THS4081 Datasheet, PDF (13/24 Pages) Texas Instruments – 175-MHz LOW-POWER HIGH-SPEED AMPLIFIERS

THS4081, THS4082
175-MHz LOW-POWER HIGH-SPEED AMPLIFIERS
SLOS274C – DECEMBER 1999 – REVISED MAY 2000
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.
+ _ȧȱ ^ȳȧ NF
10log
eni ²
2
Ȳ^{ǒ Ǔ} ȴ ^eRs
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate noise figure as:
ȧȧȧȧȱȲ ^{ȧȡȢǒ Ǔ ǒ Ǔ ȧȣȤȳȴȧȧȧȧ} _{+ ) NF 10log 1}
) ) ²
en
IN
4 kTRS
2
RS
Figure 40 shows the noise figure graph for the THS408x.
40.00
35.00
30.00
NOISE FIGURE
vs
SOURCE RESISTANCE
f = 10 kHz
TA = 25°C
25.00
20.00
15.00
10.00
5.00
0.00
10
100
1k
10k
Source Resistance – RS (Ω)
100k
Figure 40. Noise Figure vs Source Resistance
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