7 Digital Circuits

Chapter 7 Digital Circuits

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • Chapter 7Digital CircuitsAnalog signals have a continuous range of values within some specified limits and can beassociated with continuous physical phenomena.Digital signals typically assume only two discrete values (states) and are appropriate forany phenomena involving counting or integer numbers.While we were mostly interested in voltages and currents at specific points in analogcircuits, we will be interested in the information flow in digital circuits.The active elements in digital circuits are either bipolar transistors or FETs.Thesetransistors are permitted to operate in only two states, which normally correspond to twooutput voltages. Hence the transistors act as switches.Before starting we will first review number systems and Boolean algebra.7.1Number SystemsThe two digital states can be given various names: ON/OFF, true/false, high/low, 1/0, etc..The 1 and 0 notation naturally leads to the use of binary (base 2) numbers. Octal (base 8)and hexadecimal (base 16) numbers are also used since they provide a condensed numbernotation. Decimal (base 10) numbers are not of much use in digital electronics.7.1.1Binary, Octal and Hexadecimal NumbersConsider a decimal number with digits abc.We can write abc asabc10 = a× 102 + b× 101 + c× 100.(7.1)Similarly, in the binary system a number with digits abc can be written asabc2 = a× 22 + b× 21 + c× 20.(7.2)Each digit is known as a bit and can take on only two values: 0 or 1. The left most bit isthe highest-order bit and represents the most significant bit (MSB), while the lowest-orderbit is the least significant bit (LSB).131