THS4031, THS4032

100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS224C – JULY 1999 – REVISED APRIL 2000

22

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 45 shows the noise figure graph for the THS403x.

10

6

2

0

10

100

12

14

NOISE FIGURE

vs

SOURCE RESISTANCE

16

1 k

10 k

8

4

Source Resistance –

Ω

f = 10 kHz

TA = 25°C

Figure 45. Noise Figure vs Source Resistance