Logic Circuit II (Alternative Logic Gate)

Topic: Logic Circuit II (Alternative Logic Gate)

Class: SSS Two

Definition of Alternative Logic Gate

Alternative logic gates are those that are the combination of two logic gates.

Types of Alternative Logic Gates

There are generally two types:

1. NAND gate
2. NOR gate

NAND Gate

NAND gate is the combination of both an AND gate and a NOT gate. It operates the same as an AND gate but the output will be the opposite.

Logic Symbol for the NAND Gate

Truth Table for NAND Gate

INPUT A	INPUT B	OUTPUT C
0	0	1
0	1	1
1	0	1
1	1	0

NAND Gate Equation

The NAND gate equation can also be expressed by a Boolean algebra equation. For 2 input NAND gate the equation is:

The expression is the same as the AND gate with an over bar above the entire portion of the equation representing the input. This equation reads X equals A and B NOT.

NOR Gate

The NOR gate is the combination of both OR gate and a NOT gate. It operates the same as an OR gate, but the output will be the opposite.

Logic symbol for the NOR gate

Truth Table for the NOR Gate

INPUT A	INPUT B	OUTPUT C
0	0	1
0	1	0
1	0	0
1	1	0

NOR Gate Equation

The NOR gate operation can also be expressed by a Boolean algebra equation. For a 2-input NOR gate the equation is:

The expression reads X equals to A or B NOT.

Uses of Logic Gates

Logic gates can be used to:

• build digital circuit
• design complex ICs
• construct a simple alarm
• control traffic light

XOR (Exclusive-OR) gate

The XOR gate acts in the same way as the logical either/or. The output is true if either, but not both, of the input, is true. Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the input are the same.

Logic Symbol for the XOR Gate

Truth Table for the XOR Gate

INPUT A	INPUT B	OUTPUT C
0	0	0
0	1	1
1	0	1
1	1	0

XOR Equation

Or

The expression reads X equals to A or B NOT.

XOR as a Simple Comparator

The XOR gate can be used to construct a Simple Comparator. A comparator is a combinational logic circuit that compares the magnitudes of two binary quantities to determine which one has the greater magnitude. XOR can be used as a basic comparator.