THS4081, THS4082

175MHz LOW POWER HIGHSPEED AMPLIFIERS

SLOS274D − DECEMBER 1999 − REVISED JUNE 2001

13

POST OFFICE BOX 655303

APPLICATION INFORMATION

noise calculations and noise figure (continued)

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.

NF

+ 10log

e

2

ni

e

Rs

2

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

voltage, we can approximate noise figure as:

NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4kTR

S

Figure 40 shows the noise figure graph for the THS408x.

0

5

10

15

20

25

30

35

40

Source Resistance − RS (Ω)

f = 10 kHz

TA = 25°C

NOISE FIGURE

vs

SOURCE RESISTANCE

10

100

1k

10k

100k

Figure 40. Noise Figure vs Source Resistance