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	FB	Reverse Active
RB	RB	Cutoff

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?