. D IGITAL E LECTRONICS AND L OGIC G ATES In electronics circuits like amplifiers, oscillators, introduced to you in earlier sections, the signal (current or voltage) has been in the form of continuous, time-varying voltage or current. Such signals are called continuous or analogue signals . A typical analogue signal is shown in Figure.
(b) shows a pulse waveform in which only discrete values of voltages are possible. It is convenient to use binary numbers to represent such signals. A binary number has only two digits ‘ ’ (say, 0V) and ‘ ’ (say, 5V). In digital electronics we use only these two levels of voltage as shown in Fig.
. (b). Such signals are called Digital Signals . In digital circuits only two values (represented by or ) of the input and output voltage are permissible.
This section is intended to provide the first step in our understanding of digital electronics. We shall restrict our study to some basic building blocks of digital electronics (called Logic Gates ) which process the digital signals in a specific manner. Logic gates are used in calculators, digital watches, computers, robots, industrial control systems, and in telecommunications. A light switch in your house can be used as an example of a digital circuit.
The light is either ON or OFF depending on the switch position. When the light is ON, the output value is ‘ ’. When the light is OFF the output value is ‘ ’. The inputs are the position of the light switch.
The switch is placed either in the ON or OFF position to activate the light. FIGURE . (a) Analogue signal, (b) Digital signal. .
. Logic gates A gate is a digital circuit that follows curtain logical relationship between the input and output voltages. Therefore, they are generally known as logic gates — gates because they control the flow of information. The five common logic gates used are NOT, AND, OR, NAND, NOR.
Each logic gate is indicated by a symbol and its function is defined by a truth table that shows all the possible input logic level combinations with their respective output logic levels. Truth tables help understand the behaviour of logic gates. These logic gates can be realised using semiconductor devices. (i) NOT gate This is the most basic gate, with one input and one output.
It produces a ‘ ’ output if the input is ‘ ’ and vice-versa. That is, it produces an inverted version of the input at its output. This is why it is also known as an inverter . The commonly used symbol together with the truth table for this gate is given in Fig.
. . (ii) OR Gate An OR gate has two or more inputs with one output. The logic symbol and truth table are shown in Fig.
. . The output Y is when either input A or input B or both are 1s, that is, if any of the input is high, the output is high. Input Output A Y (b) FIGURE .
(a) Logic symbol, (b) Truth table of NOT gate.