A new ultrahigh-speed optically-coupled isolator compatible with TTL interfaces

OPTICALLY-COUPLED isolators based on 900-940 nm GaAs infrared emitters and phototransistors have a serious gain bandwidth limitation which is the result of the long penetration depth of infrared radiation in silicon; 45 p and 70 p for 9oy0 absorption of 900 and 940 nm, respectively. The requirement of such a large photon collection depth prevents an effective separation of the large detector capacitance from the critical feedback capacitance Cf(col1ector-basecapacitance) of the gain transistor. Thus, the large photodetector capacitance dominates the feedback capacitance of a phototransistor leading to typical rise and fall times in the range of 10 ps.