A Bond Graph Model of the Bipolar Junction Transistor
Keywords
- Bipolar Junction Transistor
- Controlled Source Modeling
- Ebers-Moll Equations
Abstract
The conventional model of the bipolar junction transistor
(BJT) uses two current sources to model the flow of current through
a transistor. The question arises: where does the power come
from that these current sources seem to inject into the circuit?
A model may contain sources representing physical power supplies
that are somewhere plugged into a wall socket. Such source
elements are perfectly reasonable components to use in a model.
Yet, an internally modulated source element somewhere within a
circuit is a dubious modeling element, as it is not clear where
it takes its power from.
The result of this article is a new bond graph model for the BJT
that transforms the modulated current sources into a non-linear
resistor. Treating these current sources as a non-linear resistor
is indeed correct, since they always dissipate power and never
generate it, i.e., they really represent sinks rather than sources.
Additionally, RS elements are added to the BJT bond graph, so that
the entropy generation by means of power dissipation is modeled
correctly. The so produced heat causes a feedback in the electrical
performance of the BJT, since the current flow through a transistor
is temperature dependent.
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