INTRODUCTION 



Pieces of electronic apparatus are complexes of 'active' and 'passive' elements. 

 Active elements are to be regarded as sources of electrical energy, whilst the 

 passive elements either consume energy or modify it in some way. For an 

 electrical circuit to be possible at least one element of each kind must be 

 present. In developing a systematic theory of such circuits a problem of the 

 chicken-and-the-egg variety arises — how to study one set of elements without 

 having first discussed the other. 



The important active elements, in practical form, are of two kinds: (1) 

 those which actually produce electrical energy — batteries, the supply mains 

 and certain transducers; (2) those which only control the flow of energy 

 derived from elsewhere, but which are treated as if they were themselves the 

 source of it — valves, transistors, and the remainder of the transducers. 



The passive elements are resistance, capacitance, and inductance, pro- 

 perties reahzed either in a specific component, e.g. a resistor, which is a 

 device for exhibiting the property of resistance, or otherwise, as in the resis- 

 tance of an electrophysiological preparation as measured between two elec- 

 trodes. It may be remarked in passing that such a preparation is to be 

 regarded as an active element of the first kind at the site of recording, but 

 as passive at a site of stimulation. 



Since it is not possible to get very far discussing the active devices without 

 introducing some passive ones it is usual to begin by considering the latter, 

 and this we shall do, beginning in the next chapter. To investigate the 

 properties of the passive elements, however, both singly and in combination, 

 we need some ideal, generalized kinds of active element which we shall 

 simply call 'generators'. A 'load' is to be understood as any passive element 

 or arrangement of such elements into which these generators may work. 



o No o 



E 



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Figure 1.1 Figure 1.2 



The ideal generators are: 



(1) The direct voltage, constant-voltage generator (Figure 1.1). This is 

 supposed to be a source of constant e.m.f. E, which will maintain this e.m.f. 

 however large or small the current it supplies. 



(2) The alternating voltage, constant-voltage generator {Figure 1.2). A 

 source of e.m.f. e = Esm cot, which, like Figure 1.1, continues to generate 

 this e.m.f. whatever the load condition. The title reads rather paradoxically 

 until it is appreciated that the constancy referred to is with respect to load 



3 



