Datasheet
SLOS238D − MAY 1999 − REVISED AUGUST 2008
www.ti.com
16
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 44 shows the noise figure graph for the THS405x.
0
5
10
15
20
25
30
35
40
NOISE FIGURE
vs
SOURCE RESISTANCE
Source Resistance - Ω
Noise Figure (dB)
100 1k 10k10 100k
f = 10 kHz
T
A
= 25°C
Figure 44. Noise Figure vs Source Resistance