III. Gm-C Filtering - Epublications - Université de Limoges

A.2 The Various Origins of Noise
A.2.a Origins of Noise
There are three main origins of noise in electronic systems [A.3].
Thermal noise comes from the thermal Brownian motion of electrons, or any other
charged particles, in an electrical component like a resistor for instance. This kind of noise is
proportional to the absolute temperature. Indeed, since there would be no motion at the
absolute zero temperature, this would mean the absence of noise.
Thermal noise power are given by the following formulas:
2
vn = 4kTRB
(A.4)
2 1
in = 4kT
B
(A.5)
R
where, k is the Boltzmann constant, T the absolute temperature given in Kelvin, B the
effective noise bandwidth and R the resistance of the material in Ω.
Shot noise originates from the corpuscular nature of the energy transport. The finite
number of charged particles, electrons for example, creates a quantized random current flow
which can be described by a Poisson distribution. It can be demonstrated that it gives the
following shot noise power:
i = 2qI
B
(A.6)
2
n d
Flicker noise is a low-frequency phenomenon, also called “1/f noise”. It is due to
conductivity variations of the material where the current flows, like the presence of
contaminants and of defects in the crystal structure of the semiconductor material. The
equivalent noise power is given by:
α
2 I
in = K B
(A.7)
β
f
K being a constant, α and β being coefficients.
A.2.b Available Input and Output Noise Powers
Input noise NI is defined as the thermal noise generated in the resistance of the signal
source. The available noise power from a resistor is then:
2
v n
N I = = kTB
(A.8)
4R
The available output noise can also be defined and will be referred as NO in the following.
- 182 -