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THS4505 Datasheet, PDF (28/38 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION, FULLY DIFFERENTIAL AMPLIFIERS
◁
THS4504
THS4505
SLOS363C – AUGUST 2002 – REVISED MARCH 2004
sumes that the voltage source is properly terminated
by the gain-setting resistors. With these scaling
factors, the amplifier's input noise power (NA) can be
calculated by summing each individual noise source
with its scaling factor. The noise delivered to the
amplifier by the source (NI) and input noise power are
used to calculate the noise factor and noise figure as
shown in equations 23 through 27.
Ni
NA
eg
Rg
Rf
ef
Si
en
Ni
No
Rs
+
Rt
ini
fully-diff
amp
So
No
es
−
et
iii
eg
Rg
Rf
ef
Figure 88. Noise Sources in a Fully
Differential Amplifier Circuit
(eni)2
2
ȡ ȣ Rg
ȧȢ ȧȤ Rf
)
Rg
)
Rg
RsRt
2ǒRs)RtǓ
(12)
(ini)2
(iii)2
4kTRt
Rg2
Rg2
2RsRG
2
ȡ ȣ Rs)2Rg
ȧȢ ȧȤ Rt
)
2RsRg
Rs)2Rg
(13)
(14)
(15)
4kTRf
ǒ Ǔ 2
Rg 2
Rf
(16)
È¡
Rg
È£2
4kTRg
ȧȢ ȧȤ 2
Rg
)
RsRt
2ǒRs)RtǓ
(17)
Figure 89. Scaling Factors for Individual
Noise Sources Assuming a Finite
Value Termination Resistor
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(eni)2
2
Rg
ȧȡȢ ȧȣȤ Rf
)
Rg
Rg
)
Rs
2
(ini)2
(iii)2
4kTRf
Rg2
Rg2
ǒ Ǔ 2
Rg 2
Rf
(18)
(19)
(20)
(21)
4kTRg
2
Rg
ȧȡȢ ȧȣȤ 2
Rg
)
Rs
2
(22)
Figure 90. Scaling Factors for Individual
Noise Sources Assuming No Termination
Resistance is Used (e.g., RT is open).
È¡
È£2
ȧȧȧȢ ȧȧȧȤ Ni + 4kTRs
2RtRg
Rt)2Rg
Rs)R2t)Rt2RRg g
(23)
Figure 91. Input Noise With a Termination Re-
sistor
ǒ Ǔ2
Ni + 4kTRs
2Rg
Rs ) 2Rg
(24)
Figure 92. Input Noise Assuming
No Termination Resistor
Noise Factor and Noise Figure Calculations
NA + SǒNoise Source Scale FactorǓ
(25)
F
+
1
)
NA
NI
(26)
NF + 10 log (F)
(27)
PC BOARD LAYOUT TECHNIQUES FOR
OPTIMAL PERFORMANCE
Achieving optimum performance with a high fre-
quency amplifier-like devices in the THS4500 family
requires careful attention to board layout parasitic and
external component types.
Recommendations that optimize performance include:
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