OPA691_15 Datasheet, PDF (11/31 Pages) Texas Instruments – Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND CURRENT FEEDBACK OPERATION
The OPA691 gives the exceptional AC performance of a
wideband current feedback op amp with a highly linear, high
power output stage. Requiring only 5.1mA quiescent current,
the OPA691 will swing to within 1V of either supply rail and
deliver in excess of 160mA at room temperature. This low
output headroom requirement, along with supply voltage
independent biasing, gives remarkable single (+5V) supply
operation. The OPA691 will deliver greater than 200MHz
bandwidth driving a 2V_PP output into 100Ω on a single +5V
supply. Previous boosted output stage amplifiers have typi-
cally suffered from very poor crossover distortion as the
output current goes through zero. The OPA691 achieves a
comparable power gain with much better linearity. The pri-
mary 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 at low gains, with improved DC accuracy,
consider the high slew rate, unity-gain stable, voltage feed-
back OPA690.
Figure 1 shows the DC-coupled, gain of +2, dual power-
supply circuit configuration used as the basis of the ±5V
Electrical Characteristic tables and Typical Characteristic
curves. 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 specifications 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 to ensure normal amplifier operation. One
optional component is included in Figure 1. In addition to the
usual power-supply de-coupling capacitors to ground, a
0.1µF capacitor is included between the two power-supply
pins. In practical PC board 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
Characteristic tables and Typical Characteristic curves.
Though not a “rail-to-rail” design, the OPA691 requires mini-
mal input and output voltage headroom compared to other
very wideband current feedback op amps. It will deliver a
3V_PP 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 2V_PP 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 combi-
nation 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 ad-
justed from the bipolar supply condition to re-optimize for a
flat frequency response in +5V, gain of +2, operation (see
Setting Resistor Values to Optimize Bandwidth). Again, on a
single +5V supply, the output voltage can swing to within 1V
of either supply pin while delivering more than 120mA output
current. A demanding 100Ω load to a mid-point bias is used
in this characterization circuit. The new output stage used in
the OPA691 can deliver large bipolar output currents into this
mid-point load with minimal crossover distortion, as shown by
the +5V supply, 3rd-harmonic distortion plots.
+5V
+V_S
0.1µF
6.8µF
+
50Ω Source
V_I
50Ω
OPA691
DIS
50Ω Load
V_O 50Ω
0.1µF
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 _OPA691
V_O 100Ω
V_S/2
R_F
453Ω
R_G
453Ω
0.1µF
FIGURE 2. AC-Coupled, G = +2, Single-Supply Specification
and Test Circuit.
OPA691
11
SBOS226D
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