RF Cascade Workbook

Copyright

1996 -
2016

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Kirt
Blattenberger,

BSEE - KB3UON

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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**

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

Complete Non-Inverting Opamp Noise Figure Formula** (for R**_{S} = R_{T})

**Complete Inverting Opamp Noise Figure Formula****
(for R**_{S}** = R**_{T})

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

V

I

R

k = Boltzmann's Constant = 1.380 662 x 10

T = Ambient temperature in Kelvin

Complete Non-Inverting Opamp Noise Figure Formula

E

I

I

R

R

R

R