What Is Logic Gate?

WHAT IS LOGIC GATE? A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V). There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR. The most common logic gates used perform the following logic functions: AND : Output is True(1) if all inputs are True (1) OR : Output is False (0) if all inputs are False (0) NOT : Output is the opposite of the single input i.e., If input is true (1) the output is false (0) If input is false (0) the output is true (1)

AND – Gate  The AND gate has two or more inputs.  The output from the AND gate is 1 if and only if all of the inputs are 1, otherwise the output from the gate is 0.  The AND gate is drawn as follows

Truth table for AND gate A 0 0 1 1

B 0 1 0 1

A.B 0 0 0 1

The output from the AND gate is written as A.B The truth table for a two-input AND gate looks like

0 0 0 1 1 0 1 1

F=0

F=0

F=0

F=1

OR-Gate    

The OR gate has two or more inputs. The output from the OR gate is 1 if any of the inputs is 1. The gate output is 0 if and only if all inputs are 0. The OR gate is drawn as follows

Truth table for OR gate A 0 0 1

B 0 1 0

A+B 0 1 1

1

1

1

0

F=0

0 0

F=1

1 1

F=1

0 1

F=1

1 NOT-Gate The NOT gate is unique in that it only has one input. It looks like,

A

The input to the NOT gate A is inverted

Ā

i.e. The binary input state of 0 gives an output of 1 and the binary input state of 1 gives an output of 0. Ā is known as "NOT A" or alternatively as the complement of A. A

Ā

0

1

1

0

Reference: O/L ICT Syllabus