SBOS875B August 2017 – July 2018 THS3491
In addition to providing higher output current drive to the load, the load-sharing configuration provides improved distortion performance. In many cases, an operational amplifier shows greater distortion performance as the load current decreases (that is, for higher resistive loads) until the feedback resistor dominates 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 (see Figure 69) driving a resistive load (RLOAD), the total series resistance (RTOT_SERIES) at the output of the amplifiers is 2 x RLOAD and each amplifier drives 2 x RLOAD. The total series resistance in the two-amplifier configuration shown in Figure 69 is the parallel combination of RS2 resistors in series with RT resistor (RTOT_SERIES = RS2 || RS2 + RT). Such configuration of resistors at the output allows for fault detection if the load is shorted to GND and can be used for filtering the signal going to the load.
Figure 69 shows two circuits: one of a single THS3491 amplifier driving a double-terminated, 50-Ω cable and one of two THS3491 amplifiers in a load-sharing configuration. In the load-sharing configuration, the two 40.2-Ω series output resistors act in parallel and in conjunction with the 30-Ω terminating resistor provide 50-Ω back-matching to the 50-Ω cable.
Figure 70 shows the normalized frequency response for the two-amplifier load-sharing configuration. The total load, RTOT_LOAD, for the configuration is the sum of RTOT_SERIES and RLOAD which is 100 Ω for the two-amplifier configuration in Figure 69. Figure 71 shows the distortion performance of the two-amplifier configuration.
Benefit of the multiple amplifier's in load-sharing configuration becomes even more evident when the total load increases. Figure 72 and Figure 73 show the HD2 and HD3 performance, respectively, in two, three, and four amplifier configurations when the RTOT_LOAD = 20 Ω. HD2 improves by almost 13 dB and 24 dB, respectively in the three and four amplifier configuration from the two-amplifier configuration, and HD3 shows an improvement of almost 15 and 19 dB in the three and four amplifier configurations, respectively.