THS4211 Datasheet, PDF (24/50 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400E – SEPTEMBER 2002 – REVISED SEPTEMBER 2009 ................................................................................................................................... www.ti.com
A modification on this circuit to include a difference
amplifier turns this circuit into a high-speed
instrumentation amplifier, as shown in Figure 84.
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
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 2nd harmonic dominates the total harmonic
distortion with a negligible 3rd harmonic component.
Focusing then on the 2nd 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.
space
space
LINEARITY: DEFINITIONS, TERMINOLOGY,
CIRCUIT TECHNIQUES, AND DESIGN
TRADEOFFS
The THS4211 features execllent 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.
Intercept points are specifications long used as key
design criteria in the RF communications world as a
metric for the intermodulation distortion performance
of a device in the signal chain (e.g., amplifiers,
mixers, etc.). Use of the intercept point, rather than
strictly the intermodulation distortion, allows simpler
system-level calculations. Intercept points, like noise
figures, can be easily cascaded back and forth
through a signal chain to determine the overall
receiver chain's intermodulation distortion
performance. The relationship between
intermodulation distortion and intercept point is
depicted in Figure 85 and Figure 86.
P_O P_O
∆f_c = f_c - f1
∆f_c = f2 - f_c
IMD₃ = P_S - P_O
P_S
P_S
f_c - 3∆f f1 f_c f2 f_c + 3∆f
f - Frequency - MHz
Figure 85.
24
Submit Documentation Feedback
Copyright © 2002–2009, Texas Instruments Incorporated
Product Folder Link(s): THS4211 THS4215