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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.