THS4021, THS4022

350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 2000

11

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 32 shows the noise figure graph for the THS402x.

Source Resistance –

Ω

NOISE FIGURE

vs

SOURCE RESISTANCE

0

2

4

6

8

10

12

14

16

10

100

1000

10000

10

10

f = 10 kHz

TA = 25°C

100

1 k

10 k

Figure 32. Noise Figure vs Source Resistance