Open Collector - MOSFET

MOSFET

The analogous connection used with MOS transistors is an open-drain connection. Open-drain outputs can be useful for analog weighting, summing, limiting, as well as digital logic. An open drain terminal is connected to ground if a high voltage (logic 1) is applied to the gate, but behaves as a high impedance if a low voltage (logic 0) is applied to the gate. This high impedance state occurs because the terminal is at a undefined voltage (floating) and as a result, such a device requires an external pull-up resistor connected to the positive voltage rail (logic 1) in order to provide a logic 1 as output.

Note that microelectronic devices using open drain signals (such as microcontrollers) may provide a 'weak' internal pull-up resistor to connect the terminal in question to a positive voltage source/rail like V_dd of the device. Such weak pullups, often on the order of 100 kiloohm, reduce power usage by keeping input signals from floating. External pullups are stronger (perhaps 3 kiloohm ) to reduce signal rise times (as with I²C) or to minimize noise (as on system RESET inputs). Internal pullups can often be disabled for cases where there is an external one, or in other cases where they are not needed.