high-frequency optocoupler

IV It is often necessary to ensure a "safe" transfer of signals from one circuit to another. The desired AC signal must be passed, but even quite high DC voltages must be completely blocked — even AC energy transfer from one circuit to the other is often highly undesirable. Typically, this sort of situation occurs where mains voltages or high DC voltages occur in one circuit, whereas the other must be "safe to touch". A standard solution, nowadays, is to use a so- called optocoupler — the desired signal is passed as light. In the circuit shown, the input signal is applied to T1. This transistor is biased to 20 mA by means of R1 . . . R3. R3 is selected so that IF (the current through the photodiode) varies from 15 mA to 25 mA as the input voltage swings 1 V peak-to- peak. Linearity can be improved at the expense of signal-to-noise ratio by reducing the IF swing. This is accomplished by increasing R3 and adding a resistor from the collector of T1 to ground to obtain the desire...