THS4271-EP Datasheet, PDF (22/44 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FEEDBACK AMPLIFIER

THS4271-EP
SGLS270C – DECEMBER 2004 – REVISED APRIL 2010
WIDEBAND, INVERTING GAIN OPERATION
Since the THS4271 is a general-purpose, wideband
voltage-feedback amplifiers, several familiar
operational amplifier applications circuits are
available to the designer. Figure 77 shows a typical
inverting configuration where the input and output
impedances and noise gain from Figure 76 are
retained in an inverting circuit configuration. Inverting
operation is one of the more common requirements
and offers several performance benefits. The
inverting configuration shows improved slew rates
and distortion due to the pseudo-static voltage
maintained on the inverting input.
5 V _+VS
100 pF
0.1 µF
+
6.8 µF
C_T
0.1 µF
R_T
130 Ω
50 Ω Source
R_g
V_I
249 Ω
R_M
61.9 Ω
+
THS4271
_
R_f
249 Ω
100 pF
V_O
499 Ω
0.1 µF 6.8 µF
+
−5 V ^−VS
Figure 77. Wideband, Inverting Gain
Configuration
In the inverting configuration, some key design
considerations must be noted. One is that the gain
resistor (R_g) becomes part of the signal channel input
impedance. If the input impedance matching is
desired (which is beneficial whenever the signal is
coupled through a cable, twisted pair, long PCB
trace, or other transmission line conductors), R_g may
be set equal to the required termination value and R_f
adjusted to give the desired gain. However, care
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must be taken when dealing with low inverting gains,
as the resulting feedback resistor value can present a
significant load to the amplifier output. For an
inverting gain of 2, setting R_g to 49.9 Ω for input
matching eliminates the need for R_M but requires a
100-Ω feedback resistor. This has an advantage of
the noise gain becoming equal to 2 for a 50-Ω source
impedance—the same as the noninverting circuit in
Figure 76. However, the amplifier output now sees
the 100-Ω feedback resistor in parallel with the
external load. To eliminate this excessive loading, it is
preferable to increase both R_g and R_f, values, as
shown in Figure 77, and then achieve the input
matching impedance with a third resistor (R_M) to
ground. The total input impedance becomes the
parallel combination of R_g and R_M.
The next major consideration is that the signal source
impedance becomes part of the noise gain equation
and hence influences the bandwidth. For example,
the R_M value combines in parallel with the external
50-Ω source impedance (at high frequencies),
yielding an effective source impedance of 50 Ω ||
61.9Ω = 27.7 Ω. This impedance is then added in
series with R_g for calculating the noise gain. The
result is 1.9 for Figure 77, as opposed to the 1.8 if R_M
is eliminated. The bandwidth is lower for the gain of
–2 circuit, Figure 77 (NG = +1.9), than for the gain of
+2 circuit in Figure 76.
The last major consideration in inverting amplifier
design is setting the bias current cancellation resistor
on the noninverting input. If the resistance is set
equal to the total dc resistance looking out of the
inverting terminal, the output dc error, due to the input
bias currents, is reduced to (input offset current)
multiplied by R_f in Figure 77, the dc source
impedance looking out of the inverting terminal is
249 Ω || (249Ω + 27.7 Ω) = 130 Ω. To reduce the
additional high-frequency noise introduced by the
resistor at the noninverting input, and power-supply
feedback, R_T is bypassed with a capacitor to ground.
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