OPA2673_1 Datasheet, PDF (22/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
WIDEBAND CURRENT-FEEDBACK
OPERATION
The OPA2673 gives the exceptional ac performance
of a wideband current-feedback op amp with a highly
linear, high-power output stage. Requiring 16mA/ch
quiescent current, the OPA2673 swings to within 1.1V
of either supply rail and delivers in excess of 460mA
at room temperature. This low output headroom
requirement, along with supply voltage independent
biasing, gives remarkable dual (±6V) supply
operation. The OPA2673 delivers greater than
450MHz bandwidth driving a 2V_PP output into 100Ω
on a single +12V supply. Previous boosted output
stage amplifiers typically suffer from very poor
crossover distortion as the output current goes
through zero. The OPA2673 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 76 shows the dc-coupled, gain of
+4V/V, dual power-supply circuit configuration used
as the basis of the ±6V 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 76, the total effective load is 100Ω || 402Ω =
80Ω.
50W Source
+6V
0.1mF
+V_S
6.8mF
+
V_I
50W
1/2
OPA2673
50W Load
V_O 50W
R_F
402W
R_G
133W
-V_S
-6V
6.8mF
0.1mF
Figure 76. DC-Coupled, G = +4V/V, Bipolar
Supply, Specification and Test Circuit
WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers
that include a disable pin is to wire multiple amplifier
outputs together, then select one of several possible
video inputs to source onto a single line. This simple
wired-OR video multiplexer can be easily
implemented using the OPA2673, as Figure 77
illustrates.
+5V
V_DIS
2kW
Video 1
75W
+5V
Power-supply
decoupling not shown.
1/2 ^DIS
OPA2673
402W
402W
-5V
475W
475W
+5V
82.5W
75W Cable
RG-59
82.5W
Video 2
75W
2kW
1/2
OPA2673
DIS
-5V
Figure 77. Two-Channel Video Multiplexer
Typically, channel switching is performed either on
sync or retrace time in the video signal. The two
inputs are approximately equal at this time. The
make-before-break disable characteristic of the
OPA2673 ensures that there is always one amplifier
controlling the line when using a wired-OR circuit
similar to that shown in Figure 77. Because both
inputs may be on for a short period during the
transition between channels, the outputs are
combined through the output impedance matching
resistors (82.5Ω in this case). When one channel is
disabled, its feedback network forms part of the
output impedance and slightly attenuates the signal in
getting out onto the cable. The gain and output
matching resistors have been slightly increased to get
a signal gain of +1V/V at the matched load and
provide a 75Ω output impedance to the cable. The
video multiplexer connection (as shown in Figure 77)
also ensures that the maximum differential voltage
across the inputs of the unselected channel do not
exceed the rated ±1.2V maximum for standard video
signal levels. The active-off line circuitry integrated
within the OPA2673 ensures that the off-channel will
stay off independently of the signal amplitude present
at the output.
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