THS4081CD datasheet(12/32 Pages) NSC

12 / 32 page

THS4081, THS4082

175MHz LOW POWER HIGHSPEED AMPLIFIERS

SLOS274D − DECEMBER 1999 − REVISED JUNE 2001

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

noise calculations and noise figure (continued)

eRg

eRf

eRs

IN+

Noiseless

IN−

eni

eno

Figure 39. Noise Model

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

ni +

) IN )

) IN–

F ø

) 4kTRs ) 4kT 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

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

overall amplifier gain (AV).

eno + eni AV + eni 1 )

(noninverting case)

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.

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

Circuits Applications Report (literature number SLVA043).

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