THS4041, THS4042

165MHz CSTABLE HIGHSPEED AMPLIFIERS

SLOS237B− MAY 1999 − REVISED FEBRUARY 2000

15

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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 64 shows the noise figure graph for the THS404x.

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 64. Noise Figure vs Source Resistance