Is there a practical difference between Open Drain and Open Collector outputs, or are the terms used interchangably? If they are indeed different, what are the contexts where each is advantageous? My hunch is that they are functionally equivalent, but Open Drain is implemented with FET technology while Open Collector is implemented with BJT technology.


1 Answer 1


That's right, open collector is BJT, open drain (C)MOS. In logic circuits where currents are low the saturation voltage of the BJT may be a bit higher than the voltage drop due to \$R_{DS(ON)}\$ for the FET, but it still will be much lower than the maximum voltage for a logic low.

Exception: you can't use a TTL open collector output as a level shifter to interface with very low voltage CMOS. This inverter can operate on a Vcc as low as 0.8 V, and the datasheet gives a maximum low level of 0.25 Vcc, that's 0.2 V. An open collector will have a voltage drop higher than that. (This device even specifies a maximum low level input voltage of 0 V, which is simply impossible.)

Apart from wired-ORing (think I2C) open drain/collector is often used to control something operating at a different voltage than the logic, often a higher voltage. With some low-voltage logic families this no longer possible, since the FETs will not tolerate the high voltage and for that reason some families will have clamping diodes built-in to protect the output FETs.

  • 7
    \$\begingroup\$ They probably forgot to say 0V +-10% :-) \$\endgroup\$ Aug 4, 2012 at 11:59
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    \$\begingroup\$ @OlinLathrop that saying in datasheets kills me. Every time I see it I throw something. \$\endgroup\$
    – Kortuk
    Aug 4, 2012 at 13:28
  • 2
    \$\begingroup\$ @Kortuk-SMASH!! \$\endgroup\$
    – vicatcu
    Aug 4, 2012 at 13:58
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    \$\begingroup\$ Hey @vicatcu, may I remind you that you're in my living room! That goes for you too, Kortuk. \$\endgroup\$
    – stevenvh
    Aug 4, 2012 at 13:59
  • \$\begingroup\$ Hmmm. Has anyone tried using the link on the TI data sheet called "Submit Documentation Feedback" yet? \$\endgroup\$
    – zebonaut
    Aug 12, 2012 at 20:35

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