Common Collector circuits are emitter followers, based on the photon energy being the same as base bias... If the emitter is at 3 volts then the base internally is at 3.6V... This is the reason why (MY testing notwithstanding) that IMO an Emitter follower is not a good circuit for an Opto-Isolator . Possibly a cheap way to provide positive logic but basically unworkable...
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