# What is Digital Circuit : Design and Its Applications

Digital circuits or digital electronics is a branch of electronics which deals with digital signals to perform the various task to meet various requirement. The input signal applied to these circuits is of digital form, which is represented in 0’s and 1’s binary language format. These circuits are designed by using logical gates like AND, OR, NOT, NANAD, NOR, XOR gates which perform logical operations. This representation helps the circuit to switch from one state to another for providing precise output. Digital circuit systems are mainly designed to overcome the disadvantage of analog systems which are slower and the output data which is obtained may contain an error.

## What is a Digital Circuit?

**Definition**: A digital circuit is designed by using a number of logic gates on a single integrated circuit – IC. The input to any digital circuit is in the binary form “0’s” and “1’s”. The output obtained on processing raw digital data is of a precise value. These circuits can be represented in 2 ways either in a combinational way or a sequential way.

### Digital Circuit Basics

Digital circuit design was first started up with a design of relays, later vacuum tubes, TTL Transistor-Transistor Logic, Emitter coupled logic, and CMOS logic. These designs use a large number of logical gates like AND, OR, NOT, etc integrated on a single IC. The input and output of digital data are represented in the logical truth table and timing diagram.

#### Logical Level

Digital data is represented in a logical format, that is in “0” and “1” format. Where logic 0 represents that the signal is low or “GND” and logic1 represents the signal is high or connected to “VCC” supply as shown below

#### Logical Truth Table

A logical truth table is a mathematical representation of the performance of digital signal when passed through the digital circuit. The table consists of 3 columns they are the clock column, input column, and the output column. For example, NOT gate logic table is represented as follows

Clock Signal | Input Logic | Output Logic |

High | 0 | 1 |

High | 1 | 0 |

#### Timing Diagram

Digital signal behavior is represented in time domain format, for example, if we consider NOT logic gate truth table the timing diagram is represented as follows when the clock is high, the input is low then the output goes high. Similarly, when the input is high then the output goes low.

#### Gates

A logic gate is an electronic component that is implemented using a Boolean function. Gates are usually implemented using diodes, transistors, and relays. There are different types of logical gates they are, AND, OR, NOT, NANAD, NOR, XOR. Among which AND, OR, NOT are basic gates and NAND and NOR are the universal gate. Let us consider AND gate representation as below, which has 2 inputs and one output.

Clock Signal | Input Logic 1 | Input Logic 2 | Output Logic |

High | 0 | 0 | 0 |

High | 0 | 1 | 0 |

High | 1 | 0 | 0 |

High | 1 | 1 | 1 |

The truth table of AND gate

There are many ways in constructing a digital circuit that is either using logical gates by creating combinational logic, a sequential logic circuit, or by a programmable logic device that uses lookup tables, or by using a combination of many IC, etc. Typically, they are designed using combinational and sequential circuit format as shown below

### Combinational Logic Circuit

It is a combination of various logic gates like AND, OR, NOT. The design of combinational logic is made in such a way that the output depends on present input and the logic is independent of time. Combinational logic circuits are classified into 3 types, they are

- Arithmetic and logical functions: Adders, Subtractors, Comparators, PLD’s
- Data Transmissions: Multiplexers, Demultiplexers, Encoders, Decoders
- Code Converters: Binary, BCD, 7-segment.

### Sequential Circuit

The design of the sequential circuit is different from the combinational circuit. In a sequential circuit, the output logic depends on both present and past input values. It also consists of a memory element which stores the processing and processed data. Sequential circuits are classified into 2 types they are,

- Synchronous Circuit
- Asynchronous Circuit

Some of the examples of sequential circuits are flip flops, clocks, counters, etc.

### Digital Circuit Design

Digital circuits can be designed in the following ways they are

- Using sequential system representation and combinational system representation
- Using the mathematical methods by reducing logical redundancy algorithms like K-Map, Boolean Algebra, QM algorithm, binary decision diagrams, etc.
- Using data flow machines which consist of registers and buses or wire. Data is communicated among various components using buses and registers. These machines are designed using hardware description languages like VHDL or Verilog.
- A computer is a general-purpose register transfer logic machine designed by using a microprogram and microsequencer processor.

### Digital Circuit Design Issues

Since the digital circuits are build up with analog components like resistors, relays, transistors, diodes, flip flops, etc. It is necessary to note that these components do not affect the behavior of the signal or data during digital circuit operation. The following are design issues which are usually observed they are,

- Problems such as glitches may occur due to inappropriate design of the system
- In-proper synchronization of a different clock signal leads to metastability in the circuit
- Digital circuits calculate more repeatedly due to high noise immunity.

#### Digital Circuit Examples

The following are the examples of digital circuits

- Mobile phones
- Radios
- Calculators, etc.

### Advantages

The following are the advantages

- Accuracy and programmability is high
- Easy to save digital data
- Immune to noise
- Many digital circuits can be integrated on a single IC
- Highly flexible
- High reliability
- A high rate of transmission
- Highly secure.

### Disadvantages

The following are the disadvantages

- They operate only on digital signals
- Consumes more energy than analog circuits
- Heat dissipation is more
- High cost.

### Applications

The following are the applications

- ADC – Analog to digital converter
- DAC – Digital to analog converter
- Signal Generator
- CRO
- A smart card, etc.

### FAQs

**1). What are digital circuits used for?**

Digital circuits are used to perform Boolean logic operations.

**2). How does digital circuit work?**

Digital circuit works with discrete signals, which are represented in the binary form of 0’s and 1’s.

**3). What are the basic components of the digital circuit?**

The basic components of the digital circuits are flip flops, diodes, transistors, Gates, etc.

**4). What is a circuit made of?**

An electronic circuit is made up of a number of passive and active components, which are connected using conducting wires.

**5). Name a few examples of Active and Passive components?**

- Examples of active components are diodes, IC, triode vacuum tubes, etc.
- Examples of passive components are resistor, capacitor, inductor, transformer, etc.

**6). Why do we use a resistor in circuits?**

We use a resistor in the circuit in order to control the current flow.

An electronic circuit is made up of a number of passive and active components, which are connected using conducting wires. They are two types of circuits they are analog circuit and digital circuit. The input to an analog circuit is a continuous variable signal, which provides signal information like current, voltage, etc. The digital circuit input signal is in a discrete time-domain format, which is represented in “0’s” and “1’s”. It provides signals strength, noise ratio, attenuation, etc properties of a digital signal. The main advantage of using digital circuits is that they are easy to implement and understand.