Noise is an unwanted electrical disturbance which gives rise to audioble or visual disturbance in communication systems and errors in digital system, the noise gets super impaired on the signal and makes it impossible to separate can be signal from noise.

Noise can be divided into two categories:

1] External Noise.

2] Internal Noise.

Atmospheric Noise : It gets produced within Earth's atmosphere it is n the form of impulses or spikes which covers a wide frequency band upto 30MHz, it becomes insignificant above 30MHz.

Extra transversial noise : It originates from the sources which exist outside the earth's atmosphere it is also called as deep space noise, it can be subdivided into

a] solar noise

b] cosmic noise.

Man made noise : It is generated due to the make + break process in a current carrying circuit, examples are electrical motors, welding machines etc. it is also called as industrial noise.

Shot noise : It is produced due to shot effect, due to shot effect, shot noise is produced in all amplifying devices rather in all achieve devices, the shot noise has a uniform spectral density like thermal noise, the exact formula for the shot noise can be obtained only for diodes, the mean square shot noise current for a diode is given as

$In^2 = 2(I + 2Io)q B$ $amperes^2$

Partition Noise : It is generated when the current gets divided between two or more paths, it is generated due to random fluctuations in the division, devices lite gallium assenide FET draw almost zero gate bias current, hence keeping partition noise to its minimum value.

Flicker noise : It will appear at frequencies below a few KHz, it is sometimes called as i/f noise, it is generated due to the fluctuations in the carrier density, this fluctuating voltage is called as flicker noise voltage.

Thermal noise : The free electrons within a conductor are always in random motion, this random motion is due to the thermal energy received y them, it is possible that an excess number of electrons may appear at one end or the other side of the conductor, as this power results from the thermal energy, it is called as "thermal noise power" the average thermal noise power is given ,

$p_n = KTB $ watts.