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NOT Gate using a Transistor

Updated: Jul 26

Logic gates can be realized using transistors. One of the most ingenious logic gates of all the #logicgates is the NOT gate and is used in many digital circuits where inverted output is required. The Logic Gates like NOR, NAND, etc. can be realized using NOT Gate. Let's learn to build a NOT Gate using a transistor.

To learn more about Logic gates Click here.

Let's learn the procedure of making a NOT gate with a transistor and see how is a NOT Gate realized with the help of a transistor.

Table of contents :

  1. What is Gate

  2. Types of Gate

  3. Truth Table

  4. What is NOT Gate

  5. Applications of NOT Gate

  6. Components Required:

6. Procedure

7. Working

What is a Gate?

In the context of digital electronics and logic circuits, a "gate" refers to a basic building block that performs a specific block that performs a specific logic operation. These logic gates are the fundamental components used to design and construct complex digital circuits, such as

microprocessors and memory units.

Each logic gate takes one or more binary inputs(usually represented as 0 or 1) and produces a binary output based on predefined logical rules. The output depends on the combination of inputs, and the behavior of each gate is described by a truth table.

Types of Gate :

  1. AND Gate: It produces a HIGH (1) output only when all of its inputs are HIGH (1).

  2. OR Gate: It produces a HIGH (1) output when at least one of its inputs is HIGH (1).

  3. NOT Gate (also called Inverter): It produces a HIGH (1) output when its input is LOW (0) and vice versa.

  4. NAND Gate: It is an AND gate followed by a NOT gate, and it produces a LOW (0) output only when all of its inputs are HIGH (1).

  5. NOR Gate: It is an OR gate followed by a NOT gate, and it produces a LOW (0) output when at least one of its inputs is HIGH (1).

  6. XOR Gate (Exclusive OR): It produces a HIGH (1) output when the number of HIGH inputs is odd.

  7. XNOR Gate (Exclusive NOR): It produces a HIGH (1) output when the number of HIGH inputs is even.

By combining different logic gates in various configurations, complex digital circuits can be built to perform various tasks and computations. These circuits form the basis of modern-day computing and electronic systems.

Truth table :

A truth table is a tabular representation that shows all possible combinations of inputs and their corresponding outputs for a given logic circuit or expression. It provides a complete and systematic way of understanding the behavior of a logic gate or a logic circuit under all possible input conditions.

In a truth table, the inputs and outputs are represented as binary values, typically 0 (LOW) and 1 (HIGH), as logic gates and digital circuits operate in a binary system. The number of rows in the truth table depends on the number of input variables, and the number of columns is always one more than the number of input variables, representing the output column.

Let us take an example of truth table:

Truth Table
Truth Table

Introduction Of NOT Gate

The NOT Gate which is also called as #Inverter gate has a triangular shape with a bubble (o) present at the top. The symbol of the NOT gate is shown below. Beyond its fundamental role in inverting binary inputs to produce corresponding outputs, the NOT gate finds wide applications in various digital circuits.

logic gates,xor gate,and gate ,nand gate,or gate,what is gate

The bubble (o) symbol present after the gate symbol can also be present before the gate symbol when an Active-low input is required. Few representations of NOT Gate are given below.

Various representation  a NOT gate
Various representation a NOT gate

The logic of the NOT Gate is inversion i.e. the input gets inverted and becomes the output. This inversion can be represented using the equation A = not(A), where A is the input, and not(A) represents the output. It is also known as Negation Gate. The truth table of logic NOT gate is given below.

                      A(Input)          Q(Output)     
                          0                  1  
                          1                  0

Where Q = not A or Ā. From the truth table it is clear that, for a NOT Gate when the input is HIGH, the output will be LOW and vice-versa.

Let's explore some of the key applications of the NOT gate and its role in shaping modern digital systems.

