Linear signal transmission with optocouplers

Until now, optocouplers have not been considered very suitable for linear, isolating circuits, owing to their nonlinearity in the current transfer ratio. In this paper, a circuit for linear signal transmission consisting of a pair of optocouplers in a feedback arrangement is analyzed. The nonlinearity of the optocouplers is practically eliminated by using coupler pairs with good tracking in their transfer function. This paper analyzes the distortion and gain of such a circuit as a function of linearity tracking, modulation index, and LED-DC-bias level. Measurements reported in this paper show that with a linearity tracking of only 10 percent, stringent distortion requirements such as for analog signal transmission in telephony can be fulfilled. The circuit achieves a distortion of 0.5 percent based on a 10 percent linearity tracking at a modulation index of 0.2 and the temperature stability of the gain is -0.09 percent/°C.