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THS6032_14 Datasheet, PDF (22/35 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER
THS6032
SLOS233F – APRIL 1999 – REVISED AUGUST 2009....................................................................................................................................................... www.ti.com
NOISE CALCULATIONS AND NOISE FIGURE
Noise can cause errors on very small signals. This is especially true for the amplifying small signals. The noise
model for current feedback amplifiers (CFB) is the same as voltage feedback amplifiers (VFB). The only
difference between the two is that the CFB amplifiers generally specify different current noise parameters for
each input, while VFB amplifiers usually only specify one noise current parameter. The noise model is shown in
Figure 49. This model includes all of the noise sources as follows:
• en = Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN– = Inverting current noise (pA/√Hz)
• eRx = Thermal voltage noise associated with each resistor (eRx = 4 kTRx)
RS
eRs
en
Noiseless
eni
+
_
eno
IN+
eRf
RF
IN−
eRg
RG
Figure 49. Noise Model
Ǹ ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ The total equivalent input noise density (eni) is calculated by using the following equation:
eni +
ǒenǓ2 ) IN )
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
(2)
Where:
k = Boltzmann's constant = 1.380658 × 10-23
T = Temperature in degrees Kelvin (273 +°C)
RF || RG = Parallel resistance of RF and RG
To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (eni) by the
overall amplifier gain (AV).
ǒ Ǔ eno + eni AV
+
eni
1
)
RF
RG
(Noninverting Case)
(3)
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 RG), 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 (RS) and the internal-amplifier noise voltage (en). 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.
For more information on noise analysis, refer to Noise Analysis in Operational Amplifier Circuits (SLVA043A).
Another noise measurement usually preferred in RF applications is 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.
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