OPA4684 Datasheet, PDF (12/33 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

APPLICATIONS INFORMATION
LOW-POWER CURRENT-FEEDBACK OPERATION
The quad-channel OPA4684 gives a new level of perfor-
mance in low-power current-feedback op amps. Using a new
input stage buffer architecture, the OPA4684 CFB_PLUS ampli-
fier 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. 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 family; for
higher output power, consider the OPA691 family.
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 50Ω with a series output
resistor. Voltage swings reported in the Electrical Character-
istics 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 || 1600Ω = 94Ω. Gain changes are most easily accom-
plished by simply resetting the R_G value, holding R_F constant
at its recommended value of 800Ω.
+5V
0.1µF
+
6.8µF
V_I
50Ω Source
R_M
1/4
50Ω ^OPA4684
50Ω
50Ω Load
R_F
800Ω
R_G
800Ω
–5V
0.1µF
+ ^6.8µF
FIGURE 1. DC-Coupled, G = +2V/V, Bipolar Supply Speci-
fications and Test Circuit.
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, 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 achieved 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 800Ω. If
the source does not require an input match to 50Ω, either
adjust R_M to get the desired load, or remove R_M and allow the
R_G resistor alone provide the input load.
+5V
0.1µF
+
6.8µF
1/4
OPA4684
50Ω
50Ω Load
50Ω Source
V_I
R_G
800Ω
R_M
53.6Ω
R_F
800Ω
0.1µF
+ ^6.8µF
–5V
FIGURE 2. DC-Coupled, G = –1V/V, Bipolar Supply Specifi-
cations and Test Circuit.
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 OPA4684 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 OPA4684 is reduced.
See Figure 3 for the AC-coupled, single +5V supply, gain of
+2V/V circuit configuration used as a basis for the +5V only
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
into this midpoint voltage bias. The input voltage can swing
to within 1.25V of either supply pin, giving a 2.5V_PP input
signal range centered between the supply pins. The input
impedance of Figure 3 is set to give a 50Ω input match. If the
source does not require a 50Ω match, remove this and drive
12
OPA4684
www.ti.com
SBOS240G