THS4021, THS4022

350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 2000

10

POST OFFICE BOX 655303

APPLICATION INFORMATION

noise calculations and noise figure (continued)

_

+

RF

RS

RG

eRg

eRf

eRs

en

IN+

Noiseless

IN–

eni

eno

Figure 31. Noise Model

e

ni +

en

2

) IN )

R

S

2

) IN–

R

F ø

R

G

2

) 4kTRs ) 4kT RF ø RG

Where:

k = Boltzmann’s constant = 1.380658

T = Temperature in degrees Kelvin (273 +

°C)

R

F

R

G

(noninverting case)

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, please refer to the

Noise Analysis section in Operational Amplifier

Circuits Applications Report (literature number SLVA043).