The Hybrid Pi model of Bipolar ( and MOSFET) devices

The hybrid Pi model is a popular model used to analyze small signal performance of active circuits based on bipolar and MOSFET transistor and other devices with appropriate modifications. The following are the descriptions of its component parts:

  1. It has three terminals ( the schematic shows a bipolar model with base, emitter and collector as its terminals. C = collector, B = base and E= emitter.
  • rbb is the base spreading resistance. The resistance between the base contact and the internal base of the transistor.
  • rb’e is the base to emitter resistance.  Represents the base current required to make up for recombination of minority carriers in the base region.
  • Ce is the emitter base diffusion capacitance.
  • Cc and rb’c represents the Early effect which accounts for the finite collector to emitter output resistance.
  • gce/ro represents the output impedance ( conductance) of the device.

This complete model can be simplified as needed for active circuits using these devices.

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