_

+

IN+

Noiseless

IN−

e

ni +

en

2

) IN )

R

S

2

) IN–

R

F ø

R

G

2

) 4 kTRs ) 4 kT RF ø RG

R

F

R

G

(noninverting case)

NF

+ 10log

e

2

ni

e

Rs

2

THS4011

THS4012

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SLOS216E – JUNE 1999 – REVISED APRIL 2010

Figure 25. Noise Model

Where:

T = Temperature in degrees Kelvin (273 + °C)

As the previous equations show, to keep noise at a minimum, small-value resistors should be used. As the

resistance term. This leads to the general conclusion that the most dominant noise sources are the source

method, noise sources smaller than 25% of the largest noise source can be effectively ignored. This can greatly

simplify the formula and make noise calculations much easier to calculate.

For more information on noise analysis, refer to the Noise Analysis section in the Operational Amplifier Circuits

Applications Report (SLVA043).

This brings up another noise measurement usually preferred in RF applications — the noise figure (NF). NF is a

measure of noise degradation caused by the amplifier. The value of the source resistance must be defined and is

typically 50

Ω in RF applications.

Because the dominant noise components are generally the source resistance and the internal amplifier noise

voltage, approximate NF as:

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