NOT Gate (Inverter) – Symbol, Truth Table

Apr 042023

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.

NOT gate Truth table

Note: The apostrophe in the following expression means inverted. So: X = A’ is the same as X = A

NOT gates in cascade

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

Cascading NOT gates

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.

The NOT gate in different technologies

74LS04 – contains 6 NOT gates. (Low-power Schottky TTL Logic Gate Chip.)
74HC04 – high-speed CMOS option. (Low-power, high-speed CMOS Logic Gate Chip)
CD4069 – widely used in electronic projects. (Low Power, High Speed standard CMOS)
CD4049 – inverter buffer with superior current handling capacity.

Example of NOT gates in TTL technology: the SN7404 integrated circuit. (6 NOT gates in a single chip)

Six Independent Hex Inverters