THS4041-Q1 Datasheet, PDF (15/23 Pages) Texas Instruments – 165-MHz C-STABLE HIGH-SPEED AMPLIFIER

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THS4041ĆQ1
165ĆMHz CĆSTABLE HIGHĆSPEED AMPLIFIER
SGLS229B − FEBRUARY 2004 − REVISED JUNE 2008
APPLICATION INFORMATION
noise calculations and noise figure (continued)
This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). The 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_{ȧȱȲǒeeRnsiǓ}²²
ȳ
ȧ
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
2
) ^ǒIN )
4 kTRS
Ȳ
RS^Ǔ2_{ȧȣȤȳȧȧȧȧȧ}
È´
Figure 64 shows the noise figure graph for the THS404x.
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 64.
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