THS3202 Datasheet, PDF (20/32 Pages) Texas Instruments – 2-GHZ, LOW DISTORTION, CURRENT FEEDBACK AMPLIFIERS

THS3202
SLOS242D − SEPTEMBER 2002 − REVISED JANUARY 2004
RS
eRs
en
Noiseless
eni
+
_
eno
IN+
eRf
Rf
IN−
eRg
Rg
www.ti.com
Figure 91. 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.
This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). Noise figure
is a measure of noise degradation caused by the amplifier. The value of the source resistance must be defined and
is typically 50 Ω in RF applications.
NF
+
ȧȱȲ ^10log
eni ²
eRs2
ȧȳȴ
Because the dominant noise components are generally the source resistance and the internal amplifier noise voltage,
we can approximate noise figure as:
ǒ Ǔ _NF
+
ȱ
10log^{ȧȧȧȧȧ}1
ȧȡȢ
)
en
ǒ 2
) IN )
4 kTRS
Ȳ
RSǓ²_{ȧȣȤȧȧȧȧȧȳ}
È´
20