THS3202_16 Datasheet, PDF (19/37 Pages) Texas Instruments – 2-GHz, LOW DISTORTION, DUAL CURRENT-FEEDBACK AMPLIFIERS

THS3202
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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 )
R_S
e_Rs
e_n
Noiseless
e_ni
+
_
e_no
IN+
e_Rf
R_f
IN−
e_Rg
R_g
Figure 78. 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, 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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