THS3061 Datasheet, PDF (15/25 Pages) Texas Instruments – LOW DISTORTION HIGH SLEW RATE CURRENT FREEBACK AMPLIFIERS

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THS3061
THS3062
SLOS394A – JULY 2002 – OCTOBER 2002
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 49. 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)
RS
eRs
en
Noiseless
eni
+
_
eno
IN+
eRf
Rf
IN–
eRg
Rg
Figure 49. Noise Model
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ ǒ Ǔ ǒ Ǔ 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
+ eni
1
)
Rf
Rg
(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_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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