Datasheet

_
+
R
F
R
S
R
G
e
Rg
e
Rf
e
Rs
e
n
IN+
Noiseless
IN−
e
ni
e
no
e
ni
+
ǒ
e
n
Ǔ
2
)
ǒ
IN ) R
S
Ǔ
2
)
ǒ
IN–
ǒ
R
F
ø R
G
ǓǓ
2
) 4 kTR
s
) 4 kT
ǒ
R
F
ø R
G
Ǔ
Ǹ
e
no
+ e
ni
A
V
+ e
ni
ǒ
1 )
R
F
R
G
Ǔ
(noninverting case)
NF + 10log
ȧ
ȧ
ȱ
Ȳ
e
2
ni
ǒ
e
Rs
Ǔ
2
ȧ
ȧ
ȳ
ȴ
THS4011
THS4012
www.ti.com SLOS216E –JUNE 1999–REVISED APRIL 2010
Figure 25. Noise Model
The total equivalent input noise density (e
ni
) is calculated by using the following equation:
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
):
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.
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, approximate NF as:
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