_

+

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

Where:

k = Boltzmann’s constant = 1.380658

T = Temperature in degrees Kelvin (273 +

°C)

R

F

R

G

(Noninverting Case)

THS3001

NOISE CALCULATIONS

Noise can cause errors on small signals. This is especially true for amplifying small signals coming over a

transmission line or an antenna. The noise model for current-feedback amplifiers (CFB) is the same as for

voltage feedback amplifiers (VFB). The only difference between the two is that CFB amplifiers generally specify

different current-noise parameters for each input, while VFB amplifiers usually only specify one noise-current

parameter. The noise model is shown in Figure 49. This model includes all of the noise sources as follows:

•

•

IN+ = Nonverting current noise (pA/

√Hz)

•

IN- = Inverting current noise (pA/

√Hz)

•

Figure 49. Noise Model

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.

20

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