The NOT gate
In digital electronics, not much could be achieved without the NOT gate, also called the inverting gate. The symbol and truth table are shown below.
The NOT gate, like the AND gate and the OR gate, is very important. This gate delivers at its output the inverse (opposite) of what is at the input.
The output of a NOT gate has the inverse value of its input. It is seen from the truth table below that the output X = A. This means that:
If we have a logical “1” at the input, there will be a logical “0” at the output and…
If we have a logical “0” at the input, there will be a logical “1” at the output.
Note: The apostrophe in the following expression means “denied”. So: X = A’ is the same as X = A
NOT gates can be cascaded. In this way, after two gates, an output equal to the original input is achieved. See the following image and the truth table
One reason to implement a circuit where you have the same thing at the output as you have at the input is the desire to achieve a delay of the original signal for a special purpose.
Example of NOT gates in TTL technology: the SN7404 integrated circuit. (6 NOT gates in a single chip)
Example of NOT gates in CMOS technology: the 4049 integrated circuit. (6 NOT gates in a single chip)