THS4051

THS4052

SLOS238D − MAY 1999 − REVISED AUGUST 2008

www.ti.com

16

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 44 shows the noise figure graph for the THS405x.

0

5

10

15

20

25

30

35

40

NOISE FIGURE

vs

SOURCE RESISTANCE

Source Resistance -

Ω

100

1k

10k

10

100k

f = 10 kHz

TA = 25°C

Figure 44. Noise Figure vs Source Resistance