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
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
no
+ e
ni
A
V
+ e
ni
ǒ
1 )
R
F
R
G
Ǔ
(Noninverting Case)
THS3001
SLOS217H –JULY 1998–REVISED SEPTEMBER 2009.................................................................................................................................................
www.ti.com
NOISE CALCULATIONS
Noise can cause errors on 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 49. This model includes all of the noise sources as follows:
• e
n
= Amplifier internal voltage noise (nV/√Hz)
• IN+ = Nonverting 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 49. 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, just 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.
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