Open drain vs push-pull transistors

Tags: electronics transistor circuits

An open drain configuration is one where the drain pin of a FET transistor is external to the IC package. This pin is either open (high impedance) or connected to ground (and thus a digital low signal). Typically open drain pins are connected to a pull-up resistor so that when the pin is open it has a digital high signal.

In the diagram below, O is the output drain pin. When the transistor is active, O is connected to ground. Otherwise, it is floating.

         ───────────────┐
                    ┌───┼─── O
                    │ D │
              G │├──┘   │
            ────┤│      │
                │├──┐   │
                    │ S │
               GND ─┴─  │
         ───────────────┘

A push-pull pin can drive its output either high or low. The transistor diagram effectively looks like an inverter:

         ───────────────┐
                Vcc ─┬─ │
                     │  │
                     │  │
                 │├──┘  │
             ┌──o┤│     │
             │   │├──┐  │
           ──┤       ├──┼─── O
             │   │├──┘  │  
             └───┤│     │
                 │├──┐  │
                     │  │
                     │  │
                GND ─┴─ │
         ───────────────┘