NOT Gate Definition
The NOT gate is a single input digital logic
gate that performs the inversion of the input signal. Therefore, it is also
known as Inverter Gate.
As per the definition of the NOT gate,
if we apply a HIGH (Logic 1) signal at the input, the output of the NOT gate
will be a LOW (Logic 0) signal, if we apply a LOW (Logic 0) signal at the input
of the NOT gate, it will produce a HIGH (Logic 1) signal at the output.
NOT Gate Symbol
The logic symbol of the NOT gate is
depicted in the following figure.
NOT Gate Truth Table
The truth table is a table of inputs and
outputs of a digital logic circuit that provides information about the outputs
for all possible combinations of input signals. The truth table of the NOT gate
is shown below:
|
Input |
Output |
|
A |
Y =
A` |
|
0 |
1 |
|
1 |
0 |
Hence, from the truth table of the NOT
gate, it is clear that the NOT gate produces a HIGH output when its input is low
and vice-versa.
NOT Gate Boolean Expression
A Boolean expression is a logical
function that results in either true or false. The Boolean expression or logical
function of the NOT gate can be obtained from its truth table, and it is given
below:
`\Y=A'=\overline{A}`
Where, Y is the output of the NOT gate,
and A is the input to the NOT gate. This expression is read as Y is equal to NOT A.
NOT Gate Circuit Diagram
The switching circuit diagram of the
NOT gate is shown in the following figure.
When the switch is open, i.e. Logic 0,
the electric current will flow from the battery through the lamp, turning the lamp on, i.e. Logic 1 output.
When the switch is closed, i.e. Logic
1, the electric current flows through the short-circuited path provided by the
switch, and no current will flow through the lamp, turning the lamp off,
i.e. Logic 0 output.
This way, the above switching circuit
realizes the NOT gate operation.
NOT Gate using Transistor
We can also realize the NOT gate by
using a BJT transistor. The circuit diagram of the NOT gate using a transistor is
shown in the following figure.
This circuit is also termed a Transistor Inverter. In this circuit,
the input signal is applied to the base terminal, a battery (+VCC)
is connected to the collector terminal through a resistor, and the emitter
terminal is connected to the ground. Here, the output is obtained at the
collector-base junction.
When the input signal A = 0 (0 V), the transistor
will be in the `off` state, and thus there is no closed path between the supply
voltage and the ground. Consequently, the total supply voltage will appear at
the output terminal `Y` of the transistor inverter. Hence, the transistor NOT gate output is HIGH or Logic 1, i.e. Y = 1.
When the input signal A = 1 (+5 V), the
transistor will be in the `on` state, and thus there is a closed path between the
supply voltage and the ground, completing the circuit. Due to this, the total supply
voltage will drop in the resistor R and no voltage will be available at the output
terminal `Y` of the transistor inverter. Hence, the transistor NOT gate output is LOW or Logic 0, i.e. Y = 0.
NOT Gate IC 7404
The most popular NOT gate IC available
in the market is the IC 7404 which contains six transistor NOT gates.
The schematic diagram of the NOT gate
IC 7404 is shown in the following figure.
NOT Gate Applications
Some important applications of NOT gate
are listed below:
- NOT gate is mainly used for inversion of a signal, i.e. to convert logic 0 or low signal into logic 1 or high signal and vice-versa.
- NOT gate is also used to design memory devices to store digital information.
- NOT gate can be also used in data transmission in communication systems.
- NOT gate can be used to create an interface between different logic families.
- NOT gates are also used in various electronic devices like oscillators, timing circuits, multivibrators, etc.
- NOT gates are also used in pulse width modulation (PWM).
Hence, this is all about NOT gate and
its various concepts such as truth table, Boolean expression, logic symbol,
applications, etc.
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