Datasheet
_
+
49.9 Ω
100 Ω
V
O+
V
I+
_
+
49.9 Ω
100 Ω
V
O−
499 Ω
249 Ω
249 Ω
100 Ω
V
I−
_
+
100 Ω
V
O
V
I+
_
+
R
g1
R
f1
100 Ω
V
I-
_
+
R
f2
49.9 Ω
R
f1
R
g2
R
g2
R
f2
49.9 Ω
THS4271
THS4271
THS4271
V
O
+
1
2
ǒ
1 )
2R
f1
R
g1
Ǔ
ǒ
V
i)
–V
i–
Ǔ
ǒ
R
f2
R
g2
Ǔ
THS4271
THS4275
www.ti.com
SLOS397F –JULY 2002–REVISED OCTOBER 2009
A Low-Noise Receiver With the THS4271
THEORY AND GUIDELINES
A combination of two THS4271 amplifiers can create
Distortion Performance
a high-speed, low-distortion, low-noise differential
receiver circuit as depicted in Figure 83. With both
The THS4271 provides excellent distortion
amplifiers operating in the noninverting mode of
performance into a 150-Ω load. Relative to alternative
operation, the circuit presents a high load impedance
solutions, it provides exceptional performance into
to the source. The designer has the option of
lighter loads, as well as exceptional performance on a
controlling the impedance through termination
single 5-V supply. Generally, until the fundamental
resistors if a matched termination impedance is
signal reaches very high frequency or power levels,
desired.
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 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 THS4271 features excellent distortion
performance for monolithic operational amplifiers.
This section focuses on the fundamentals of
Figure 83. A High Input Impedance, Low-Noise,
distortion, circuit techniques for reducing nonlinearity,
Differential Receiver
and methods for equating distortion of operational
amplifiers to desired linearity specifications in RF
receiver chains.
A modification on this circuit to include a difference
amplifier turns this circuit into a high-speed
Amplifiers are generally thought of as linear devices.
instrumentation amplifier, as shown in Figure 84.
The output of an amplifier is a linearly-scaled version
Equation 1 calculates the output voltage for this
of the input signal applied to it. However, amplifier
circuit.
transfer functions are nonlinear. Minimizing amplifier
nonlinearity is a primary design goal in many
applications.
Figure 84. A High-Speed Instrumentation
Amplifier
(1)
space
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