I'm trying to understand why the following circuit, which corresponds to an open collector not gate, needs to use two diodes. The circuit I'm taking about is below:

enter image description here

I think I understand the one just after the Vcc, I guess it's for preventing the current to flow to ground when A is high, am I right? Why can't you just remove the Vcc instead of using that diode?

I'm trying to figure out what is the purpose of the one that is between ground and B, but I don't get why is it necessary to prevent the curent to flow from B to ground.

  • \$\begingroup\$ First, take into account that the schematic you're posting is a simplified schematic. Now imagine what would happen if you'd connect something to B, and this 'something' would pull B below your GND... Or if this 'something' would be connected to Vdd, a higher voltage than your Vcc, and it would lift B above your Vcc... \$\endgroup\$
    – MartinF
    Commented Aug 12, 2018 at 12:18

1 Answer 1


The diode to the right is hard to avoid in chip construction of MOSfet circuits where the substrate is grounded :
Presentation on theme: "Metal Oxide Semiconductor Field Effect Transistor (MOSFET) PPT Slides, Dr. M A ISLAM IIUCPresentation on theme: "Metal Oxide Semiconductor Field Effect Transistor (MOSFET)" PPT Slides, Dr. M A ISLAM IIUC

The upper diode to Vcc is not part of the MOSfet structure - it is added by the chip designer. It does limit the possible applications of this open-drain output, since a load above Vcc would be limited when this diode forward-biases.
However, the upper diode does help protect the drain from over-voltage (above Vcc). It may be a weak diode only meant for occasional events, like static discharge.

Sometimes, a chip designer would not use that circuit. For example, a chip meant to be powered by Vcc=3.3V might be advertised to have "5V-tolerant I/O". This diode would not be present as shown.

  • 2
    \$\begingroup\$ Someone put a lot of effort into drawing that illustration. If you did it, thank you! If someone else did it, could you please provide a link to the original work and/or an attribution? \$\endgroup\$ Commented Aug 12, 2018 at 13:15
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    \$\begingroup\$ If you're going to steal someone else's diagram instead of drawing your own, you need to provide attribution. Otherwise, this is plagiarism, and we do not allow it here. \$\endgroup\$
    – Dave Tweed
    Commented Aug 12, 2018 at 13:48
  • \$\begingroup\$ @ElliotAlderson Attribution added to diagram - should it also go into text? \$\endgroup\$
    – glen_geek
    Commented Aug 12, 2018 at 16:12
  • \$\begingroup\$ Sorry, I'm not seeing the attribution in the diagram. Maybe the text would be better. \$\endgroup\$ Commented Aug 12, 2018 at 16:32
  • \$\begingroup\$ @ElliotAlderson I can't find it on the diagram too, but I can see it when editing. Attribution has been added to text body. \$\endgroup\$
    – glen_geek
    Commented Aug 12, 2018 at 16:47

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