eno + eni AV + eni 1 )

R

F

R

G

(Noninverting Case)

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

× 10–23

T = Temperature in degrees Kelvin (273 +

°C)

RF || RG = Parallel resistance of RF and RG

_

+

RF

RS

RG

eRg

eRf

eRs

en

IN+

Noiseless

IN–

eni

eno

THS6012

SLOS226F – SEPTEMBER 1998 – REVISED JUNE 2012

www.ti.com

NOISE CALCULATIONS AND NOISE FIGURE

Noise can cause errors on very small signals. This is especially true for the amplifying small signals. The noise

model for current feedback amplifiers (CFB) is the same as voltage feedback amplifiers (VFB). The only

difference between the two is that the 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 41. This model includes all of the noise sources as follows:

•

en = Amplifier internal voltage noise (nV/√Hz)

•

IN+ = Noninverting current noise (pA/

√Hz)

•

IN- = Inverting current noise (pA/

√Hz)

•

eRX = Thermal voltage noise associated with each resistor (eRX = 4 kTRx)

Figure 41. Noise Model

The total equivalent input noise density (eni) is calculated by using the following equation:

To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (eni) by the

overall amplifier gain (AV).

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

closed-loop gain is increased (by reducing RG), the input noise is reduced considerably because of the parallel

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

resistor (RS) and the internal amplifier noise voltage (en). Because noise is summed in a root-mean-squares

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.

This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). Noise

figure 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.

20

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