THS4211 Datasheet, PDF (23/41 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

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3.9 pF
50 Ω Source
V_I
392 Ω
57.6 Ω
392 Ω
5V
_
THS4211
+
49.9 Ω
V_O
33 pF
-5 V
Figure 82. A Two-Pole Active Filter With
Two Poles Between 90 MHz and 100 MHz
A Low-Noise Receiver With the THS4211
A combination of two THS4211 amplifiers can create
a high-speed, low-distortion, low-noise differential re-
ceiver circuit as depicted in Figure 83. With both
amplifiers operating in the noninverting mode of
operation, the circuit presents a high load impedance
to the source. The designer has the option of
controlling the impedance through termination re-
sistors if a matched termination impedance is desired.
100 Ω
V_I+
+
_
49.9 Ω
V_O+
392 Ω
787 Ω
392 Ω
100 Ω
_
100 Ω
V_I-
+
49.9 Ω
V_O-
Figure 83. A High Input Impedance, Low Noise,
Differential Receiver
A modification on this circuit to include a difference
amplifier turns this circuit into a high-speed instru-
mentation amplifier, as shown in Figure 84.
THS4211
THS4215
SLOS400D – SEPTEMBER 2002 – REVISED NOVEMBER 2004
100 Ω
V_I-
+
R_g2
R_f2
THS4211
_
R_f1
R_g1
100 Ω
V_I+
R_f1
_
THS4211
+
_
R_g2
THS4211
+
R_f2
49.9 Ω
V_O
49.9 Ω
Figure 84. A High-Speed Instrumentation
Amplifier
ǒ Ǔ ǒ Ǔ V_O +
1
2
1
)
2R_f1
R_g1
ǒ_{Vi)–Vi–}Ǔ
R_f2
R_g2
(1)
THEORY AND GUIDELINES
Distortion Performance
The THS4211 provides excellent distortion perform-
ance into a 150-Ω load. Relative to alternative sol-
utions, it provides exceptional performance into
lighter loads, as well as exceptional performance on a
single 5-V supply. Generally, until the fundamental
signal reaches very high frequency or power levels,
the 2^nd harmonic dominates the total harmonic distor-
tion with a negligible 3^rd harmonic component. Focus-
ing then on the 2^nd harmonic, increasing the load
impedance directly improves distortion. The total load
includes the feedback network; in the noninverting
configuration (Figure 75) this is the sum of R_f and R_g,
while in the inverting configuration (Figure 76), only R_f
needs to be included in parallel with the actual load.
LINEARITY: DEFINITIONS, TERMINOLOGY,
CIRCUIT TECHNIQUES, AND DESIGN
TRADEOFFS
The THS4211 features execllent distortion perform-
ance for monolithic operational amplifiers. This sec-
tion focuses on the fundamentals of distortion, circuit
techniques for reducing nonlinearity, and methods for
equating distortion of operational amplifiers to desired
linearity specifications in RF receiver chains.
Amplifiers are generally thought of as linear devices.
The output of an amplifier is a linearly-scaled version
of the input signal applied to it. However, amplifier
transfer functions are nonlinear. Minimizing amplifier
nonlinearity is a primary design goal in many appli-
cations.
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