OPA3681 Datasheet, PDF (11/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND CURRENT-FEEDBACK OPERATION
The OPA3681 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
current, the OPA3681 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 re-
markable single (+5V) supply operation. The OPA3681 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 OPA3681 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
performance (bandwidth and distortion) is relatively inde-
pendent of signal gain.
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
|| 998Ω. The disable control line (DIS) is typically left open
to guarantee normal amplifier operation. One optional com-
ponent 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 optionally added capacitor will typi-
cally 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 OPA3681 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. 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 OPA3681 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
1/3
OPA3681
DIS
50Ω Load
V_O 50Ω
0.1µF
R_F
499Ω
R_G
499Ω
–V_S
–5V
6.8µF
0.1µF
+5V
+V_S
806Ω
+
0.1µF
6.8µF
0.1µF
DIS
V_I
57.6Ω 806Ω
1/3
V_O 100Ω
OPA3681
V_S/2
R_F
499Ω
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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OPA3681