Datasheet

_
+
R
g
2.49 kΩ
R
T
1.24 kΩ
0.1 µF 6.8 µF
−V
S
50-Ω Source
+
V
I
0.1 µF 6.8 µF
+
+V
S
V
O
C
T
0.1 µF
R
f
2.49 kΩ
R
M
51.1 Ω
To Load
_
+
49.9 Ω
50-Ω Source
V
I
+V
S
V
O
R
f
2 kΩ
R
g
2 kΩ
+V
S
2
+V
S
2
_
+
2 kΩ
50-Ω Source
V
I
V
S
V
O
R
f
2 kΩ
51.1 Ω
R
g
R
T
R
T
+V
S
2
+V
S
2
To Load
To Load
R
T
C
T
Power Supply Bypassing
Not Shown For Simplicity
THS4281
www.ti.com SLOS432A –APRIL 2004–REVISED NOVEMBER 2009
WIDEBAND, INVERTING OPERATION Another consideration in inverting amplifier design is
setting the bias current cancellation resistor (R
T
) on
Figure 71 shows a typical inverting configuration
the noninverting input. If the resistance is set equal to
where the input and output impedances and noise
the total dc resistance presented to the device at the
gain from Figure 68 are retained with an inverting
inverting terminal, the output dc error (due to the
circuit gain of –1 V/V.
input bias currents) is reduced to the input offset
current multiplied by R
T
. In Figure 71, the dc source
impedance presented at the inverting terminal is 2.49
kΩ || (2.49 kΩ + 25.3 Ω) ≈ 1.24 kΩ. To reduce the
additional high-frequency noise introduced by the
resistor at the noninverting input, R
T
is bypassed with
a 0.1-μF capacitor to ground (C
T
).
SINGLE-SUPPLY OPERATION
The THS4281 is designed to operate from a single
2.7-V to 16.5-V power supply. When operating from a
single power supply, care must be taken to ensure
the input signal and amplifier are biased appropriately
to allow for the maximum output voltage swing and
not violate V
ICR
. The circuits shown in Figure 72
shows inverting and noninverting amplifiers
configured for single-supply operation.
Figure 71. 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
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 example,
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. The 100-Ω feedback
resistor, in parallel with the external load, causes
excessive loading on the amplifier output. To
eliminate this excessive loading, it is preferable to
increase both R
g
and R
f
values, as shown in
Figure 71, and then achieve the input matching
Figure 72. DC-Coupled Single Supply Operation
impedance with a third resistor (R
M
) to ground. The
total input impedance is the parallel combination of
R
g
and R
M
.
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