va≈Vs.

Since the output of a voltage sensitive linear amplifier should be proportional to
the voltage at its input stage, we can write

Vout = AVa

≈ AVs. (8.1.2)

HereAis the gain of the amplifier, which depends on the type of amplifier and the
external components.

(^1) The terms impedance and resistance can be used interchangeably when the current is not changing with
respect to time. If the current changes with time, the effects of circuit and cable capacitance and inductance
also come into play. It is a good idea to always use the term impedance since it characterizes the system
better than the simple resistance.