Datasheet

OPA3691
SBOS227E
12
www.ti.com
APPLICATIONS INFORMATION
WIDEBAND CURRENT-FEEDBACK OPERATION
The OPA3691 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
current, the OPA3691 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 OPA3691 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 OPA3691 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 independent of signal gain.
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 || 998Ω. 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 decoupling capacitors to ground, a 0.01µF
capacitor is included between the two power-supply pins. In
practical PC board layouts, this optionally added capacitor
will typically improve the 2nd-harmonic distortion perfor-
mance 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 Characteristics. Though not a
rail-to-rail
design, the OPA3691 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 be-
tween the supply pins. The input impedance matching resis-
tor (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 120mA output current.
A demanding 100Ω load to a midpoint bias is used in this
characterization circuit. The new output stage used in the
OPA3691 can deliver large bipolar output currents into this
midpoint load with minimal crossover distortion, as shown by
the +5V supply, 3rd-harmonic distortion plots.
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifica-
tion and Test Circuit.
FIGURE 2. AC-Coupled, G = +2, Single-Supply, Specifica-
tion and Test Circuit.
1/3
OPA3691
+5V
+
DIS
–5V
50Ω Load
50Ω
50ΩV
O
V
I
50Ω Source
R
G
499Ω
R
F
499Ω
+
6.8µF
0.1µF 6.8µF
0.1µF
0.01µF
+V
S
–V
S
1/3
OPA3691
+5V
+V
S
DIS
V
S
/2
806Ω
100ΩV
O
V
I
57.6Ω
806Ω
R
F
499Ω
R
G
499Ω
0.1µF
0.1µF
6.8µF
+
0.1µF