OPA4684M Datasheet, PDF (13/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
APPLICATION INFORMATION
low-power current-feedback operation
The quad-channel OPA4684 gives a new level of performance in low-power current-feedback op amps. Using
a new input stage buffer architecture, the OPA4684 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
performance of gain bandwidth independence. This low-power amplifier also delivers exceptional output
power—it’s ±4V swing on ±5V supplies with > 100-mA output drive gives excellent performance into standard
video loads or doubly-terminated 50-Ω 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; while for higher output power, consider the OPA691 family.
Figure 41 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 50 Ω with a
resistor to ground and the output impedance is set to 50 Ω with a series output resistor. Voltage swings reported
in the Electrical Characteristics are taken directly at the input and output pins while load powers (dBm) are
defined at a matched 50ĆΩ load. For the circuit of Figure 41, the total effective load will be 100 Ω || 1600 Ω =
94 Ω. Gain changes are most easily accomplished by simply resetting the R_G value, holding R_F constant at its
recommended value of 800 Ω.
Figure 41. DC-Coupled, G = +2V/V, Bipolar Supply Specifications and Test Circuit
Figure 42 shows the DC-coupled, gain of −1V/V, dual power-suuply 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 42 to set
the input impedance equal to 50 Ω. 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 50ĆΩ input match. A point will be reached where R_G will equal 50 Ω, R_M is removed, and
the input match is set by R_G only. With R_G fixed to achieve an input match to 50 Ω, 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 800 Ω. If the source does not require an input match to 50 Ω, either adjust R_M to get
the desired load, or remove it and let the R_G resistor alone provide the input load.
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