Thermal noise - definition
Thermal noise - definition
Thermal noise (thermal noise or Johnson-Nyquist noise) is a type of electrical noise generated as a result of the chaotic thermal motion of charge carriers (electrons) in conductors and resistive elements located at temperatures above absolute zero. This phenomenon is fundamental and does not depend on external voltage or current sources - it occurs even in perfectly linear and passive electrical systems.
Thermal noise was theoretically described by John B. Johnson and confirmed mathematically by Harry Nyquist. It is a stochastic process with a flat power spectrum over a wide range of frequencies, meaning that its energy is uniformly distributed over the analysed bandwidth - which is why it is often equated with white noise. Its power spectral density depends linearly on the absolute temperature and the value of the resistance, and proportionally on the bandwidth, in which the noise is measured.
In electronic systems, thermal noise manifests itself as a voltage or current that interferes with useful signals. Its presence limits receiver sensitivity, measurement accuracy and signal-to-noise ratio (SNR) in analogue and digital systems. As it cannot be completely eliminated, it becomes important to design circuits with a minimum level of inherent noise, and to use filtering, shielding and cooling techniques.
Thermal noise is of practical importance in many fields - from telecommunications to measurement electronics to solid-state physics. Its accurate modelling makes it possible to analyse the performance limits of amplifiers, detectors, sensors and transmission paths. In modern electronic systems, it is often included as a key component of background noise, providing a benchmark for evaluating the quality of components and devices.
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