Physics and Engineering of Radiation Detection

516 Chapter 8. Signal Processing

8.9.B NoiseinSpecificComponents..................

B.1 NoiseinAmplifiers.......................

All amplifiers exhibit inherent internal noise, the level of which depends on the way
they have been designed and constructed. However the noise at an amplifier input is
particularly troublesome because it gets amplified by the same amplification factor
as the input signal. Determination of the amplifier noise requires consideration
of both internal and external noise sources. The noise sources at the input of an
amplifier in a typical detection system include

amplifier’s internal noise sources.

shot noise due to detector bias,

Johnson noise due to series resistance, and

Johnson noise due to parallel resistance.

va

Vout

Rp

Rs

ijp

isd

vjs

Amplifier/Shaper

HV

Detector

Figure 8.9.3: Circuit for equivalent noise determination of an amplifier.

The equivalent noise at the input of an amplifier therefore consists of these four main
components as shown in Fig.8.9.3. Here the amplifier’s internal noise source has
been represented as an input voltage noise source having spectral voltage density
v^2 a. The thermal voltage noise power density of the series resistor is represented
by vjs. The noise current power density through the parallel resistor has been
represented byi^2 jp. The bias voltage applied to the detector acts as a source of
shot noise, with an spectral noise current densityi^2 sb. The last noise source is the
shunt or parallel resistor with a spectral noise current density ofisp. The amplifier’s
equivalent internal noise voltage spectral density can be assumed to be composed
to two components: a random or white noisev^2 asand the 1/fnoise characteristic of
electronic systems. Hence we can write

v^2 a=v^2 as+

A

f

, (8.9.18)