In a two terminal network, noise in a frequency interval 
may be
characterized
using either an equivalent e.m.f. generator
in series with the device or a current generator
in parallel. An equivalent noise resistance,
Rn, and an equivalent noise current, Ieq, may be defined
where T is the room temperature and k is Boltzmann's constant.
Figure 1: Equivalent circuit of the detector and amplifier for noise
considerations
The equivalent circuit for a detector and amplifier system is shown in figure 1 and the noise components for an FET amplifier input stage are given below.
where gm is the transistor transconductance.
The equivalent noise charge of a circuit with shaping time 
, is then
where S1 and S2 depend upon the type of shaping used. Thus an an optimum shaping time exists. If S1 and S2 are of similar magnitude and the leakage current term dominates, a short shaping time is required. The minimum obtainable equivalent noise charge may be limited by the 1/f noise if this term is large.
