THS4021_16 Datasheet, PDF (10/29 Pages) Texas Instruments – 350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4021
THS4022
SLOS265C – SEPTEMBER 1999 – REVISED JULY 2007
R_S
e_Rs
e_n
Noiseless
e_ni
+
_
e_no
IN+
e_Rf
R_F
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IN–
e_Rg
R_G
Figure 31. Noise Model
S0277-01
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ 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 of the amplifier, just 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, see the Noise Analysis in Operational Amplifier Circuits application
report (SLVA043).
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.
NF
+
10log^{ȧȧȱȲǒ}_{eeRnsi Ǔ}²²
ȳ
ȧȧ
È´
10
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