Abstract: An opto-coupler for isolating electrical signals constituted by an electrically responsive radiation source or emitting device, such as a light emitting diode (LED), optically coupling to a radiation sensitive detector for generating an electrical output in response to an electrical signal applied to the radiation source. The detector includes a PN junction positioned within a semiconductor body for generating maximum output current at high switching speeds with attendant minimization of capacitance. Increased values of detector current generation allow economical, reliable, and uncomplicated integrated circuit amplifying means to be responsive to the generated detector output current for generating increased current and voltage output signals, for example, TTL compatible, without speed degradation and readily implementable on a single small semiconductor integrated circuit chip or substrate.