Subject : Principles Of Communication

Topic: Fourier Transform and Noise

Difficulty: Medium

Shot noise:

• The shot noise is produced due to shot effect.

• Due to the shot effect, shot noise is produced in all the amplifying devices rather in all the active devices.

• The shot noise is produced due to the random variations in the arrival of electrons (holes) at the output electrode of an amplifying device.

• Therefore it appears as a randomly varying noise current superimposed on the output.

• 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

  $I_2^n$ = 2(I+2$I_0$) q B ${amperes}^2$

Where I = direct current across the junction

$I_0$= reverse saturation current

q = electron charge

B = effective noise bandwidth in Hz.

Equivalent Noise temperature:

• The equivalent noise temperature of a system is defined as the temperature at which the noisy resistor has to be maintained so that by connecting this resistor to the input of a noiseless version of the system; it will produce the same amount of noise power at the system output as that produced by the actual system.

• The equivalent noise temperature is used in dealing with the UHF and microwave low noise antennas, receivers or devices.

• Equivalent Noise temperature $T_{eq}$ at Amplifier Input:

The noise at the input of the amplifier input is given as follows:

$P_{na}$ = (F-1) k $T_o$ B

This is the noise contributed by the amplifier. This noise power can be alternatively represented by some fictitious temperature $T_{eq}$ such that,

k $T_{eq}$ B= (F-1) k $T_o$ B

Thus the equivalent noise temperature of the amplifier is given by,

$T_{eq}$ = (F-1) $T_o$