NOT Gate
A NOT gate has a single input voltage or signal and a single output voltage or signal like any other. This gate is called a NOT gate because the state of the output voltage and the input voltage are never the same. That is, if the input voltage of the NOT gate is low, the output voltage is high, and if the input voltage is high, the output voltage is low. This is why this gate is also known as an inverter.
Working principle of NOT gate
How the NOT gate works is straightforward to understand from the figure. A circuit has a switch A and a bulb connected in parallel. obviously,
- When the switch is in the off position the bulb is on, i.e., the output is available.
- The bulb does not turn on when the switch is in the on position; In this case, the output is zero, i.e., no output is available.
So, this circuit works like a NOT gate.
NOT gate in electronic circuit
An electronic NOT gate with one input is shown in the figure. The simplified form of this circuit is also shown. Incidentally, a NOT gate cannot be constructed with diodes alone.
An electronic NOT gate with one input is shown in the figure. The simplified form of this circuit is also shown. Incidentally, a NOT gate cannot be constructed with diodes alone.
The input voltage and output voltage are denoted by A and Y respectively. Suppose, the two possible states of the input and output voltages are low (say, 0V) and high (say, 5V). RB and RC are two resistors and VCC (=5V) is permanently connected to the battery circuit. The gate can be in either of the following two states.
- A is low: In this case the output voltage is high. As shown in the figure, if A is low, the values of RB and RC are taken such that the transistor is in cut-off condition i.e., no current flows through RC. This causes Y to be high for VCC.
- A is high: In this case the output voltage is low. According to the figure, if A is high, the transistor is in saturation for RB and RC, i.e., maximum current flows through RC. This causes the point P to be low, i.e., Y is low.
Truth table
| Input | Output |
|---|---|
A |
Y |
0 |
1 |
1 |
0 |
Symbol
Two NOT gate symbols are shown in the figure. Digital circuits can be drawn using either of these two symbols, but the first symbol is more commonly used.
Boolean algebra related to NOT gate
In Boolean algebra, a ‘ – ‘ symbol on A denotes a ‘NOT’ process. The Boolean algebraic equation for the NOT gate shown in the figure is,
Y = Ā
This equation is read as – “Y equals not A”.
When Ā = 0, then Y = 1
When Ā = 1, then Y = 0
NOT output of a digital input
As shown in the figure, for every low level or 0-level of the A input (in the DF and GH time intervals) the Y output goes to a high level i.e., 1-level. On the other hand, for 1-levels of A (in CD, FG, and HJ recesses) Y is at 0-level.
Special note
- OR, AND, and NOT gates are called basic logic gates. Because any other logic gate is some combination of these three basic gates.
- Not all logic gates can be constructed using any of the three basic gates. For example, an AND gate or a NOT gate cannot be obtained by combining multiple OR gates. However, NOR and NAND gates can be made by combining OR, AND, and NOT gates. The peculiarity of this dual gate is that, given a large number of NOR data, all kinds of logic gates can be made by specially combining them—as is the case with NAND gates. This is why NOR and NAND gates are called universal logic gates, although neither of them are fundamental gates.
- De Morgan’s theorem is used to construct various logic gates using multiple basic gates.
De Morgan’s theorem
- (A ∪ B)’ = A’ ∩ B’
- (A ∩ B)’ = A’ ∪ B’