fast opto-coupler

The opto-coupler in the normal com- mon emitter circuit at the output of a phototransistor is invariably too slow for use in data communication. Its great advantage remains, of course, the excellent isolation between transmitter and receiver. To retain the advantage, the phototransistor has been integrated into a cascode circuit, as shown in figure 1. The photograph illustrates data transfer in a conventional circuit (top) and in the cascode circuit — the fast opto-coupler — (bottom) at a fre- quency of about 30 kHz. The cascode circuit"s faster operation is due to the transistor"s internal Miller capacitance being of no consequence as the collector voltage remains con- stant. The result is a faster transistor. The base of T2 is biased at about 1.5 V by voltage divider R1/R2. Capacitor C1ensures that, even with rapid fluctuations in current, this voltage remains stable. If you consider T2 as an emitter follower, it is clear that the collector of Ti is always pro- vided with a ...