Noise temperature

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In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce that level of Johnson–Nyquist noise, thus:

${\displaystyle {\frac {P_{\text{N}}}{B}}=k_{\text{B}}T}$

where:

• ${\displaystyle P_{\text{N}}}$ is the noise power (in W, watts)
• ${\displaystyle B}$ is the total bandwidth (Hz, hertz) over which that noise power is measured
• ${\displaystyle k_{\text{B}}}$ is the Boltzmann constant (1.381×10⁻²³ J/K, joules per kelvin)
• ${\displaystyle T}$ is the noise temperature (K, kelvin)

Thus the noise temperature is proportional to the power spectral density of the noise, ${\displaystyle P_{\text{N}}/B}$. That is the power that would be absorbed from the component or source by a matched load. Noise temperature is generally a function of frequency, unlike that of an ideal resistor which is simply equal to the actual temperature of the resistor at all frequencies.