Noise figure was introduced by H. T. Friis in 1944 . It is a measure of degradation of signal’s SNR due to noise added by a circuit when the signal is passing through it. Noise figure() is noise factor() expressed in decibel.
Noise factor() of a circuit is defined as the ratio of SNR at input() to SNR at output() of the circuit.
Signal power at the input of the circuit
Signal power at the output of the circuit
Noise power at the input of the circuit
Total noise power at the output of the circuit
Noise power added by the circuit referred to the output
Power gain of the circuit
|Noise Factor (F)||1||1.25||1.414||1.6||2||4||–|
|Noise Figure (N)||0||1||1.5||2||3||6||dB|
NF of a Resistive load
If the load impedance() is power matched for source resistance(), then . Its conversion gain is -6dB.
For example a 6dB RF attenuator or pad has noise figure of 6dB and conversion gain of -6dB. If a signal enters into a attenuator or pad, then the signal is attenuated by 6dB while the noise floor remains constant. Therefore the signal to noise ratio through the pad is degraded by 6dB.
The Noise figure of a passive device is same as the of the conversion gain(in dB sense).
Thermal noise power of a passive device at temperature () is given by
Noise temperature is another way of representing available noise power of a component or amplifier. Noise temperature () is the equivalent temperature at which a resistor connected at the input of the component (noiseless) produce the same noise as the real component or amplifier. Noise temperature is equivalent temperature but not the real temperature of the amplifier
By definition, noise factor of the amplfier is given by
Therefore noise temperature in terms of noise figure and absolute temperature is
Cascaded Noise Figure
For a n-stage cascaded system shown in Figure 5, the total noise factor () is given by
Noise factor of -stage
Gain of -stage;
The cascaded noise figure () is given by
 H. T. Friis, “Noise Figure of Radio Receivers,” Proceeding of the IRE., vol. 32, pp. 419-422, July 1944.