2.1 Introduction

When the word "translinear" was coined in 1975 s it was with a very specific purpose in mind. A novel class of circuits had gradually emerged following the discovery of a few special cases in 1967 and first reported at the 1968 ISSCC [2]. These circuits could only be accurately realized using monolithic BJTs, since isothermal operation and tight matching of device geometry and doping profiles were essential. Hence, although such circuits might have been conceivable in the early '60s, they certainly would not have been realizable in the days of discrete devices. These circuits were based on the remarkable fact that the transconductance of a BJT is linearly proportional to its collector current, hence the term trans-linear. Further, the word carried with it the connation of "lying somewhere between familiar home territories of the linear circuit and the formidable terrains of the nonlinear". Indeed, in addition to their well-known application in multipliers and other nonlinear circuits, translinear concepts are found embedded in many contemporary linear integrated circuits. The most familiar example is the current mirror; the classical four-transistor Class-AB output stage of almost any op-amp can be viewed in translinear terms; the current-conveyor, used in the recently-rediscovered current-feedback amplifier, is yet another example.

While the term "translinear" has this more general application, the inputs and outputs of what will be called "strict-TL" circuits are entirely in the form of currents, and no voltages other than the junction voltages are involved. The...