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.


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



E_NI = Input noise voltage

I_BN = Non-inverting input noise current

I_BI = Inverting noise current

R_S = Source resistor

R_T = Termination resistor

R_F = Feedback resistor

R_G = Gain setting resistor