THS3091 Datasheet, PDF (34/45 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3091, THS3095
SLOS423H – SEPTEMBER 2003 – REVISED DECEMBER 2015
www.ti.com
matching resistance (refer to Figure 71) driving a resistive load (RL), each series resistance is 2*RL and each
amplifier drives 2*RL . A convenient indicator of whether an op amp will function well in a load sharing
configuration is the characteristic performance graph of harmonic distortion versus load resistance. Such graphs
can be found in most of TI’s high-speed amplifier data sheets. These graphs can be used to obtain a general
sense of whether or not an amplifier will show improved distortion performance in load sharing configurations.
Two test circuits are shown in Figure 71, one for a single THS3091 amplifier driving a double-terminated, 50-Ω
cable and one with two THS3091 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.
Figure 72 and Figure 73 show the 32-MHz, 18-VPP sine wave output amplitudes for the single THS3091
configuration and the load sharing configuration, respectively, measured using an oscilloscope. An ideal sine
wave is also included as a visual reference (the dashed red line). Figure 72 shows visible distortion in the single
THS3091 output. In the load sharing configuration of Figure 73, however, no obvious degradation is visible.
Figure 74 and Figure 75 show the 64-MHz sine wave outputs of the two configurations from Figure 8. While the
single THS3091 output is clearly distorted in Figure 74, the output of the load sharing configuration in Figure 75
shows only minor deviations from the ideal sine wave.
The improved output waveform as a result of load sharing is quantified in the harmonic distortion versus
frequency graphs shown in Figure 76 and Figure 77 for the single amplifier and load sharing configurations,
respectively. While second-harmonic distortion remains largely the same between the single and load sharing
cases, third-harmonic distortion is improved by approximately 8 dB in the frequency range between 20 MHz to 64
MHz.
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