A Bond Graph Model of the Bipolar Junction Transistor

Keywords

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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Last modified: July 12, 2005 -- © François Cellier