OPA2691ID Datasheet, PDF (11/30 Pages) Texas Instruments – Dual Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND CURRENT-FEEDBACK OPERATION
The OPA2691 gives the exceptional AC performance of a
wideband current-feedback op amp with a highly linear, high-
power output stage. Requiring only 5.1mA/ch quiescent cur-
rent, the OPA2691 will swing to within 1V of either supply rail
and deliver in excess of 160mA tested at room temperature.
This low output headroom requirement, along with supply
voltage independent biasing, gives remarkable single (+5V)
supply operation. The OPA2691 will deliver greater than 200MHz
bandwidth driving a 2Vp-p output into 100Ω on a single +5V
supply. Previous boosted output stage amplifiers have typically
suffered from very poor crossover distortion as the output
current goes through zero. The OPA2691 achieves a compa-
rable power gain with much better linearity. The primary
advantage of a current-feedback op amp over a voltage-
feedback op amp is that AC performance (bandwidth and
distortion) is relatively independent of signal gain. For similar
AC performance with improved DC accuracy, consider the high
slew rate, unity-gain stable, voltage-feedback OPA2690.
Figure 1 shows the DC-coupled, gain of +2, dual power-
supply circuit configuration used as the basis of the ±5V
Electrical Characteristics and Typical Characteristics. 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 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 || 804Ω = 89Ω. The disable control line (DIS) is
typically left open (SO-14 only) to ensure normal amplifier
operation. One optional component is included in Figure 1. In
addition to the usual power-supply decoupling capacitors to
ground, a 0.01µF capacitor is included between the two
power-supply pins. In practical printed circuit board (PCB)
layouts, this optional added capacitor will typically improve
the 2nd-harmonic distortion performance by 3dB to 6dB.
Figure 2 shows the AC-coupled, gain of +2, single-supply
circuit configuration used as the basis of the +5V Electrical
Characteristics and Typical Characteristics. Though not a
rail-to-rail design, the OPA2691 requires minimal input and
output voltage headroom compared to other very wideband
current-feedback op amps. It will deliver a 3Vp-p output
swing on a single +5V supply with greater than 150MHz
bandwidth. 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 2 establishes an input midpoint bias
using a simple resistive divider from the +5V supply (two
806Ω resistors). The input signal is then AC-coupled into this
midpoint voltage bias. The input voltage can swing to within
1.5V of either supply pin, giving a 2Vp-p input signal range
centered between the supply pins. The input impedance
matching resistor (57.6Ω) used for testing is adjusted to give
a 50Ω input match when the parallel combination of the
biasing divider network is included. The gain resistor (R_G) is
AC-coupled, giving the circuit a DC gain of +1—which puts
the input DC bias voltage (2.5V) on the output as well. The
feedback resistor value has been adjusted from the bipolar
supply condition to re-optimize for a flat frequency response
in +5V operation, gain of +2, operation (see the Setting
Resistor Values to Optimize Bandwidth section). Again, on a
single +5V supply, the output voltage can swing to within 1V
of either supply pin while delivering more than 75mA output
current. A demanding 100Ω load to a midpoint bias is used
in this characterization circuit. The new output stage used in
the OPA2691 can deliver large bipolar output currents into
this midpoint load with minimal crossover distortion, as shown
by the +5V supply, 3rd-harmonic distortion plots.
+5V
0.1µF
+V_S
6.8µF
+
50Ω Source
V_I
50Ω
1/2
OPA2691
DIS
50Ω Load
V_O 50Ω
0.01µF
R_F
402Ω
R_G
402Ω
–V_S
–5V
6.8µF
0.1µF
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifica-
tion and Test Circuit.
+5V
+V_S
806Ω
+
0.1µF
6.8µF
0.1µF
DIS
V_I
57.6Ω 806Ω
1/2
V_O 100Ω
OPA2691
V_S/2
R_F
453Ω
R_G
453Ω
0.1µF
FIGURE 2. AC-Coupled, G = +2, Single-Supply Specification
and Test Circuit.
OPA2691
11
SBOS224D
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