THS3491 Datasheet, PDF (17/28 Pages) Texas Instruments – 900-MHz, High-Power Output, Current Feedback Amplifier

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THS3491
SBOS875 – AUGUST 2017
Typical Application (continued)
9.2.1 Design Requirements
Use two THS3491 amplifiers in a parallel load-sharing circuit to improve distortion performance.
Table 3. Design Parameters
DESIGN PARAMETER
V_OPP
R_LOAD
VALUE
20 V
100 Ω
9.2.2 Detailed Design Procedure
In addition to providing higher output current drive to the load, the load sharing configuration can also provide
improved distortion performance. In many cases, an operational amplifier shows better distortion performance as
the load current decreases (that is, for higher resistive loads) until the feedback resistor starts to dominate the
current load. In a load sharing configuration of N amplifiers in parallel, the equivalent current load that each
amplifier drives is 1/N times the total load current. For example, in a two-amplifier load sharing configuration with
matching resistance (refer to Figure 14) driving a resistive load (R_LOAD), each series resistance is 2 × R_LOAD and
each amplifier drives 2 × R_LOAD.
Two test circuits are shown in Figure 14, one for a single THS3491 amplifier driving a double-terminated, 50-Ω
cable and one with two THS3491 amplifiers in a load sharing configuration. In the load sharing configuration, the
two 100-Ω series output resistors act in parallel to provide 50-Ω back-matching to the 50-Ω cable.
10 Power Supply Recommendations
The THS3491 can operate off a single supply or with dual supplies as long as the input CM voltage range
(CMIR) has the required headroom (4.3 V) to either supply rail. Supplies should be decoupled with low
inductance, often ceramic, capacitors to ground less than 0.5 inches from the device pins. The use of ground
plane is recommended, and as in most high-speed devices, it is advisable to remove ground plane close to
device sensitive pins such as the inputs. An optional supply decoupling capacitor across the two power supplies
(for split supply operation) improves second harmonic distortion performance.
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