Transistor or photovoltaic output optoisolators use light to transmit information across an electrical insulation barrier, usually for safety or functional reasons. They are distinguished from other optoisolator types by their use of a simple phototransistor or photovoltaic cell (solar cell) as an output device. The outputs of these devices do not require an external power source for operation and are analog in character, allowing their use for transmitting analog information between circuits which cannot be electrically connected.