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

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

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:
• Cutoff:

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• 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?
• What is the Triode region?
• For the Saturation region, why is 0.2V the magic number for V_CE?