Datasheet

_
+
THS4271
R
g
249 Ω
R
T
130 Ω
100 pF
0.1 µF 6.8 µF
−V
S
−5 V
50 Ω Source
+
V
I
100 pF
0.1 µF 6.8 µF
+
+V
S
5 V
V
O
C
T
0.1 µF
R
f
249 Ω
R
M
61.9 Ω
499 Ω
THS4271
THS4275
www.ti.com SLOS397F –JULY 2002–REVISED OCTOBER 2009
WIDEBAND, INVERTING GAIN OPERATION must be taken when dealing with low inverting gains,
as the resulting feedback resistor value can present a
Since the THS4271 and THS4275 are
significant load to the amplifier output. For an
general-purpose, wideband voltage-feedback
inverting gain of 2, setting R
g
to 49.9 Ω for input
amplifiers, several familiar operational amplifier
matching eliminates the need for R
M
but requires a
applications circuits are available to the designer.
100-Ω feedback resistor. This has an advantage of
Figure 76 shows a typical inverting configuration
the noise gain becoming equal to 2 for a 50-Ω source
where the input and output impedances and noise
impedance—the same as the noninverting circuit in
gain from Figure 75 are retained in an inverting circuit
Figure 75. However, the amplifier output now sees
configuration. Inverting operation is one of the more
the 100-Ω feedback resistor in parallel with the
common requirements and offers several
external load. To eliminate this excessive loading, it is
performance benefits. The inverting configuration
preferable to increase both R
g
and R
f
, values, as
shows improved slew rates and distortion due to the
shown in Figure 76, and then achieve the input
pseudo-static voltage maintained on the inverting
matching impedance with a third resistor (R
M
) to
input.
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 76, as opposed to the 1.8 if R
M
is eliminated. The bandwidth is lower for the gain of
–2 circuit, Figure 76 (NG=+1.9), than for the gain of
+2 circuit in Figure 75.
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
Figure 76. Wideband, Inverting Gain
inverting terminal, the output dc error, due to the input
Configuration
bias currents, is reduced to (input offset current)
multiplied by R
f
in Figure 76, the dc source
impedance looking out of the inverting terminal is 249
In the inverting configuration, some key design
Ω || (249Ω + 27.7 Ω) = 130 Ω. To reduce the
considerations must be noted. One is that the gain
additional high-frequency noise introduced by the
resistor (R
g
) becomes part of the signal channel input
resistor at the noninverting input, and power-supply
impedance. If the input impedance matching is
feedback, R
T
is bypassed with a capacitor to ground.
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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