Applications of NOT Gate are :

1. Signal Inversion and Signal Conditioning:

The primary function of the NOT gate is to invert binary signals. It is commonly used to convert a positive logic signal (HIGH = 1, LOW = 0) into its complement (LOW = 1, HIGH = 0) or vice versa. This inversion is often essential in signal processing and communication circuits to ensure that signals are compatible with different parts of a system.

2. Logic Level Conversion:

In digital circuits, different devices or components may operate at varying voltage levels. The NOT gate comes in handy for logic level conversion, enabling seamless interfacing between circuits operating at different voltage levels. By using the NOT gate, signals can be translated to match the desired logic levels, ensuring smooth communication between different parts of the system.

3. Clock Signal Generation and Division:

The NOT gate plays a crucial role in clock signal generation and frequency division. By using a feedback loop with a suitable delay element, the NOT gate can be configured to generate stable clock signals with specific frequencies. Moreover, cascading multiple NOT gates can be used to divide the frequency of an input clock signal, allowing for precise timing control in digital systems.

4. Memory and Flip-Flop Circuits:

The NOT gate is an integral component in the design of memory and flip-flop circuits. These circuits are essential for storing and retaining binary data in sequential logic systems. NOT gates are used within these circuits to control the state of the memory cells and flip-flops, enabling data storage, retrieval, and sequential operation.

5. Logical Operations and DeMorgan's Theorem:

In combinational logic circuits, the NOT gate plays a key role in performing various logical operations. By combining NOT gates with other logic gates like AND, OR, and XOR, engineers can design complex circuits that perform arithmetic, comparison, and decision-making operations. DeMorgan's theorem, which states that the complement of a logical expression is obtained by inverting AND and OR operations while swapping them, further demonstrates the importance of NOT gates in logical operations.

6. Error Detection and Correction:

In digital systems, error detection and correction are critical for data integrity and reliability. The NOT gate is employed in error detection circuits to detect faulty or incorrect data and trigger appropriate corrective actions. It plays a crucial role in parity checking, checksum calculations, and error flag generation.

In conclusion, the NOT gate is a fundamental yet powerful logic gate with a myriad of applications in digital circuits. Its ability to invert binary signals and complement logical expressions makes it indispensable in various electronic systems. From signal inversion and level conversion to clock generation and logical operations, the NOT gate continues to be a cornerstone in digital circuit design, contributing to the robustness and efficiency of modern electronic devices.

Truth Table Of NOT Gate:

Truth Table of NOT Gate
Truth Table of NOT Gate

Components Required (NOT GATE)

The list of components needed to design a NOT gate using n-p-n transistor are:

1. NPN transistor (PNP transistor can also be used)

NPN Transistor
NPN Transistor

NPN Transistor is a Bipolar Junction Transistor (BJT) that is most commonly used in circuits. NPN transistor is designed by sandwiching a P-type semiconductor between two N-type semiconductors (Vice-versa for PNP Transistor). NPN and PNP both transistors have three terminals: Base, Emitter, and Collector.

To know more about Transistors click here.

2. 10KΩ resistor & 4-5KΩ resistor

10k Ohm Resistor
10k Ohm Resistor
5k Ohm Resistor
5k Ohm Resistor

Resistors are passive devices that oppose the flow of current or divide the voltage through the circuit. The resistor values used for the proper functioning of NOT gate using transistor are 10k Ohm and 4-5k Ohms.

To know more about Resistors click here

3. LED (Light Emitting Diode)


Light Emitting Diode is the most commonly used light source. It is a semiconductor, when current flows through it, the LED emits light. Here in this project, it is used to see the output.

4. Breadboard


The basic component of any circuit-building process is Breadboard. The connections between any components can be made using Breadboard. The input sensors or output display devices are connected to the microcontroller using wired connections using the breadboard. The holes in the breadboard are in series. They are available in various sizes like full-sized, half-sized, and mini breadboards.

