Datasheet

OPA684
12
SBOS219D
www.ti.com
APPLICATIONS INFORMATION
LOW-POWER, CURRENT-FEEDBACK OPERATION
The OPA684 gives a new level of performance in low-power,
current-feedback op amps. Using a new input stage buffer
architecture, the OPA684 CFB
plus
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 allows both
the bandwidth and distortion to remain nearly constant over
gain, moving closer to the ideal current feedback perfor-
mance of gain bandwidth independence. 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. Single +5V supply operation is also
supported with similar bandwidths but with reduced output
power capability. For lower quiescent power in a CFB
plus
amplifier, consider the OPA683, while for higher output
power, consider the OPA691.
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 test purposes, the input imped-
ance is set to 50Ω with a resistor to ground, and the output
impedance is set to 50Ω with a series output resistor. Voltage
swings reported in the characteristics are taken directly at the
input and output pins while load powers (dBm) are defined at
a matched 50Ω load. For the circuit of Figure 1, the total
effective load will be 100Ω || 2000Ω = 95Ω. Gain changes are
most easily accomplished by simply resetting the R
G
value,
holding R
F
constant at its recommended value of 1kΩ. The
disable control line (
DIS ) is typically left open to give normal
amplifier operation. It may, however, be asserted LOW to
reduce the amplifier supply current to 100µA typically.
Figure 2 shows the DC coupled, gain of –1V/V, dual power-
supply circuit used as the basis of the Inverting Typical
Characteristics. Inverting operation offers several perfor-
mance benefits. Since there is no common-mode signal
across the input stage, the slew rate for inverting operation
is higher and the distortion performance is slightly improved.
An additional input resistor, R
M
, is included in Figure 2 to set
the input impedance equal to 50Ω. The parallel combination
of R
M
and R
G
set the input impedance. As the desired gain
increases for the inverting configuration, R
G
is adjusted to
achieve the desired gain, while R
M
is also adjusted to hold a
50Ω input match. A point will be reached where R
G
will equal
50Ω, R
M
is removed, and the input match is set by R
G
only.
With R
G
fixed to achieve an input match to 50Ω, increasing
R
F
will increase the gain. This will, however, quickly reduce
the achievable bandwidth as the feedback resistor increases
from its recommended value of 1kΩ. If the source does not
require an input match of 50Ω, either adjust R
M
to the get the
desired load, or remove it and let the R
G
resistor alone
provide the input load.
These circuits show ±5V operation. The same circuits can be
applied with bipolar supplies from ±2.5V to ±6V. Internal
supply independent biasing gives nearly the same perfor-
mance for the OPA684 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 OPA684 is reduced.
R
F
1kΩ
OPA684
+5V
–5V
DIS
50Ω
R
M
50Ω
R
G
1kΩ
50Ω Source
50Ω Load
V
I
0.1µF6.8µF
0.1µF6.8µF
+
+
FIGURE 1. DC Coupled, G = +2V/V, Bipolar Supply Speci-
fications and Test Circuit.
FIGURE 2. DC Coupled, G = –1V/V, Bipolar Supply, Speci-
fication and Test Circuit.
R
F
1kΩ
OPA684
+5V
–5V
DIS
50Ω
R
M
52.3Ω
R
G
1kΩ
50Ω Load
50Ω Source
0.1µF 6.8µF
0.1µF 6.8µF
+
+
V
I
Figure 3 shows the AC-coupled, single +5V supply, gain of
+2V/V circuit configuration used as a basis only for the +5V
Electrical and Typical Characteristics. 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 non-inverting
input. The input signal is then AC coupled into this midpoint