2

Boolean algebra

2.1 Introduction

In a digital system the electrical signals that are used have two voltage levels which may, for example, be 5 and 0 volts. The electrical devices used in these systems can generally exist indefinitely in one of these two possible voltage states, providing the power supply is maintained. For example, a bipolar transistor that is non-conducting in a 5 volt system will have approximately 5 volts between collector and emitter. However, when the transistor is turned on and is conducting, it can be arranged, with a suitable choice of load, that the voltage between collector and emitter is approximately zero. The two voltage levels employed in a digital circuit can be arbitrarily assigned values of 0 and 1. The two ...

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