INTRODUCTION OF ANALOG ELECTRONICS :
‘Electronics‘ is one of those words which most of us recognize, but which is quite hard to define. However, we usually mean apparatus and systems which use devices which amplify and process electrical signals, and which need a source of power in order to work. Most of the devices which do this
are transistors. In the early days of electronics, the devices were vacuum-tubes (‘valves’) in which a stream of electrons was emitted from a heated cathode, via a control grid, towards an anode.
This is probably where the name ‘electronics’ came from. (Beams of electrons in a vacuum are still used in the cathode-ray tubes used for displays in television receivers and
computer monitors, and in microwave devices called magnetrons and travelling-wave tubes.) The electrical signal from a microphone is an example of an analog signal; its waveform (graph of voltage against time) has a similar shape to the waveform of the sound waves which it ‘picks up’.
The converse process, that of converting an electrical signal into a sound wave, also involves analog signals. The electrical analog signal is fed to a loudspeaker, which produces a sound waveform which is an analog of the original sound. An example of a digital signal is that recorded on a compact disc (CD). If an analog signal from a microphone is to be recorded, then it must first be converted to digital form.
This involves sampling the analog signal at a frequency much higher than the highest analog signal frequency, and then converting the sample amplitudes into corresponding digital codes represented by a series of electrical pulses. Further coding is used, first to ‘compress’ the total data and then to convert it into longer sequences for immunity against corruption.
These sequences are stored on the disc as tiny ‘blips’ representing binary data. All of the digital circuits use transistors, in sub-circuits called gates and flip-flops. So, both types of electronics have transistors in common. The design of the gate and flip-flop circuits, for ever-higher speed and lower power dissipation, depends heavily on the same circuit concepts as the analog circuits designed for higher frequencies and lower power dissipation.
These are concepts such as the equivalent circuit of the transistor, stored charge, stray capacitance and inductance, input and output impedance, electrical noise and the Uke. Interconnections between sub-assemblies, whether for digital signals in the form of pulses, or for analog signals, must be designed with a knowledge of transmission line theory when high frequencies or high data rates (which incur high frequencies) are present.
So, a great deal of the analog material of this book forms also the basic material of digital circuit design. The analog signal processing in the book is mirrored in digital signal processing, much of which is modelled on analog prototypes, and uses the same design theory, modified for digital implementation.
Thus, a good grounding in the theory of analog electronics is not only useful in its own right, but provides much of the background skills for the design of digital systems. The following examples illustrate the roles of analog and digital electronics in various systems.
A multimedia computer is an example of the way that analog devices, and analog signals, fit into a digital system. The multimedia material is recorded on a CD used as read-only memory (ROM), so these discs are commonly called CD-ROM. Greater-capacity discs are called DVD (digital video
disc or digital versatile disc). The contents may be audio (sound), still pictures, video (moving pictures) or animation. In many cases it is a combination of all of these. The sound is recorded as a digitally-coded version of the original analog sound signal, as for ordinary compact discs.