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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Noise temperature ： ウィキペディア英語版 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: :$\}\; \backslash ,\; =\; \backslash ,\; k\_B\; \backslash ,\; T$ where: *$P$ is the power (in watts) *$B$ is the total bandwidth (Hz) over which that noise power is measured *$k\_B$ is the Boltzmann constant (1.381×10−23 J/K, joules per kelvin) *$T$ is the noise temperature (K) Thus the noise temperature is proportional to the power spectral density of the noise, $P/\; B$. 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. ==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 ''vn'', or as a noiseless component in parallel with a noisy current source producing a current of ''in''. This equivalent voltage or current corresponds to the above power spectral density $\}$, and would have a mean squared amplitude over a bandwidth ''B'' of: :$\backslash over\; \}\; =\; 4\; k\_B\; R\; T$ :$\backslash over\; \}\; =\; 4\; k\_B\; G\; T$ 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). 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 doesn't 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Noise temperature」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.