![]() 5.3.3(c) the base/collector junction is being used as a photodiode. When the phototransistor output is taken from the base connection (with the emitter left unconnected) as shown in Fig. ![]() Consequently these specialised devices are considerably more expensive than simple general-purpose optocouplers. ![]() 5.3.1 can therefore claim both better performance and stability over the lifetime of the circuit. Optocouplers such as the I元00 from Vishay illustrated in Fig. Variations can occur due to changes in ambient temperature and to ageing of the infrared LED. Without some sort of feedback, variations in the current transfer ratio will affect the performance of the optocoupler. Using feedback from a second photodiode with characteristics closely matched to those of the output photodiode also overcomes a basic problem with optocouplers. In audio systems isolation between inputs and higher voltage/current equipment is usually provided by audio transformers, however it is also possible to use specialised audio optocouplers such as the I元00, which uses an infra red LED to illuminate one photodiode as an output device and a second photodiode to provide feedback, ensuring improved linearity and wider frequency range than phototransistor or photoresistor alternatives.
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