The 6N138 is an optically coupled isolator. It consists of an infrared emitting diode and an integrated high gain photon detector to provide 3000Vdc electrical insulation, 500V/µS common mode transient immunity and extremely high current transfer ratio between input and output. Separate pins for the photodiode and output stage result in TTL compatible saturation voltages and high speed operation. Where desired the VCC and VO terminals may be tied together to achieve conventional photodarlington operation. A base access terminal allows a gain bandwidth adjustment to be made.