Chapter 4

Bipolar Junction Transistor

Base, Emitter, Collector

• NPN: Not Pointing iN
• PNP: Pointing iN Please
• The arrow is for the emitter current.

• For an NPN the current flows into the collector. For a PNP the current flows out of the collector.

Large-Signal DC models

BJT Regions Base-Emitter Collector-Base Region FB FB Saturation FB RB Active RB RB Cutoff RB FB Reverse Active

Region Constraints Region NPN PNP Saturation $I_{B}>0$ and $\beta I_{B}>I_{C}>0$ $I_{B}>0$ and $\beta I_{B}>I_{C}>0$ Active $I_{B}>0$ and $V_{CE}>0.2$ $I_{B}>0$ and $V_{CE}<-0.2$ Cutoff $V_{BE}<0.5$ and $V_{BC}<0.5$ $V_{BE}>-0.5$ and $V_{BC}>-0.5$

• Active: The normal mode of operation. This mode has the largest common-emitter current gain.
• Reverse Active: The collector and emitter roles are reversed. Most BJTs are not symmetrical, thus $\alpha$ and $\beta$ take on different values.
• Saturation: High current from the emitter to collector. Logical "on".
• Cutoff: Very little current flow. Logical "off".

• What are the Saturation, Reverse Active and Cutoff regions used for?
• Why do we always seem to use a common emitter configuration?
• What does the Reverse Active large-signal model look like? How about graphically?
• For the Saturation region, why is 0.2V the magic number for V_CE?

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