  Bipolar junction transistors (BJTs) have been at the core of linear
electronics from its beginnings. Although their properties can be well
represented transport model equations, design and analysis approaches have, to
a good extent, been limited to using current-equispaced horizontal and parallel
isolines of the characteristic surface. Here, we resort to the geometrical
structure imposed on BJTs behavior by the Early effect and voltage as a means
to derive a simple, intuitive and more complete respective model that, though
excluding cut-off and saturation regimes, can simplify the design and
characterization of BJTs and respective circuits, including integrated devices.
The approach involves using a beam of isolines converging at the Early voltage
as the model of a BJT. The angles of the isolines, another important aspect of
BJTs, are experimentally verified often to vary almost in linear fashion with
the base current, endowing the proposed Early model with an intrinsic geometric
nature. An experimental-numerical methodology is suggested for the Early model
estimation, and applied to 12 real-world small signal NPN BJTs. Interesting
results are obtained, including the identification of diverse Early voltage
values for different BJT types and the observation, for the considered BJTs, of
the almost linear variation of the isoline angles with the input current. A
case example of transfer function estimation, as well as total harmonic
distortion analysis, are also provided in order to illustrate the potential of
the proposed methodology for approximating, under certain circumstances, the
operation of the common emitter configuration. The proposed model also has good
potential as a didactic aid in physics and electronics programs, complementing
the currently adopted modeling approaches.
