Noise Models For Circuit Elements

There are three main fundamental noise mechanism -thermal,shot,and flicker.

Thermal Noise is due to the thermal excitation of charge carriers in conductor.This noise has white spectral density and is proportional to absolute temperature.It is not dependent on a bias dependent on bias condition (dc bias current) and it occurs in all resistors(including conductors) above absolute zero.Thus,thermal noise places fundamental limits on the dynamic range achievable in electronics circuits. It should be mentioned here that thermal noise is also referred to as Johnson or Nyquist noise .

Shot Noise was first studied by W.Schottky using vacuum-tube diode,but shot noise also occur in pn junctions.This noise occurs because the dc bias current is not continuous and smooth but instead is a result of pulses of current is caused by individual flow of current.It can also be modeled as a white noise source.Shot noise is also typically larger than thermal noise and is sometimes used to create white nise generators.