Friis Noise Equation

Friis Formula for Noise factor calculates the total noise factor. It was created by Danish-American electrical engineer Harald T. Friis. Friis noise equation is used to calculate the overall noise figure of a receiver, which can be composed of a number of stages. For each stage, it has own noise factor where its impedances are matched. Noise factor measures amplifier ideally and gains the noise produced by the source. The resulting noise factor is expressed as ratio.

Friis Formula for Noise Factor


Ftotal = F1 + (F2-1) / G1 + (F3-1) / G1 * G2 + (F4 - 1) / G1 * G2 * G3 + . . . + (Fn-1) / G1 * G2 . . . Gn-1 Freceiver = FLNA + (Frest - 1) / GLNA

Ftotal = Total noise factor
Frest = Overall noise factor
Freceiver = Noice Factor at receiver End
FLNA = Dominated by the noise factor of the LNA
GLNA = Power Gain of the LNA

Friis Formula for Noise Factor can be expressed as decibels when the noise factor equation is converted from the resulting ratio.

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