The NOT gate

The first logic gate I'm going to cover is the NOT gate, the symbol for which is:

The NOT gate allows us to invert a signal. This is useful when we want to have the opposite of what a circuit outputs.

Let's say that we're using some combinational logic further in our circuit and want to have an output which only works when signal A is off or low and signal B is on or high. I'm sure that a complicated circuit could be built which combines a 0 and a 1 to produce the required output, but it's much much simpler to just invert one of the signals and go from there into our combinational logic.

This is the simplest of the logic gates because it can be built with just a single transistor for the semiconductor component.

In the circuit, consider the input labelled A.

When there is no voltage applied, the transistor is turned off. With the transistor off, not conducting, there is a potential difference across the transistor with reference to ground measured at NOT-A. As this is a positive (in this case) voltage, it can be used as a logic 1.

When there is a voltage at A, this will bias the transistor base on and cause the transistor to conduct. This means that it acts effectively as a simple wire and therefore there will be no potential difference at NOT-A. No voltage means a logic 0.

From the above, we can build a truth table:

A	NOT-A
0	1
1	0