Bipolar Junction Transistors
Bipolar Junction Transistors
Bipolar Junction Transistors
Bipolar Junction
Transistors
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Introduction
• The invention of transistor was the beginning of a
technological revolution that is still continuing
• Two basic types of transistors
- Bipolar junction transistor (BJT)
- Field effect transistor (FET)
• The BJT is used in two broad areas
- as a linear amplifier to boost or amplify signal
- as an electronic switch
• The term bipolar refers to use of both electrons and
holes as current carriers in transistor structure
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Bipolar Junction Transistor Structure
• The BJT is constructed with three doped
semiconductor regions separated by two pn junctions.
• The three regions are called emitter, base and
collector.
- One type consists of two n regions separated by a p
region (npn)
- Other type consists of two p regions separated by a n
region (pnp)
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Bipolar Junction Transistor Symbols
• The base is a thin lightly doped region compared to the
heavily doped emitter and moderately doped collector
regions.
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Bipolar Junction Transistor Biasing
• BJT to operate properly as an amplifier, the two pn
junctions must be correctly biased with external dc
voltages.
• Base-Emitter (BE) -> forward biased
• Base-Collector (BC) -> reverse biased
• The collector is more positive than base, which is more
positive than emitter
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Bipolar Junction Transistor Biasing
• Base-Emitter (BE) -> forward biased
• Base-Collector (BC) -> reverse biased
• The voltages are reversed to maintain the forward
reverse bias.
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Bipolar Junction Transistor Operation (npn)
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Bipolar Junction Transistor Operation (npn)
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Bipolar Junction Transistor Current
• The direction of conventional current is in the direction
of the arrow on the emitter terminal.
• The emitter current is the sum of the collector current
and the small base current i.e. I I +I
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Bipolar Junction Transistor Current
• When transistor is connected to dc bias voltage
- forward biases the base emitter junction
- reverse biases the base collector junction
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Bipolar Junction Transistor Current
• DC Beta ( )
- The dc current gain of a transistor is the ratio of the dc
collector current ( ) to the dc base current ( ) and is
designated dc beta ( ).
- Range value: 20 < < 200
=
• DC Alpha (α )
- The ratio of the dc collector current ( ) to the dc emitter
current ( ) is the dc alpha (α ).
- Range value: 0.95<α <0.99 or greater but << 1
- The reason is that IC is always slightly less than by
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the amount of i.e. ( = + ).
α =
Bipolar Junction Transistor Current
Question: Determine the dc current gain and the
emitter current for a transistor where 50 µA and
3.65 mA ?
Solution:
.
= = = 73
µA
= + = 3.65mA + 50 µA = 3.70 mA
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Bipolar Junction Transistor Circuit Analysis
: dc base current : dc emitter current : dc collector current
: dc voltage at base with respect to emitter
: dc voltage at collector with respect to base
: dc voltage at collector with respect to emitter
: forward biases the junction
: reverse biases the junction
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Bipolar Junction Transistor Circuit Analysis
• When the base-emitter junction is forward-biased it is forward-
biased diode and has forward voltage drop
V ≌ 0.7 V
Since emitter is ground (0V), KVL law, the voltage across
=
Also, by Ohms law , =
Substituting for , =
" #" $
Solving for , =
Voltage at collector w.r.t ground emitter is
= %
Drop across , =
Voltage at collector w.r.t emitter is
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=
Voltage across reverse biased collector base junction
=
Bipolar Junction Transistor Circuit Analysis
Question: Determine I , I , I , V , V , and V , when
transistor has a β' = 150.
Solution:
V ≌ 0.7 V
Solving for I ,
()) #()$ # .+
I = = = 430µA
*) ,
I β' I = 150*430 = 64.5mA
I I + I 64.5mA + 430μA 64.9mA
Solving V and V ,
V =V I R 10-(64.5mA)(100Ω) 3.
V =V V =3.55-0.7=2.85V
Since the collector is at a higher voltage than the base, the 17
collector-base junction is reverse-biased.
Bipolar Junction Transistor
Collector Characteristic Curves
• The collector characteristic curves show relationship of
the three transistor currents.
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Bipolar Junction Transistor
Collector Characteristic Curves
• Consider point A on characteristic curve
- Assume V is set to produce value of I and V = 0
- Both BE and BC junction are forward biased then
transistor is in saturation region.
i.e. base is 0.7V while emitter and collector are at 0 V
- I is through BE junction
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Bipolar Junction Transistor
Collector Characteristic Curves
• Consider point A and B on characteristic curve
- Saturation is the state of a BJT in which I has
reached a maximum and is independent of I .
- When V is increased, V increases as I increases.
- V remains less than 0.7 V due to the forward
biased BC junction
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Bipolar Junction Transistor
Collector Characteristic Curves
• Consider point B and C on characteristic curve
- When V exceeds 0.7 V, the BC junction becomes
reverse biased and the transistor goes in active region.
- Once the BC junction is reverse-biased, I levels off and
remains essentially constant for a given value of I as
V continues to increase
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Bipolar Junction Transistor
Collector Characteristic Curves
• Consider point C on characteristic curve
- When V reaches high voltage, the reverse-biased BC
junction goes into breakdown and I increases rapidly.
- A transistor should never be operated in this
breakdown region.
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Bipolar Junction Transistor
Collector Characteristic Curves
• By setting up other values of base current, a family of
collector curves is developed.
• β' is the ratio of collector current to base current
I
β'
I
• Family of I versus V
curves for several values of
I (I , < I < < I , etc.)
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Bipolar Junction Transistor
Collector Characteristic Curves
• The DC load line represents the circuit that is external to
the transistor. It is drawn by connecting the saturation
and cutoff points.
• The region between the saturation and cutoff points is
called the active region.
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