5. Power supply

+5V Power supply
+5V Power supply

This is a power supply board that takes DC power from the normal circuit board and converts its required voltage values and supplies it to the circuit built. Here we are using +5V DC power.

6. PUSH button

PUSH Button
PUSH Button

PUSH Buttons are simple devices that are used to control switching. It is easy to use. It opens or closes the circuit when connected in a #circuit.

7. Connecting (Jumper) Wires

Jumper Wires
Jumper Wires

Jumper wires are the main components that are used to establish the connections between different devices of the circuit.



The logic between a NOT Gate's input and output is inverting. So to build a NOT Gate using a transistor, we have to build a #transistor circuit that produces an output that is the inverse of the input value. The transistor is used as a switch to build a NOT Gate and for this tutorial, the transistor used is BC547 NPN Transistor.

To know the working of a transistor

BC547 NPN Transistor
BC547 NPN Transistor

The circuit for building a NOT Gate using an NPN transistor, BC547 (Q1) is easy to understand. The Collector terminal of the NPN transistor is connected to +5V power supply through a 5KΩ (R1) resistor. The #base terminal of the transistor is connected to the input A with a 10KΩ (R2) resistor in between them and the emitter terminal of the transistor is connected to the GND.

For a NOT Gate, the output Ā is taken at the #collector terminal of the transistor and is then connected to an #LED to observe the output. The same is illustrated in the circuit below.

NOT Gate using a Transistor
NOT Gate using a Transistor

The board (red color) on the right side of the image given below is the +5V power supply which is connected to the #breadboard and further, the circuit is connected as explained above. If a PNP transistor is used then just interchange the connections of the #Emitter with the Collector in the above circuit and the rest of the things should be connected in the same manner.

Connect all the components in the same way as shown in the below schematic, to build a NOT Gate.

Circuit Diagram
Circuit Diagram


The working of NOT Gate using a transistor can be understood clearly when implemented practically. The hardware connections for the whole circuit shown above when connected practically is as shown below:

Hardware connections
Hardware connections

The working of a NOT gate can be understood by taking two cases from the truth table mentioned above. We would be successful in making a NOT gate using a transistor if the output produced is inverted with respect to the given input.

Let’s see the two cases and understand how the NOT Gate using transistor circuit works.

Case 1: Input A = 0

If the connections exactly as shown above, then for providing a LOW input at base terminal A, you would have to remove the wire connected between the 10KΩ resistor and power supply for the base terminal. For more ease, connect a PUSH Button instead of using the wire between the power supply and 10KΩ resistor, and then, you would only need to press the push button to provide HIGH or LOW input.

Case 1
Case 1

When we have Input A = 0 connected to the base terminal of the transistor, the electrical signal would not pass from the collector to the emitter terminal. So, the 5V power supply connected to the collector terminal cannot pass current to the emitter terminal, being the base value 0.

Now, the output is drawn from the collector terminal and since the power supply (5V) is ON, the current supply would go directly to the output. The output is further connected to the LED and receiving a HIGH input, the LED turns ON that indicates the output high.

Case 1 when Input A = 0
Case 1 when Input A = 0

If you want to increase the brightness of the LED, then you may reduce the resistance value of R1 resistor to around 1k-2kΩ and observe the difference.

Case 2: Input A = 1

In the second case, when the input is HIGH i.e. A = 1 and so, the base would allow passing the electrical signal from the collector to the emitter terminal of the transistor. Now, when a +5V supply is given to the collector, the base would allow the current to pass to the emitter terminal which is connected to the ground.

The output terminal connected to the collector terminal gets a LOW value because the collector terminal is connected to the ground through the emitter terminal of the transistor. So, the LED receives a LOW(0) input and would be OFF when input at the base is HIGH.

In both cases, we got the inverted output (with their respective inputs) as in the truth table of the NOT Gate and hence, we have successfully built a NOT Gate using a transistor.

The video below explains the working of NOT Gate using Transistor.



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