Datasheet
SLOS274D − DECEMBER 1999 − REVISED JUNE 2001
13
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
ȧ
ȧ
ȱ
Ȳ
e
2
ni
ǒ
e
Rs
Ǔ
2
ȧ
ȧ
ȳ
ȴ
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate noise figure as:
NF + 10log
ȧ
ȧ
ȧ
ȧ
ȧ
ȱ
Ȳ
1 )
ȧ
ȡ
Ȣ
ǒ
e
n
Ǔ
2
)
ǒ
IN ) R
S
Ǔ
2
ȧ
ȣ
Ȥ
4kTR
S
ȧ
ȧ
ȧ
ȧ
ȧ
ȳ
ȴ
Figure 40 shows the noise figure graph for the THS408x.
0
5
10
15
20
25
30
35
40
Source Resistance − R
S
(Ω)
f = 10 kHz
T
A
= 25°C
NOISE FIGURE
vs
SOURCE RESISTANCE
10 100 1k 10k
Noise Figure (dB)
100k
Figure 40. Noise Figure vs Source Resistance