In electronic books, the small signal model of a BJT is given as:
I've just studied two-port networks theory and, as an application, another book I'm reading shows this picture:
1st question: why do we have a voltage-controlled voltage generator on the left (which is absent in the first image)?
2nd question: why do we have a current-controlled current generator on the right? I've been always told that the BJT is a "transconductance amplifier" (= collector current is controlled by the base-emitter voltage, not by the base current, as shown indeed in the first image)