Datasheet

_
+
THS4211
R
g
392 Ω
R
T
200 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
392 Ω
R
M
57.6 Ω
499 Ω
THS4211
THS4215
SLOS400E –SEPTEMBER 2002–REVISED SEPTEMBER 2009 ................................................................................................................................... www.ti.com
WIDEBAND, INVERTING GAIN OPERATION dealing with low inverting gains, as the resultant
feedback resistor value can present a significant load
Since the THS4211 and THS4215 are
to the amplifier output. For an inverting gain of 2,
general-purpose, wideband voltage-feedback
setting R
g
to 49.9 Ω for input matching eliminates the
amplifiers, several familiar operational-amplifier
need for R
M
but requires a 100-Ω feedback resistor.
applications circuits are available to the designer.
This has the advantage that the noise gain becomes
Figure 76 shows a typical inverting configuration
equal to 2 for a 50-Ω source impedance—the same
where the input and output impedances and noise
as the noninverting circuit in Figure 75. However, the
gain from Figure 75 are retained in an inverting circuit
amplifier output now sees the 100-Ω feedback
configuration. Inverting operation is a common
resistor in parallel with the external load. To eliminate
requirement and offers several performance benefits.
this excessive loading, it is preferable to increase
The inverting configuration shows improved slew
both R
g
and R
f
, values, as shown in Figure 76, and
rates and distortion due to the pseudo-static voltage
then achieve the input matching impedance with a
maintained on the inverting input.
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 || 57.6
Ω = 26.8 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 inverting
gain-of-2 circuit in Figure 76 (NG=+1.9), than for the
noninverting 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) × R
f
in Figure 76, the dc source impedance looking out of
In the inverting configuration, some key design
the inverting terminal is 392 Ω || (392 Ω + 26.8 Ω) =
considerations must be noted. One is that the gain
200 Ω. To reduce the additional high-frequency noise
resistor (R
g
) becomes part of the signal-channel input
introduced by the resistor at the noninverting input,
impedance. If input impedance matching is desired
and power-supply feedback, R
T
is bypassed with a
(beneficial when the signal is coupled through a
capacitor to ground.
cable, twisted pair, long PCB trace, or other
transmission line conductor), R
g
may be set equal to
the required termination value and R
f
adjusted to give
the desired gain. However, care must be taken when
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