The IL201/IL202/IL203 are optically coupled pairs employing a gallium arsenide infrared LED and a silicon NPN phototransistor. Signal information, including a DC level, can be transmitted by the device while maintaining a high degree of electrical isolation between input and output. The IL201/IL202/IL203 can be used to replace relays and transformers in many digital interface applications, as well as analog applications such as CRT modulation.