THS4082 Datasheet, PDF (13/32 Pages) National Semiconductor (TI) – 175 MHz High Speed AMPLIFIERS

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THS4081, THS4082
175ĆMHz LOWĆPOWER HIGHĆSPEED AMPLIFIERS
SLOS274D − DECEMBER 1999 − REVISED JUNE 2001
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_{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 40 shows the noise figure graph for the THS408x.
NOISE FIGURE
vs
SOURCE RESISTANCE
40
f = 10 kHz
35 _{TA = 25°C}
30
25
20
15
10
5
0
10
100
1k
10k
100k
Source Resistance − RS (Ω)
Figure 40. Noise Figure vs Source Resistance
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