THS4051 Datasheet, PDF (16/36 Pages) Texas Instruments – 70-MHz HIGH-SPEED AMPLIFIERS

THS4051
THS4052
SLOS238D − MAY 1999 − REVISED AUGUST 2008
APPLICATION INFORMATION
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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^{ȧȧȱȲǒ}_{eeRnsi Ǔ}²²
ȳ
ȧȧ
È´
Because the dominant noise components are generally the source resistance and the internal amplifier noise voltage, we
can approximate noise figure as:
ǒ Ǔ _NF
+
ȱ
10log^{ȧȧȧȧȧ}1
ȧȡȢ
)
en
ǒ 2
) IN )
4 kTRS
Ȳ
RSǓ²_{ȧȣȤȧȧȧȧȧȳ}
È´
Figure 44 shows the noise figure graph for the THS405x.
NOISE FIGURE
vs
SOURCE RESISTANCE
40
35
f = 10 kHz
TA = 25°C
30
25
20
15
10
5
0
10
100
1k
10k
Source Resistance - Ω
100k
Figure 44. Noise Figure vs Source Resistance
16