THS4271-EP Datasheet, PDF (25/44 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FEEDBACK AMPLIFIER

THS4271-EP
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A Low-Noise Receiver With the THS4271
A combination of two THS4271 amplifiers can create
a high-speed, low-distortion, low-noise differential
receiver circuit as depicted in Figure 84. 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
resistors if a matched termination impedance is
desired.
100 Ω
V_I+
+
_
49.9 Ω
V_O+
249 Ω
499 Ω
249 Ω
100 Ω
_
100 Ω
V_I−
+
49.9 Ω
V_O−
Figure 84. 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
instrumentation amplifier, as shown in Figure 85.
Equation 1 calculates the output voltage for this
circuit.
100 Ω
V_I-
+
R_g2
R_f2
THS4271
_
R_f1
R_g1
100 Ω
V_I+
R_f1
_
THS4271
+
_
R_g2
THS4271
+
R_f2
49.9 Ω
V_O
49.9 Ω
SGLS270C – DECEMBER 2004 – REVISED APRIL 2010
space
THEORY AND GUIDELINES
Distortion Performance
The THS4271 provides excellent distortion
performance into a 150-Ω load. Relative to alternative
solutions, 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 second harmonic dominates the total harmonic
distortion with a negligible third harmonic component.
Focusing then on the second harmonic, increasing
the load impedance improves distortion directly. The
total load includes the feedback network; in the
noninverting configuration (Figure 76) this is the sum
of R_f and R_g, while in the inverting configuration
(Figure 77), only R_f needs to be included in parallel
with the actual load.
LINEARITY: DEFINITIONS, TERMINOLOGY,
CIRCUIT TECHNIQUES, AND DESIGN
TRADEOFFS
The THS4271 features excellent distortion
performance for monolithic operational amplifiers.
This section 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
applications.
Figure 85. 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)
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