![]() ![]() ![]() ![]() Using feedback from a second photodiode with characteristics closely matched to those of the output photodiode also overcomes a basic problem with optocouplers. Without some sort of feedback, variations in the current transfer ratio will affect the performance of the optocoupler. Variations can occur due to changes in ambient temperature and to ageing of the infrared LED. Optocouplers such as the I元00 from Vishay illustrated in Fig. 5.3.1 can therefore claim both better performance and stability over the lifetime of the circuit. Consequently these specialised devices are considerably more expensive than simple general-purpose optocouplers. When the phototransistor output is taken from the base connection (with the emitter left unconnected) as shown in Fig. 5.3.3(c) the base/collector junction is being used as a photodiode. As the phototransistor now does not function as an amplifier the 'Miller Effect' (where the value of the junction capacitance is multiplied by the current gain of the transistor) does not apply, so the effectively large capacitance of the base/collector junction is greatly reduced, allowing the optocoupler to work at higher speeds, which means in the case of audio applications, a wider bandwidth is available, although this mode of operation greatly reduces the output signal amplitude. In any of the configurations shown in Fig 5.3.3, the choice of load resistor has a significant effect on the output signal the higher the value of R L the greater the amplitude of the output signal but the narrower the bandwidth, so the value of R L chosen is a compromise that depends on the purpose of the circuit.įor the 4N25 to provide isolation for audio signals the input to the infrared LED must be appropriately biased with a DC voltage, so that when a modulating AC (audio) signal is applied, the current through the LED can be varied without the optocoupler output reaching either saturation or cut off. This is really an extension of the linear mode of operation, and can be applied using either one of two basic configurations, phototransistor or photodiode.įig. ![]()
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