Mar 112019
 

Logic AND gate

The logic AND gate is one of the simplest gates in Digital Electronics. The symbols for a 2 and a 3 input AND gates are shown in the following image.

The first one is the symbol of a 2-input AND gate and the second one is the symbol of a 3-input AND gate.

2-input and 3-input logic AND Gate - Truth Tables

2-input and 3-input logic AND Gate – Truth Tables

The 2-input logic AND gate is the most known, although it can have many more inputs (A, B, C, etc.), but it only has one output: Q. The 2-input and the 3-input AND gates have the above truth tables.

It is easy to see that the output Q is “1” (logical 1, high level) when the input A and B are “1”. In other words, the output Q is “1” when A and B inputs are “1”. The AND gate is represented by Boolean algebra as: Q = A*B or Q = AB. A 3-input logic AND gate implemented with switches is shown in the following diagram.

Logic AND Gate implemented using Switches

Logic AND Gate implemented using Switches

Truth table of a 3 input logic AND gate.  O = Open and C = Closed

This logic gate may have many inputs. We can create a multiple input AND gate connecting 2-input AND gates in cascade. The problem connecting AND gates in cascade is that the propagation time (time needed by the signal to travel from the input to the output) increases.

How to build a 3 input logic AND gate with 2 input AND gates

We can build a 3-input AND gate from 2-input AND gates in cascade as shown in the diagram below.

3-input Logic AND gate using 2-input AND gates

The truth table is similar to the one shown above, where switches are used.

Using the same metod we can implement a 4 or more inputs logic AND gates.

Multiple-input AND Gate

A 6-input AND Gate can be made using the following circuit diagram:

Multiple-input AND Gate

6-input Logic AND Circuit Equation

This diagram uses one 3-input AND gate and a three 2-input AND gate.

The TTL 7408 Quad 2-input AND Gate Pinout

TTL 7408 Quad 2-input AND gate IC

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