Noise temperature

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:

where:

Thus the noise temperature is proportional to the power spectral density of the noise, ${\displaystyle P/B}$. 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.

A noisy component may be modelled as a noiseless component in series with a noisy voltage source producing a voltage of v_n, or as a noiseless component in parallel with a noisy current source producing a current of i_n. This equivalent voltage or current corresponds to the above power spectral density ${\displaystyle {{P} \over {B}}}$, and would have a mean squared amplitude over a bandwidth B of:

where R is the resistive part of the component's impedance or G is the conductance (real part) of the component's admittance. Speaking of noise temperature therefore offers a fair comparison between components having different impedances rather than specifying the noise voltage and qualifying that number by mentioning the component's resistance. It is also more accessible than speaking of the noise's power spectral density (in watts per hertz) since it is expressed as an ordinary temperature which can be compared to the noise level of an ideal resistor at room temperature (290 K).

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