Johnson–Nyquist noise

Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the <a href="/facts/Electronic_noise/Izbv38Rr">electronic noise</a> generated by the <a href="/facts/Thermal_energy/eVaKAC7K">thermal agitation</a> of the <a href="/facts/Charge_carrier/wpjjql4x">charge carriers</a> (usually the <a href="/facts/Electron/L0KBo5cW">electrons</a>) inside an <a href="/facts/Electrical_conductor/BSuyupLP">electrical conductor</a> at equilibrium, which happens regardless of any applied <a href="/facts/Voltage/EN2po6N2">voltage</a>. Thermal noise is present in all <a href="/facts/Electrical_circuit/zt94kTW0">electrical circuits</a>, and in sensitive electronic equipment (such as <a href="/facts/Radio_receiver/5olDTxkg">radio receivers</a>) can drown out weak signals, and can be the limiting factor on sensitivity of electrical measuring instruments. Thermal noise is proportional to <a href="/facts/Absolute_temperature/JXeJJPa6">absolute temperature</a>, so some sensitive electronic equipment such as <a href="/facts/Radio_telescope/SxIXUowL">radio telescope</a> receivers are cooled to <a href="/facts/Cryogenic/cw7dHAkp">cryogenic</a> temperatures to improve their <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a>. The generic, statistical physical derivation of this noise is called the <a href="/facts/Fluctuation-dissipation_theorem/1z4noQYj">fluctuation-dissipation theorem</a>, where generalized <a href="/facts/Electrical_impedance/I06NSXtN">impedance</a> or generalized <a href="/facts/Electric_susceptibility/TumGY966">susceptibility</a> is used to characterize the medium.

Thermal noise in an <a href="/facts/Ideal_resistor/JrsGReM6">ideal resistor</a> is approximately <a href="/facts/White_noise/wJ0SR21n">white</a>, meaning that its power <a href="/facts/Spectral_density/q3Rfs7b0">spectral density</a> is nearly constant throughout the <a href="/facts/Frequency_spectrum/q3Rfs7b0">frequency spectrum</a> (Figure 2). When limited to a finite bandwidth and viewed in the <a href="/facts/Time_domain/1w6mP2MU">time domain</a> (as sketched in Figure 1), thermal noise has a nearly <a href="/facts/Normal_distribution/UapjjPyQ">Gaussian amplitude distribution</a>. 
For the general case, this definition applies to <a href="/facts/Charge_carriers/wpjjql4x">charge carriers</a> in any type of conducting <a href="/facts/Transmission_medium/yT1uXLmq">medium</a> (e.g. <a href="/facts/Ion/wrbwhF3z">ions</a> in an <a href="/facts/Electrolyte/4YE8bIVa">electrolyte</a>), not just <a href="/facts/Resistor/JrsGReM6">resistors</a>. Thermal noise is distinct from <a href="/facts/Shot_noise/Vblzt9ux">shot noise</a>, which consists of additional current fluctuations that occur when a voltage is applied and a macroscopic current starts to flow.

Johnson–Nyquist noise open-in-new

Johnson–Nyquist noise