# Opamp Noise Figure

These equations calculate the noise figure of an opamp using parameters provided by vendors at the operating frequency and gain configurations. Be sure to note units both for the input parameters and for the equations, or you will end up with really bad results. I originally got these formulas from an engineering desk reference, but a source accessible by everyone is "Noise Analysis for High-Speed Op Amps," by Texas Instruments (see page 9).

Approximation Formula

VNOISE = Noise voltage density, i.e.  nV/sqrt(Hz)
INOISE = Noise current density, i.e.  pA/sqrt(Hz)
RS = Source resistance, i.e. 50 Ω
k = Boltzmann's Constant = 1.380 662 x 10-23 J/K
T = Ambient temperature in Kelvin

Complete Non-Inverting Opamp Noise Figure Formula (for RS = RT)

Complete Inverting Opamp Noise Figure Formula (for RS = RT)

ENI = Input noise voltage

IBN = Non-inverting input noise current

IBI = Inverting noise current

RS = Source resistor

RT = Termination resistor

RF = Feedback resistor

RG = Gain setting resistor

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