THS6032 Datasheet, PDF (25/31 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

THS6032
LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER
SLOS233C – APRIL1999 – REVISED MARCH 2000
APPLICATION INFORMATION
noise calculations and noise figure (continued)
As the previous equations show, to keep noise at a minimum, small value resistors should be used. As the
closed-loop gain is increased (by reducing R_G ), the input noise is reduced considerably because of the parallel
resistance term. This leads to the general conclusion that the most dominant noise sources are the source
resistor (R_S ) and the internal amplifier noise voltage (e_n ). Because noise is summed in a root-mean-squares
method, noise sources smaller than 25% of the largest noise source can be effectively ignored. This can greatly
simplify the formula and make noise calculations much easier to calculate.
For more information on noise analysis, please refer to Noise Analysis in Operational Amplifier Circuits,
literature number SLVA043A
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.
ȱ ȳ _e ²
ȧ ȧ ^{NF + 10log ni}
Ȳ^ǒe_Rs^Ǔ2 ȴ
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^{ȧȧȧȧ}1
ȧȡȢ
)
en
2 _{) ǒIN )}
4 kTR_S
Ȳ
RS^Ǔ2_{ȧȣȤȳȧȧȧȧ}
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