Circuit compensates optocoupler temperature coefficient

When using an optocoupler in a linear application, you should consider its gain drift with temperature. Traditional single- and dual-transistor-output devices have a notable gain drift with temperature. In recent years, some temperature-compensated optocouplers have appeared. However, another option is to use two optocouplers or a dual optocoupler with appropriate feedback to make the drift of one device cancel the drift of the other. The circuit in Figure 1 accomplishes that task by using a differential amplifier with the drift treated as a common-mode signal. In operation, it is interesting to apply a dc signal to the input and use digital voltmeters to simultaneously monitor the output of each optocoupler and the differential amplifier. Apply a heat gun and observe the individual outputs change rapidly while the amplifier output moves much more slowly. This result occurs even with optocouplers from different manufacturers. With optocouplers of the same type, you can observe good drift cancellation. Parts from the same manufacturer and dual devices give outstanding results. You can use individual optocouplers instead of dual devices to meet safety-agency spacing requirements.

To examine the method in control-system terms, consider Figure 2, which shows one amplifier, a, in the forward path and another amplifier, b, in the feedback path. Also consider the following equation:

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