Chapter 4
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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.
How a BJT works
Large-Signal DC models
Base-Emitter
Collector-Base
Region
Notes
FB
FB
Saturation
I
B
>
0
{\displaystyle I_{B}>0}
and
β
I
B
>
I
C
>
0
{\displaystyle \beta I_{B}>I_{C}>0}
FB
RB
Active
I
B
>
0
{\displaystyle I_{B}>0}
and
V
C
E
>
0.2
{\displaystyle V_{CE}>0.2}
RB
FB
Reverse Active
RB
RB
Cutoff
Region
NPN
PNP
Saturation
I
B
>
0
{\displaystyle I_{B}>0}
and
β
I
B
>
I
C
>
0
{\displaystyle \beta I_{B}>I_{C}>0}
I
B
>
0
{\displaystyle I_{B}>0}
and
β
I
B
>
I
C
>
0
{\displaystyle \beta I_{B}>I_{C}>0}
Active
I
B
>
0
{\displaystyle I_{B}>0}
and
V
C
E
>
0.2
{\displaystyle V_{CE}>0.2}
I
B
>
0
{\displaystyle I_{B}>0}
and
V
C
E
<
−
0.2
{\displaystyle V_{CE}<-0.2}
Cutoff
V
B
E
<
0.5
{\displaystyle V_{BE}<0.5}
and
V
B
C
<
0.5
{\displaystyle V_{BC}<0.5}
V
B
E
>
−
0.5
{\displaystyle V_{BE}>-0.5}
and
V
B
C
>
−
0.5
{\displaystyle 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
α
{\displaystyle \alpha }
and
β
{\displaystyle \beta }
take on different values.
Saturation
:
Cutoff
:
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?
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