THS3001IDR Datasheet, PDF (20/39 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIER

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)
R_S
e_Rs
e_n
Noiseless
e_ni
+
_
e_no
IN+
e_Rf
R_F
IN−
e_Rg
R_G
Figure 49. Noise Model
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.
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