TABLE OF CONTENTS This Chapter will describe many examples of translinear circuits, which come as close to true current-mode operation as any circuit can.
At the outset, however, it must be stated that the term "current-mode circuit" has no rigorous meaning. The behavior of electrical networks is always the result of an interplay between voltage and current. Choice of impedance level is frequently crucial to successful implementation of a signal-processing function. In designing with the silicon bipolar junction transistor (BJT), biasing plays an important role in setting impedance levels. The small-signal view of circuit design requires that strongly nonlinear devices be represented by linearized models, to allow well-known linear mathematical methods to be used in analysis. In this traditional view, the BJT is often viewed as a current-controlled current-source, in which the finite common-emitter current gain, beta, is regarded as a key parameter in determining circuit behavior. It is often assumed to be more or less constant with collector current, I C, so presenting the transistor as essentially linear in nature. This "beta-view" diminishes the emphasis on the far more important role of V BE, which is often taught as being only an approximate characteristic of the device: the words "about 700mV for a typical silicon transistor" are to be found in too many text-books.
In fact, the relationship between I C and V BE is at the very heart of the BJT. Figure 2.1 shows that this relationship can be viewed in two equivalent ways;
