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Two inputs are connected already. How should you connect the third input so that you can use a 3 input AND gate as a 2 input one.

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Connect the 3rd input to either of the other inputs.

schematic

simulate this circuit – Schematic created using CircuitLab

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Connect the extra input to the power supply so it is hardwired to a logic 1. Connecting two inputs together increases the loading on the signal connected to those inputs, causing slower transitions. If possible, choose the input that connects to the gate of the NMOS transistor that is closest to ground as the spare input. Permanently tying that input high will cause the internal capacitance at the drain of this transistor to be held at a solid low level and should improve the rising transition of the AND gate output.

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  • \$\begingroup\$ That's an interesting point but I don't think it should matter in this case, especially if it's for a single gate. Many fancy exclusive gates can be built like that, by crossing inputs together of simpler gates i.e. XOR gate \$\endgroup\$ – Iancovici Sep 22 '13 at 20:11
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    \$\begingroup\$ For the most part, direct connection to Vcc is fine; however, for some older logic families a pullup resistor should be used. From a 1997 TI app note Designing With Logic: "Devices with multiple-emitter inputs (SN74 and SN74S series) are exceptions. Since no voltage greater than 5.5 V should be applied to the inputs (because if exceeded, the base-emitter junction at the inputs breaks down), the inputs of these devices must be connected to the supply voltage VCC via series resistor" \$\endgroup\$ – Tut Sep 23 '13 at 10:57
  • \$\begingroup\$ The pullup resistor was imposed as a design rule when I worked at a large electronics company years ago. I'm not totally sure why it is necessary since the standard SN74 and SN74S series supply is 5V. Presumably extra protection from spikes or maybe voltage overshoot due to poor regulator response, although it may also be desired for automated board testing. \$\endgroup\$ – Tut Sep 23 '13 at 11:05

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