Electronic Logic Gates - Basic Building Blocks

A logic gate is an electronic circuit which produces output signal depending on the input signal and its internal circuitry. Gates are the basic logic elements whose output signal is a simple Boolean operation of its input signal.

Binary information is represented in digital computers by physical quantities called signals. Electrical signals such as voltage exist in either one of the two recognizable states. The two states represent a binary variable that can be equal to 1 or 0. For example, a signal of 5 volts represents binary 1 and 0.2 volt represents binary 0.

Binary logic is described in algebraic or tabular form, the manipulation of binary information is done by logic circuits called gates. Gates are blocks of hardware that produce signals at binary 1 or 0 when input logic requirements are satisfied. A variety of logic gates are commonly used in digital computer systems. Each gate has a distinct graphic symbol and its operation can be described by means of an algebraic expression. The input-output relationship of the binary variables for each gate can be represented in tabular form by a truth table.

Each gate has one or two binary input variables. The input variables are designated by symbols A and B and binary output is designated by variable x. The AND gate produces the AND logic function, that is, the output is TRUE(1) if input A and input B both are true, otherwise, the output is FALSE (0). We can either use a dot between the variables or an AND operation symbol between them. AND gates may have more than two inputs, and as we have already said that the output is TRUE (1) if and only if all inputs are TRUE(1).

The OR gate produces the inclusive function, that is the output of OR gate is TRUE(1) if any of the inputs is TRUE(1), otherwise, the output is FALSE(0). The algebraic symbol of the OR function is + as that of arithmetic addition. OR gate may have more than two inputs, and obviously, the output is TRUE(1) if any input is TRUE(1).

The inverter circuit inverts the logic sense of the input. It produces Complement function, i.e. the output is FALSE if the input is TRUE and vice-versa. The algebraic symbol used for the logic complement is either a prime or a bar over the variable symbol.

The small circle in the output of the symbol of an inverter shows a logic complement. A triangle symbol by itself implements a buffer circuit. A buffer does not produce any particular logic function i.e. the output of the function is the same as the input. The main purpose of the buffer is to drive other gates that require a large amount of power.

The NAND function is the inverter of the AND function. The graphic symbol of NAND gate consists of an AND graphic symbol followed by a small circle. The designation NAND is an abbreviation of NOT-AND. The NOR gate is the complement of the OR gate and uses an OR graphic symbol followed by a small circle.

Both NAND and NOR gates may have more than two inputs and the output is always the complement of the AND or OR function, respectively. Each element of the digital circuit, i.e. OR, AND, XOR etc. can be implemented using either NAND or NOR gates. Therefore, these two gates are also known as universal gates. Any gate can be implemented by these two gates either singly or in combination. It is also worth consideration that these two gates are easy to be manufactured in terms of technical convenience.

The graphic symbol of exclusive OR gate is similar to the OR gate except for the additional curved line on the input side. The output of this gate is TRUE if any input is TRUE but excludes the combination when both inputs are TRUE. The exclusive NOR is the complement of the exclusive OR. The graphic symbol of exclusive NOR gate is indicated by the small circle in the graphic symbol of exclusive OR gate. The output of this gate is TRUE only if both inputs are TRUE or both inputs are FALSE. The exclusive OR and exclusive NOR gates are rarely found with more than two inputs. They are commonly available with two inputs.