OPA2681 Datasheet, PDF (11/21 Pages) Burr-Brown (TI) – Dual Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND CURRENT FEEDBACK OPERATION
The OPA2681 gives the exceptional AC performance of a
wideband current feedback op amp with a highly linear, high
power output stage. Requiring only 6mA/ch. quiescent cur-
rent, the OPA2681 will swing to within 1V of either supply
rail and deliver in excess of 135mA guaranteed at room
temperature. This low output headroom requirement, along
with supply voltage independent biasing, gives remarkable
single (+5V) supply operation. The OPA2681 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
OPA2681 achieves a comparable power gain with much
better linearity. The primary advantage of a current feedback
op amp over a voltage feedback op amp is that AC perfor-
mance (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 OPA2680.
Figure 1 shows the DC coupled, gain of +2, dual power
supply circuit configuration used as the basis of the ±5V
Specifications and Typical Performance 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 speci-
fications 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 guarantee 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 Specifica-
tions and Typical Performance Curves. Though not a “rail-
to-rail” design, the OPA2681 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, 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 75mA
output current. A demanding 100Ω load to a midpoint bias
is used in this characterization circuit. The new output stage
used in the OPA2681 can deliver large bipolar output cur-
rents into this midpoint load with minimal crossover distor-
tion, as shown by the +5V supply, 3rd harmonic distortion
plots.
+5V
0.1µF
+V_S
6.8µF
+
50Ω Source
V_I
50Ω
1/2
OPA2681
DIS
50Ω Load
V_O 50Ω
+5V
+V_S
806Ω
+
0.1µF
6.8µF
0.1µF
DIS
V_I
57.6Ω 806Ω
1/2
V_O 100Ω
OPA2681
V_S/2
0.1µF
R_F
402Ω
R_F
499Ω
R_G
402Ω
–V_S
–5V
6.8µF
0.1µF
R_G
499Ω
0.1µF
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi-
cation and Test Circuit.
FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica-
tion and Test Circuit.
®
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OPA2681