THS4011, THS4012

290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS

SLOS216B – JUNE 1999 – FEBRUARY 2000

16

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 26 shows the noise figure graph for the THS401x.

20

5

0

10

100

25

NOISE FIGURE

vs

SOURCE RESISTANCE

30

1 k

100 k

15

10

Source Resistance –

Ω

f = 10 kHz

TA = 25°C

10 k

Figure 26. Noise Figure vs Source Resistance