Full Subtractor in Digital Electronics

In this lecture, we will learn about the Full Subtractor, its circuit diagram, full subtractor circuit, full subtractor truth table, full subtractor using half subtractor, full subtractor expression, K-map of the full subtractor and many more in a very detailed analysis.

Subtracting the two binary digits is the most basic operation performed by digital computers. For this subtraction, there are binary adders used.

There are two types of binary Subtractors:

  1. Half Subtractor and
  2. Full Subtractor

In this section, we will learn digital circuits which are used to subtract two binary numbers called Full f subtractors. We already learn the Half subtractor in the previous lecture, you can learn from the above link.

Note: Before getting the knowledge of the subtractors, you can also learn the digital circuits that add the two binary numbers.

There are two types of binary adders:

  1. Half adders and
  2. Full adders

What is a Full Subtractor

As we have seen in the Half subtractor lecture, there is a disadvantage of half subtractor which is overcome by the Full subtractor.

The Full subtractor is a combinational circuit with three inputs A, B, and Bin, and two outputs namely D and B.

A is the minuend, B is the subtrahend, Bin is the borrow produced by the previous stage, D is the difference output and B is the horror output.

Full Subtractor Block Diagram

The block diagram of a full subtractor is shown in the Figure below:

Full subtractor block diagram

Full Subtractor Truth Table

The Full Subtractor Truth Table is one that gives the relationship between the input and output of a logic circuit. The following is the truth table of the full-subtractor:

Full Subtractor Truth Table

Full Subtractor K Map

The equations for the difference bit (D) and the output borrow bit (B) can be found using the K-Map (or Karnaugh Map), a technique for simplifying Boolean algebra.

full subtractor k map

Full Subtractor Expression

From the above K-map of full subtractor, we can write the boolean expression for sull subtractor as per below:

Simplification of Difference Output (D):

\mathbf{ D = \bar A \bar B B_{in} + \bar A B \bar B_{in} + A \bar B \bar B_{in} + AB B_{in}}
\mathbf{ D = B_{in}(\bar A \bar B + AB) + \bar B_{in} (\bar A B + A \bar B)}
\mathbf{ \therefore D = B_{in}(\overline{A \oplus B}) + \bar B_{in} (A \oplus B)}

Let \mathbf{A \oplus B = C}, then

\mathbf{ \therefore D = B_{in}\bar C + \bar B_{in} C}
\mathbf{ \therefore D = B_{in} \oplus A \oplus B}

Simplification of Borrow Output (B):

From the above k-map, we can write the Borrow output as,

\mathbf{ B = \bar A B_{in} + \bar A B + BB_{in}}

Full Subtractor Circuit Diagram

we can realize the full subtractor using two XOR gates, two NOT gates, two AND gates, and one OR gate. The circuit diagram of the full subtractor is shown in the below figure:

Full Subtractor Circuit Diagram

We can write the expression for the Borrow output as

\mathbf{B = (\overline {A \oplus B} ) B_{in} + \bar A B}

We can realize the above equation same as the Borrow output as we have seen above.


borrow output of full subtractor

Full Subtractor Using Half Subtractor

The below figure shows the implementation of a full subtractor using a half subtractor. To implement the Full subtractor using the half subtractor we have to use 2 half subtractors and one OR Gate.

Full Subtractor Using Half Subtractor

Full Subtractor Using NAND Gates

We can realize the full subtractor circuit using NAND gates only as shown in the figure below:

Full Subtractor Using NAND Gates

From the logic circuit of the full subtractor using NAND Gates, we can see that 9 NAND Gates are required to realize the full subtractor using NAND Gates.

Applications of Full Subtractor

The full subtractor has the following notable applications:

  • In the ALU (Arithmetic Logic Unit) of computer CPUs, full subtraction is employed.
  • For arithmetic operations like subtraction, full subtractors are frequently employed in electronic calculators and many other digital devices.
  • Different microcontrollers employ full subtractors for arithmetic subtraction.
  • They are utilized in PC program counts and timers.
  • In processors, full subtractors are also used to compute addresses, tables, etc.
  • Systems based on networking and DSP (Digital Signal Processing) also use full subtractors.

FAQs on Full Subtractor

What is a full subtractor?

A full subtractor is a combinational circuit that performs subtraction involving three bits, namely A (minuend), B (subtrahend), and Bin (borrow-in).

What is a full subtractor and its application?

Full subtractors are used in ALU (Arithmetic Logic Unit) in computer CPUs. Full subtractors are extensively used to perform arithmetical operations like subtraction in electronic calculators and many other digital devices.

What are the advantages of a full subtractor?

 We can cascade single-bit full Subtractor circuits and could subtract two multiple-bit binary numbers.

How many OR gates are required in a full subtractor?

The full subtractor can be implemented using two half subtractors and an OR gate.

What is the difference between a half subtractor and a full subtractor?

The difference between a half subtractor and a full subtractor is the output of the half subtractor is the Ex-OR of two inputs. However, the difference output of the full subtractor is the XOR of three inputs.

Sr. NoSubject Wise Interview Questions and Answers
1.Top 100 Analog Electronics Interview Questions And Answers
2.Top 100 Digital Electronics Interview Questions And Answers
3.Top 100 EDC Interview Questions and Answers
4.Communication Interview Questions and Answers (Topic Wise)
5.Transducer Interview Questions And Answers
6.Electrical and Electronics Measurements Interview Questions and Answers
7.Top 100 Communication System Interview Questions
8.Satellite Communication Interview Questions And Answers
9.Computer Networking Interview Questions and Answers
10.Optical Fiber Communication Interview Questions and Answers
11.Physics Interview Question with answer
12.Machine Learning Interview Questions and Answers

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

telegram logo Join Our Telegram Group!