THS4011 Datasheet, PDF (13/36 Pages) Texas Instruments – 290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS

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R_S
e_Rs
e_n
e_ni
IN+
THS4011
THS4012
SLOS216E – JUNE 1999 – REVISED APRIL 2010
Noiseless
+
_
e_no
e_Rf
R_F
IN−
e_Rg
R_G
Figure 25. 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 +
ǒ_enǓ² _{) IN )}
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
Where:
k = Boltzmann's constant = 1.380658 × 10^-23
T = Temperature in degrees Kelvin (273 + °C)
R_F || R_G = Parallel resistance of R_F and R_G
To get the equivalent output noise density of the amplifier, multiply the equivalent input noise density (e_ni) by the
overall amplifier gain (A_V):
ǒ Ǔ eno + eni AV
+
e_ni
1
)
R_F
R_G
(noninverting case)
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, refer to the Noise Analysis section in the Operational Amplifier Circuits
Applications Report (SLVA043).
This brings up another noise measurement usually preferred in RF applications — the noise figure (NF). NF 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^{ȧȧȱȲǒ}_{eeRnsi Ǔ}²²
ȳ
ȧȧ
È´
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, approximate NF as:
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