BASIC DIGITAL CIRCUITS
In a digital system there are only a few basic operations performed, irrespective of the
complexities of the system. These operations may be required to be performed a number of
times in a large digital system like digital computer or a digital control system, etc. The basic
operations are AND, OR, NOT, and FLIP-FLOP. The AND, OR, and NOT operations are
discussed here and the FLIP-FLOP, which is a basic memory element used to store binary
information (one bit is stored in one FLIP-FLOP).
The And Operation
A circuit which performs an AND operation is shown in Fig. 1.2. It has N inputs (N
≥ 2) and one output. Digital signals are applied at the input terminals marked A, B, …, N, the
other terminal being ground, which is not shown in the diagram. The output is obtained at the
output terminal marked Y (the other terminal being ground) and it is also a digital signal. The
AND operation is defined as : the output is 1 if and only if all the inputs are 1.
Mathematically, it is written as
Y = A AND B AND C … AND N
= ABC …N
Fig. 1.2 The standard symbol for an AND gate
where A, B, C, … N are the input variables and Y is the output variable. The variables are
binary, i.e. each variable can assume only one of the two possible values, 0 or 1. The binary
variables are also referred to as logical variables.
Equation (1.1) is known as the Boolean equation or the logical equation of the AND gate.
The term gate is used because of the similarity between the operation of a digital circuit and a
gate. For example, for an AND operation the gate opens (Y = 1) only when all the inputs are
present, i.e. at logic 1 level.
Truth Table Since a logical variable can assume only two possible values (0 and 1),
therefore, any logical operation can also be defined in the form of a table containing all