THS4042 Datasheet, PDF (14/34 Pages) National Semiconductor (TI) – 165-MHz C-STABLE HIGH-SPEED AMPLIFIER

THS4041, THS4042
ą
165ĆMHz CĆSTABLE HIGHĆSPEED AMPLIFIERS
SLOS237B− MAY 1999 − REVISED FEBRUARY 2000
APPLICATION INFORMATION
noise calculations and noise figure (continued)
RS
eRs
en
Noiseless
eni
+
_
eno
IN+
eRf
RF
IN−
eRg
RG
Figure 63. Noise Model
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ eni + ǒenǓ}² _{) ǒIN ) RSǓ}² _{) IN– ǒRF ø RGǓ} ² _{) 4 kTRs ) 4 kTǒRF ø RGǓ}
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 to calculate.
For more information on noise analysis, please refer to the Noise Analysis section in Operational Amplifier
Circuits Applications Report (literature number SLVA043).
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