• The noise factor(F) of an amplifier or any network is defined in terms of the signal

  

• The temperature to calculate the noise power is assumed to be the room temperature.

• The$ \frac{S}{N}$ at the input will always be greater than at the output. This is due to the noise added by the amplifier.

• Therefore the noise factor is the means to measure the amount of noise added and it will always be greater than one.

• The ideal value of noise factor is unity.

• The noise factor F is sometimes frequency dependent. Then its value determined at one frequency is known as the spot noise factor and the frequency must be stated along with the spot noise factor.

Noise Figure:

• Sometimes the noise factor is expressed in decibels. When noise factor is expressed in decibels it is called noise figure.

Noise Figure = $F_{dB}$ =$10log_{10}⁡F$

• Substituting the expression for the noise factor we get

• The ideal value of noise figure is 0 Db.