EE-231 Electronics I: Engr. Dr. Hadeed Ahmed Sher
EE-231 Electronics I: Engr. Dr. Hadeed Ahmed Sher
EE-231 Electronics I: Engr. Dr. Hadeed Ahmed Sher
Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, TOPI 23460
hadeed@giki.edu.pk
March 7, 2018
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 1 / 27
Overview
2 BJT datasheet
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Bipolar Junction Transistor (BJT)
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Bipolar Junction Transistor (BJT)
Three terminals are named as emiiter (E), collector (C) and base (B).
Emitter is heavily doped and collector and base lightly doped.
Base has the least doping usually 1:10 or less compared to the outer layers.
The base is made very thin usually at a ratio of 150:1 with respect to the
total width.
BJT is called bipolar because both the holes and electrons participate in
the injection process.
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 4 / 27
Bipolar Junction Transistor (BJT) BJT Operation
BJT operation
Same is for NPN transistor except that electrons flow and biasing supplied
are reversed.
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 5 / 27
Bipolar Junction Transistor (BJT) BJT Operation
BJT operation
The cumulative effect of the forward biased and reverse biased junctions is
shown below.
BJT operation
From the KVL we know that emitter current consist of collector and base
current and therefore, the largest current in the device.
IE = IC + IB (1)
Collector current however consist of two parts,
Where, IC (inj) is the collector current due to carriers injected into the base
and ICBO is the current that flows due to thermally generated carriers.
The term ICBO refers to current from collector to base while the emitter is
disconnected. Note that this is temperature dependent.
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Bipolar Junction Transistor (BJT) Common base configuration
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Bipolar Junction Transistor (BJT) Common base configuration
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Bipolar Junction Transistor (BJT) Common base configuration
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Bipolar Junction Transistor (BJT) Common base configuration
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Bipolar Junction Transistor (BJT) Common base configuration
Looking on the output characteristics the cutoff mode is the one in which
IC =0.
Cutoff region
Emitter base and collectorbase junctions, both are reverse-biased.
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 12 / 27
Bipolar Junction Transistor (BJT) Common base configuration
Alpha (α)
It is the ratio of collector current to emitter current.
Alpha in DC mode
IC
αdc = (3)
IE
For DC mode this is fixed and usually its value varies between 0.9-0.998.
For AC mode it is written as
Alpha in AC mode
∆IC
αac = (4)
∆IE
Biasing
If the base current IB is assumed to be zero then the biasing condition can
be demonstrated as shown below.
For NPN transistor the current direction is opposite as well as the biasing
voltages are reversed.
Note that the arrow in a BJT defines the direction of conventional current
flow. Biasing voltage should not exceed the breakdown voltage as shown
in Fig.10
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Bipolar Junction Transistor (BJT) Common emitter configuration
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Bipolar Junction Transistor (BJT) Common emitter configuration
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Bipolar Junction Transistor (BJT) Common emitter configuration
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Bipolar Junction Transistor (BJT) Common emitter configuration
α(0A) ICBO
IC = + = 250ICBO (7)
1 − 0.996 1 − 0.996
If ICBO =1µA then IC =0.25mA as shown in output characteristics.In
general for IB =0
ICBO
ICEO = (8)
1−α
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 18 / 27
Bipolar Junction Transistor (BJT) Common emitter configuration
Beta (β)
For DC mode its value varies between 50-400. For AC mode it is written as
Alpha in AC mode
∆IC
βac = (10)
∆IB
Engr. Dr. Hadeed Ahmed Sher (FEE, GIKI) Week 6 Resources March 7, 2018 19 / 27
Bipolar Junction Transistor (BJT) Common emitter configuration
We know that
β = αβ + α = (β + 1)α (14)
IE = IC + IB (11)
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Bipolar Junction Transistor (BJT) Common emitter configuration
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Bipolar Junction Transistor (BJT) Common emitter configuration
Breakdown region
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Bipolar Junction Transistor (BJT) Common collector configuration
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Bipolar Junction Transistor (BJT) Limits of operation
Limits of operation
Maximum rating of the device given
in datasheet should not be violated.
It is governed by
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BJT datasheet
BJT datasheet
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BJT datasheet
BJT datasheet
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BJT datasheet
BJT datasheet
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