## 2. Types of Noise sources:

There are several types of noise sources in electrical circuits. However, we discuss only threee important noise sources here.

1. Thermal or Johnson Noise

2. Shot Noise

3. 1/f Noise (Also called Flicker or Pink noise)

1. Thermal Noise: This is the noise generated by thermal agitation of electrons in a conductor.

The noise power, P, in watts, is given by P = kT D f,

Where, k is Boltzmann's constant in joules per Kelvin,

T is the conductor temperature in Kelvin, and

D f is the bandwidth in hertz.

Note that thermal noise power, per hertz, is equal throughout the frequency spectrum, depends only on k and T. The Thermal noise is also called Johnson's noise.

2. Shot Noise: Shot noise normally occurs when there is a potential barrier (voltage differential). PN junction diode is an example that has potential barrier. When the electrons and holes cross the barrier, shot noise is produced. For example, a diode, a transistor, and vacuum tube will all produce Shot noise. On the other hand, a resistor normally does not produce Shot noise since there is no potential barrier built within a resistor. Current flowing through a resistor will not exhibit any fluctuations. However, current flowing through a diode produces small fluctuations. This is due to electrons (in turn, the charge) arriving in quanta, one electron at a time. The current flow is not continuous, but limited by the quantum of the electron charges.

When a mean current I flows, Shot noise leads to current fluctuations given by

I^{2}(f)^{ }= 2eI^{0}A^{2}/Hz

Where, e (1.6 x 10-19 coulombs). is the electronic charge.

Shot noise is proportional to the current passing through the devise.

3. 1/f (one-over-f) Noise: 1/f noise is found in many natural phenomena such as nuclear radiation, electron flow through a conductor, or even in the environment. In electrical engineering, it is called also Flicker noise.

1/f noise is refers to a time series with random fluctuations. The power spectra S(f) as a function of the frequency f is given by

S(f) = 1/f ^{b}

Where, b is very close to 1.

However, note that this formula may not be applicable under all circumstances.

Flicker noise is more prominent in FETs, and bulky resistors.

d. White Noise: White noise is the noise that has constant magnitude of power over frequency. Examples of White noise are Thermal noise, and Shot noise.