Electronic Circuits for Mechatronics ELCT609
Lecture 4: Bipolar Junction Transistor
Dr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: eman.azab@guc.edu.eg
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Bipolar Junction Transistor (BJT)
Physical Structure and I-V Characteristics
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Physical Structure
 Two back to back PN Junctions
 NPN or PNPTransistor
 Three terminal device, for NPN:
Base (P-Type), Emitter and
Collector (N-Type)
 Base-Emitter Junction
 Base-Collector Junction
 Emitter doping is higher than
Collector doping
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
NPN BJT Transistor
PNP BJT Transistor
BJT I-V characteristics
1.
BJT NPN Transistor in Cutoff Mode
Base-Emitter Junction is Reverse biased
Base-Collector Junction is Reverse biased
BEJ
Reverse
Q
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BCJ
Reverse
Cutoff
Mode
IC  I B  I E  0
BJT I-V characteristics
2.
BJT NPN Transistor in Active Mode
Base-Emitter Junction is Forward biased
Base-Collector Junction is Reverse biased
BEJ
Forward
Q
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BCJ
Reverse
Active
Mode
BJT I-V characteristics
2.
BJT NPN Transistor
Active Mode
in
 Base-Emitter Junction is Forward
biased and electrons pass through
the Base to the collector due to the
Base small area
 Electrons from the Emitter are
collected at the Collector side
 The transistor’s Collector current
can be modeled by a current I  I exp  VBE 


C
s
dependent current source
V
 T 
 Is depends on doping and width of
the Base
I C  F I B
𝑉𝐵𝐸 ≅ 0.7𝑉
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Q
BJT I-V characteristics
3.
NPN Transistor in Reverse Active mode
Base-Collector Junction is Forward biased
Q
Base-Emitter Junction is reverse biased
Emitter and collector reverse their roles
However, BJT has an asymmetrical physical structure
The current gain from Base to Emitter is very small
BEJ
Reverse
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BCJ
Forward
𝑉𝐵𝐶 ≅ 0.5𝑉
Reverse
Active
Mode
R  F IE  R IB
BJT I-V characteristics
4.
NPN Transistor in Saturation Mode
Both junctions are Forward biased
𝑉𝐵𝐸 = 0.7𝑉
𝑉𝐵𝐶 = 0.5𝑉
The total current is the EBJ diffusion current subtracted from CBJ
diffusion current
BEJ
Forward
BCJ
Forward
Saturation
Mode
BJT could be used as a closed switch in Saturation mode
I C  F I B
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Q
VCE  0.2V
BJT NPN I-V Characteristics
 IC versus VBE and VCE
VBE
IC = Is exp
VT
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
For Active
ONLY
Q
BJT NPN I-V Characteristics
 IC versus VCE
The Early effect
iC  IS exp(
VBE  vCE 
) 1 

VT 
VA 
ro 
iC
V
 A
vCE IC
Q
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Large Signal Model in Active Mode
Q
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT PNP Physical Structure
 BJT PNP Transistor
 PNP is the NPN Complementary
structure
 Two back to back PN Junctions
 Three terminal device: Base (Ntype), Emitter and Collector (Ptype)
 Emitter-Base Junction
 Collector-Base Junction
 Emitter doping is higher than
Collector doping
 Same I-V Characteristics as NPN
Transistor
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Q
PNP BJT Transistor
BJT PNP Physical Structure
 BJT PNP Transistor in Active Mode
Q
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EBJ
Forward
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
CBJ
Reverse
Active
Mode
BJT Modes of Operation
Electrical Equations of BJT
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT NPN Modes of Operation
Mode
BEJ
BCJ
Equations
Condition
Cutoff
Reverse
Reverse
𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵 =0
𝑉𝐵𝐸 < 0.7
𝑉𝐵𝐶 < 0.5
Reverse
𝑉𝐵𝐸 = 0.7
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 = 𝛽𝐹 𝐼𝐵 = 𝛼𝐹 𝐼𝐸
𝛽𝐹
𝛼𝐹 =
1 + 𝛽𝐹
𝑉𝐵𝐶 < 0.5
Or
𝑉𝐶𝐸 > 0.2
Forward
𝑉𝐵𝐸 = 0.7
𝑉𝐵𝐶 = 0.5
𝑉𝐶𝐸 = 0.2
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 < 𝛽𝐹 𝐼𝐵
Forward
𝑉𝐵𝐶 = 0.5
𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵
𝐼𝐸 = 𝛽𝑅 𝐼𝐵 = 𝛼𝑅 𝐼𝐶
𝛽𝑅
𝛼𝑅 =
1 + 𝛽𝑅
𝑉𝐵𝐸 < 0.7
Active
(Forward)
Forward
Q
Saturation
Reverse
Active
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Forward
Reverse
Q
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT PNP Modes of Operation
Mode
EBJ
CBJ
Equations
Condition
Cutoff
Reverse
Reverse
𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵 =0
𝑉𝐸𝐵 < 0.7
𝑉𝐶𝐵 < 0.5
Reverse
𝑉𝐸𝐵 = 0.7
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 = 𝛽𝐹 𝐼𝐵 = 𝛼𝐹 𝐼𝐸
𝛽𝐹
𝛼𝐹 =
1 + 𝛽𝐹
𝑉𝐶𝐵 < 0.5
Or
𝑉𝐸𝐶 > 0.2
Forward
𝑉𝐸𝐵 = 0.7
𝑉𝐶𝐵 = 0.5
𝑉𝐸𝐶 = 0.2
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 < 𝛽𝐹 𝐼𝐵
Forward
𝑉𝐶𝐵 = 0.5
𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵
𝐼𝐸 = 𝛽𝑅 𝐼𝐵 = 𝛼𝑅 𝐼𝐶
𝛽𝑅
𝛼𝑅 =
1 + 𝛽𝑅
𝑉𝐸𝐵 < 0.7
Active
(Forward)
Forward
Q
Saturation
Reverse
Active
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Forward
Reverse
Q
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Calculating DC operating point
Solved Exercise
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Solved Example
 Find the DC Operating point of the Transistors?
Given: VBE=0.7V ,β=10
(Ans.: IB=0.023mA, IC=0.23mA, IE=0.253mA, VCE=9.54V, Active )
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Solution Steps:
Identify the BJT Type
2. Place the terminals name on the circuit
3. Write a KVL in the INPUT Loop
1.

Input loop for BJT is any loop containing VBE
Assume the BJT mode (most of the time active)
5. Calculate the currents and voltages
6. Write KVL in the OUTPUT loop
4.

7.
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Output loop for BJT is any loop containing VCE
Verify your assumption!
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Example
 Find the DC Operating point of the Transistors?
Given: VEB=0.7V ,β=10
(Ans.: IB=93µA, IC=0.93mA, IE=1.023mA, VEC=8.14V, Active )
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo