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.

As shown in Figure 9-1 for example, in a two-amplifier load sharing configuration with matching resistance 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. Figure 7-9 and Figure 7-10 show more information. 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.

Figure 9-1 shows two test circuits: 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 9-2 and Figure 9-3 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 9-2 shows visible distortion in the single THS3091 output. In the load sharing configuration of Figure 9-3, however, no obvious degradation is visible.

Figure 9-4 and Figure 9-5 show the 64-MHz sine wave outputs of the two configurations from Figure 9-1. While the single THS3091 output is clearly distorted in Figure 9-4, the output of the load sharing configuration in Figure 9-5 shows only minor deviations from the ideal sine wave.