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Noise in Electrical Circuits Noise in Electronic Components

Noise Figure Measurement - A Practical Approach

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 Johnsonís 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

I2(f) = 2eI0A2/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.

Noise in Electrical Circuits Noise in Electronic Components
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