Implementing XOR Function with Transistors: A Comprehensive Guide

Implementing the XOR function using transistors is a fundamental task in digital electronics. This tutorial provides a detailed explanation on how to achieve the XOR function using a combination of NMOS and PMOS transistors. Understanding this concept is crucial for engineers and enthusiasts in the field of digital circuits and logic design.

What is XOR Function?

The XOR (exclusive OR) function is a logical operation that outputs true only when inputs differ. Specifically, for a two-input XOR gate, the truth table is as follows:

A B A XOR B 0 0 0 0 1 1 1 0 1 1 1 0

Transistor-Level Implementation

A common method to implement an XOR function using transistors involves using a combination of NMOS and PMOS transistors. This technique allows for the creation of a logic gate that performs the XOR operation based on the input conditions.

NMOS and PMOS Configuration

To implement the XOR function using transistors, you need to configure the following:

Use two PMOS and two NMOS transistors: These will form the core of the XOR gate.

Circuit Design

The circuit design involves arranging the transistors in a specific way to achieve the XOR function:

Connect PMOS transistors in parallel: This means the sources of both PMOS transistors are connected to Vdd, and their gates are connected to inputs A and B, respectively. Both drains are connected to the output. Connect NMOS transistors in series: This means the sources of both NMOS transistors are connected to ground, and their gates are connected to inputs A and B, respectively. Both drains are connected to the output of the upper NMOS transistor (N1).

Here is a schematic representation of the connections:

Schematic

PMOS Transistors: P1: Source connected to Vdd, Gate connected to A, Drain connected to the output. P2: Source connected to Vdd, Gate connected to B, Drain connected to the output. NMOS Transistors: N1: Source connected to ground, Gate connected to A, Drain connected to the output. N2: Source connected to ground, Gate connected to B, Drain connected to N1.

Operation

The operation of the XOR function using transistors can be explained through different input conditions:

When A 0 and B 0: Both PMOS transistors (P1 and P2) are ON (conducting): Both NMOS transistors (N1 and N2) are OFF. Output 0: Since both PMOS transistors are conducting, the output is grounded. When A 0 and B 1: P1 is ON, P2 is OFF: Both NMOS transistors (N1 and N2) are OFF. Output 1: Since P1 is conducting, current flows through P1, and the output is at high voltage. When A 1 and B 0: P1 is OFF, P2 is ON: Both NMOS transistors (N1 and N2) are OFF. Output 1: Since P2 is conducting, current flows through P2, and the output is at high voltage. When A 1 and B 1: Both PMOS (P1 and P2) are OFF, and both NMOS (N1 and N2) are ON: Output 0: Since both NMOS transistors are conducting, the output is grounded.

Conclusion

This configuration allows you to implement the XOR function effectively using transistors. By combining series and parallel arrangements of NMOS and PMOS transistors, you can control the output based on various input conditions. This fundamental building block is widely used in digital electronics and forms the basis of complex logic circuits.