Datasheet
_
+
R
f
R
S
R
g
e
Rg
e
Rf
e
Rs
e
n
IN+
Noiseless
IN−
e
ni
e
no
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
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
ǒ1 )
R
f
R
g
Ǔ(Noninverting Case)
THS3202
www.ti.com
SLOS242F –SEPTEMBER 2002–REVISED JANUARY 2010
NOISE CALCULATIONS
Noise can cause errors on very small signals. This is especially true for amplifying small signals coming over a
transmission line or an antenna. The noise model for current-feedback amplifiers (CFB) is the same as for
voltage feedback amplifiers (VFB). The only difference between the two is that 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 78. This model includes all of the noise sources as follows:
• e
n
= Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN− = Inverting current noise (pA/√Hz)
• e
Rx
= Thermal voltage noise associated with each resistor (e
Rx
= 4 kTR
x
)
Figure 78. Noise Model
The total equivalent input noise density (e
ni
) is calculated by using the following equation:
To get the equivalent output noise 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
F
and 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.
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