Editing Noise temperature (section)

==Noise voltage and current==
A noisy component may be modelled as a noiseless component in series with a noisy voltage source producing a voltage of  {{math|''v''<sub>n</sub>}}, or as a noiseless component in parallel with a noisy current source producing a current of  {{math|''i''<sub>n</sub>}}. This equivalent voltage or current corresponds to the above power spectral density <math>\frac{P} {B}</math>, and would have a mean squared amplitude over a bandwidth {{mvar|B}} of:

:<math>\begin{align}
  \frac{\bar{v}_\text{n}^2}{B} &= 4 k_\text{B} R T \\
  \\
  \frac{\bar{i}_\text{n}^2}{B} &= 4 k_\text{B} G T
\end{align}</math>

where {{mvar|R}} is the resistive part of the component's [[electrical impedance|impedance]] or {{mvar|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&nbsp;K).

Note that one can only speak of the noise temperature of a component or source whose impedance has a substantial (and measurable) resistive component. Thus it does not make sense to talk about the noise temperature of a capacitor or of a voltage source. The noise temperature of an [[amplifier]] refers to the noise that would be added at the amplifier's ''input'' (relative to the input impedance of the amplifier) in order to account for the added noise observed following amplification.