Applied Electronic II Adama University EENG 2202 1 Electrical Engineering CHAPTER 6 DIGITAL ELECTRONICS INTRODUCTION Our world is an analog world. Measurements that we make of the physical objects around us are never in discrete units, but rather in a continuous range. We talk about physical constants such as 2.718281828… or 3.141592…. To build analog devices that can process these values accurately is next to impossible. Even building a simple analog radio requires very accurate adjustments of frequencies, voltages, and currents at each part of the circuit. To make things simpler, we work with a digital abstraction of our analog world. Instead of working with an infinite continuous range of values, we use just two values! Yes, just two values: 1 and 0, on and off, high and low, true and false, black and white, or whatever you want to call it. It is certainly much easier to control and work with two values rather than an infinite range. We call these two values a binary value for the reason that there are only two of them. A single 0 or a single 1 is then a binary digit or bit . This sounds great, but we have to remember that the underlining building block for our digital circuits is still based on an analog world. This chapter discusses how digital circuits are implemented at the physical level. As you know, transistors are the fundamental building blocks for all digital circuits. They are the actual physical devices that implement the binary switch and, therefore, also for the logic gates. There are many different transistor technologies for creating a digital circuit. Some of these technologies are the diode-transistor logic (DTL), transistor-transistor logic (TTL), bipolar logic, and complementary metal-oxide semiconductor (CMOS) logic. Among them, the most widely used is the CMOS technology. 6.1 BASIC AND DERIVED LOGIC GATES The three basic building blocks of digital circuits are the NOT, the OR, and the AND gates. NOR, NAND, XOR, and XNOR gates are examples of derived logic gates. The operation of these gates is explained in the upcoming sections. a. NOT Gate (inverter) The NOT gate simply inverts its input, so a 0 input will produce a 1 output, and a 1 becomes 0 . Logic symbol for NOT gate is shown in fig. 6.1. x y Figure 6.1 Logic symbol for NOT gate
Applied Electronic II Adama University EENG 2202 2 Electrical Engineering b. OR & NOR Gates OR gate takes two or more inputs and produce an output of 1 if at least one of this inputs is 1 , and 0 if all are 0’ s. NOR gate is derived by combining OR gate and Inverter which does the opposite of OR gate. NOR gate give an output of 1 whenever all the inputs are 0 ’s. The logic symbol for two-input OR & NOR gates is depicted in fig. 6.2. x y F x y F (a) OR Gate Symbol (b) NOR Gate Symbo Figure 6.2 Logic symbols for OR & NOR gates Truth table for OR & NOR Gate is given in the table of figure 6.3.
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