BJT Small-Signal Model (2): Hybrid-π Model vs. T Model

This video is a tutorial on the small-signal model of Bipolar Junction Transistors (BJTs), specifically comparing the Hybrid-π model and the T equivalent circuit model. The instructor, Vincent Chang, begins by reviewing the concept of transconductance, emphasizing the small-signal approximation where the AC input voltage is much smaller than the thermal voltage. He then recaps the two important resistances: the resistance looking into the base (rπ) and the resistance looking into the emitter (re), and their relationship. The video also reviews the MOSFET hybrid-π and T models to draw parallels. For the BJT, the instructor presents the Hybrid-π model with voltage-controlled current source and the T model with current-controlled current source, highlighting the resistance looking into the emitter (re). He explains that both models are equivalent and can be used interchangeably depending on convenience. The video concludes by mentioning that there are alternative versions of these models (model 2) and that the next lecture will cover second-order models.