THS4021 Datasheet, PDF (11/22 Pages) Texas Instruments – 350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4021, THS4022
350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 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 32 shows the noise figure graph for the THS402x.
NOISE FIGURE
vs
SOURCE RESISTANCE
16
f = 10 kHz
14 _{TA = 25°C}
12
10
8
6
4
2
0
1100
100
1010k0
101000k0
Source Resistance – Ω
Figure 32. Noise Figure vs Source Resistance
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