Figure 2 shows the DC-coupled, gain of –1V/V, dual power-
supply circuit used as the basis of the inverting Typical
Characteristics for each channel. Inverting operation offers
several performance benefits. Since there is no common-
mode signal across the input stage, the slew rate for inverting
operation is typically higher and the distortion performance is
slightly improved. An additional input resistor, RM, is included
in Figure 2 to set the input impedance equal to 50Ω. The
parallel combination of RM and RG set the input impedance.
As the desired gain increases for the inverting configuration,
RG is adjusted to achieve the desired gain, while RM is also
adjusted to hold a 50Ω input match. A point will be reached
where RG will equal 50Ω, RM is removed, and the input match
is set by RG only. With RG fixed to achieve an input match to
50Ω, increasing RF will increase the gain. However, this will
reduce the achievable bandwidth as the feedback resistor
increases from its recommended value of 953Ω. If the source
does not require an input match to 50Ω, either adjust RM to
get the desired load, or remove it and let the RG resistor
alone provide the input load.
APPLICATIONS INFORMATION
LOW-POWER, CURRENT-FEEDBACK OPERATION
The dual channel OPA2683 gives a new level of perfor-
mance in low-power, current-feedback op amps. Using a
new input stage buffer architecture, the OPA2683 CFBPLUS
amplifier holds nearly constant AC performance over a wide
gain range. This closed-loop internal buffer gives a very low
and linearized impedance at the inverting node, isolating the
amplifier’s AC performance from gain element variations.
This low impedance allows both the bandwidth and distortion
to remain nearly constant over gain, moving closer to the
ideal current- feedback performance of gain bandwidth inde-
pendence. This low-power amplifier also delivers exceptional
output power—its ±4V swing on ±5V supplies with > 100mA
output drive gives excellent performance into standard video
loads or doubly-terminated 50Ω cables. This dual-channel
device can provide adequate drive for several emerging
differential driver applications with exceptional power effi-
ciency. Single +5V supply operation is also supported with
similar bandwidths but reduced output power capability. For
higher output power in a dual current-feedback op amp,
consider the OPA2684, OPA2691, or OPA2677.
+5V
Figure 1 shows the DC-coupled, gain of +2, dual power-
supply circuit used as the basis of the ±5V Electrical and
Typical Characteristics for each channel. For test purposes,
the input impedance is set to 50Ω with a resistor to ground,
and the output impedance is set to a 1kΩ load. Voltage
swings reported in the characteristics are taken directly at the
input and output pins. For the circuit of Figure 1, the total
effective load will be 1kΩ || 1.9kΩ = 656Ω. Gain changes are
most easily accomplished by simply resetting the RG value,
holding RF constant at its recommended value of 953Ω.
+
0.1µF
6.8µF
1/2
OPA2683
VO
1kΩ
RG
953Ω
RF
953Ω
50Ω Source
VI
RM
52.3Ω
0.1µF
6.8µF
+
–5V
+5V
FIGURE 2. DC-Coupled, G = –1V/V, Bipolar Supply Specifi-
+
cations and Test Circuit.
0.1µF
6.8µF
VI
These circuits show ±5V operation. The same circuit can be
applied with bipolar supplies from ±2.5V to ±6V. Internal
supply independent biasing gives nearly the same perfor-
mance for the OPA2683 over this wide range of supplies.
Generally, the optimum feedback resistor value (for nomi-
nally flat frequency response at G = +2) will increase in value
as the total supply voltage across the OPA2683 is reduced
from ±5V.
50Ω Source
RM
50Ω
1/2
OPA2683
VO
1kΩ
RF
953Ω
RG
953Ω
0.1µF
6.8µF
See Figure 3 for the AC-coupled, single +5V supply, gain of
+2V/V circuit configuration used as a basis only for the +5V
Electrical and Typical Characteristics for each channel. The
key requirement of broadband single-supply operation is to
maintain input and output signal swings within the usable
voltage ranges at both the input and the output. The circuit
of Figure 3 establishes an input midpoint bias using a simple
resistive divider from the +5V supply (two 10kΩ resistors) to
the noninverting input. The input signal is then AC-coupled
+
–5V
FIGURE 1. DC-Coupled, G = +2V/V, Bipolar Supply Speci-
fications and Test Circuit.
OPA2683
SBOS244H
13
www.ti.com
TI [ TEXAS INSTRUMENTS ]
