Many electronic systems may be regarded as having a pair of input terminals and a pair of output terminals, between which a signal is processed (see figure 4.1). For example, a signal might be amplified between the input and output terminals. This approach is valid for devices such as transistors that have only three terminals because one terminal is common to both the input and output. The electronic system responds to external input circuits or drives external load circuits by means of the terminal quantities Vi,Ii, Vo and Io and its behaviour is specified by the interdependences of these variables. Once these dependences are established, the response of the system to input and output connections can be deduced independently of any knowledge of the detailed internal action or circuitry. Relationships between Vi,Ii, Vo and Io can be represented graphically. Plots of Vi,Ii, Vo and Io against one another are called static characteristics or just characteristics for brevity. Particularly useful are Vi versus Ii, Vo versus Io and Io versus Ii. These are appropriately called input, output and transfer characteristics, respectively. Notice that the usual convention is adopted in which Ii and Io are defined as positive when they flow into the input and output terminals (see figure 4.1).
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