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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


Opamp Noise Figure equation formula - RF Cafe



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

Opamp Noise Figure equation formula - RF Cafe



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

Opamp Noise Figure equation formula - RF Cafe
Opamp Noise Figure equation formula - RF Cafe


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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RF Cascade Workbook is a very extensive system cascaded component Excel workbook that includes the standard Gain, NF, IP2, IP3, Psat calculations, input & output VSWR, noise BW, min/max tolerance, DC power cauculations, graphing of all RF parameters, and has a graphical block diagram tool. An extensive User's Guide is also included. - Only $35.
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