Datasheet
APPLICATIONS INFORMATION
WIDEBAND VOLTAGE-FEEDBACK
1/2
OPA2889
+5V
+V
S
-V
S
+
DIS
-5V
50 LoadW
50W
350W
50WV
O
V
D
V
I
50 SourceW
R
G
750W
R
F
750W
+
6.8 Fm
0.1 Fm 6.8 Fm
0.1 Fm
0.1 Fm
OPA2889
SBOS373B – JUNE 2007 – REVISED AUGUST 2008 .......................................................................................................................................................
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OPERATION
The OPA2889 provides an exceptional combination
of high output power capability in a dual, wideband,
unity-gain stable, voltage-feedback op amp using a
new high slew rate input stage. Typical differential
input stages used for voltage-feedback op amps are
designed to steer a fixed-bias current to the
compensation capacitor, setting a limit to the
achievable slew rate. The OPA2889 uses a new input
stage that places the transconductance element
between two input buffers, using the output currents
as the forward signal. As the error voltage increases
across the two inputs, an increasing current is
delivered to the compensation capacitor. This
configuration provides high slew rate (250V/ µ s) while
consuming very low quiescent current (460 µ A/ch).
This exceptional full-power performance comes at the
price of a slightly higher input noise voltage than
alternative architectures. The 8.4nV/ √ Hz input voltage
noise for the OPA2889 is exceptionally low for this
Figure 50. DC-Coupled, G = +2, Bipolar Supply,
type of input stage.
Specification and Test Circuit
Figure 50 shows the dc-coupled, gain of +2V/V, dual
power-supply circuit configuration used as the basis
Figure 51 illustrates the ac-coupled, gain of +2V/V,
of the ± 5V Electrical Characteristics and ± 5V Typical
single-supply circuit configuration used as the basis
Chararacteristics . This illustration is for one channel;
of the +5V Electrical Characteristics and +5V Typical
the other channel is connected similarly. For test
Chararacteristics . Though not a rail-to-rail design, the
purposes, the input impedance is set to 50 Ω with a
OPA2889 requires minimal input and output voltage
resistor to ground and the output impedance is set to
headroom compared to other very wideband
100 Ω . Voltage swings reported in the Electrical
voltage-feedback op amps. It delivers a 2.8V
PP
output
Characteristics are taken directly at the input and
swing on a single +5V supply with > 50MHz
output pins, while output powers (dBm) are at the
bandwidth. The key requirement of broadband
matched 50 Ω load. For the circuit of Figure 50 , the
single-supply operation is to maintain input and
total effective load will be 100 Ω || 1.5k Ω . The disable
output signal swings within the usable voltage ranges
control line (MSOP-10 package only) is typically left
at both the input and the output. The circuit of
open for normal amplifier operation. Two optional
Figure 51 establishes an input midpoint bias using a
components are included in Figure 50 . An additional
simple resistive divider from the +5V supply (two
resistor (350 Ω ) is included in series with the
698 Ω resistors). Separate bias networks would be
noninverting input. Combined with the 25 Ω dc source
required at each input. The input signal is then
resistance looking back towards the signal generator,
ac-coupled into the midpoint voltage bias. The input
this resistor gives an input bias current cancelling
voltage can swing to within 1.1V of either supply pin,
resistance that matches the 375 Ω source resistance
giving a 2V
PP
input signal range centered between
seen at the inverting input (see the DC Accuracy and
the supply pins. The input impedance matching
Offset Control section). In addition to the usual
resistor (59 Ω ) used for testing is adjusted to give a
power-supply decoupling capacitors to ground, a
50 Ω input load when the parallel combination of the
0.1 µ F capacitor is included between the two
biasing divider network is included.
power-supply pins. In practical printed circuit board
(PCB) layouts, this optional-added capacitor typically
improves the 2nd-harmonic distortion performance by
3dB to 6dB.
16 Submit Documentation Feedback Copyright © 2007 – 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2889