There are two types of transistors: pnp and npn. The terminals are labeled emitter (E), base (B), and collector (C). In operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. The collector current is comprised of majority and minority carriers. There are three operating regions: active, cutoff, and saturation. Transistors can be configured in common-base, common-emitter, or common-collector circuits.

1. Chapter 3:
Bipolar Junction Transistors

2. Transistor Construction
Transistor Construction
There are two types of transistors:
• pnp
• npn
The terminals are labeled:
• E - Emitter
pnp
pnp
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
• E - Emitter
• B - Base
• C - Collector
npn
npn
2
2

3. Transistor Operation
Transistor Operation
With the external sources, VEE and VCC, connected as shown:
• The emitter-base junction is forward biased
• The base-collector junction is reverse biased
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Electronic Devices and Circuit Theory, 10/e
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3
3

4. Currents in a Transistor
Currents in a Transistor
The collector current is comprised of two
B
I
C
I
E
I +
+
+
+
=
=
=
=
Emitter current is the sum of the collector and
base currents:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The collector current is comprised of two
currents:
minority
CO
I
majority
C
I
C
I +
=
4
4

5. Common
Common-
-Base Configuration
Base Configuration
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The base is common to both input (emitter–base) and
output (collector–base) of the transistor.
5
5

6. Common
Common-
-Base Amplifier
Base Amplifier
Input Characteristics
Input Characteristics
This curve shows the relationship
between of input current (IE) to input
voltage (VBE) for three output voltage
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Electronic Devices and Circuit Theory, 10/e
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voltage (VBE) for three output voltage
(VCB) levels.
6
6

7. This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
Common
Common-
-Base Amplifier
Base Amplifier
Output Characteristics
Output Characteristics
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
an output voltage (VCB) for
various levels of input
current (IE).
7
7

8. Operating Regions
Operating Regions
• Active – Operating range of the
amplifier.
• Cutoff – The amplifier is basically
off. There is voltage, but little
current.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
current.
•
• Saturation – The amplifier is full on.
There is current, but little voltage.
8
8

9. E
I
C
I ≅
Approximations
Approximations
Emitter and collector currents:
Base-emitter voltage:
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Electronic Devices and Circuit Theory, 10/e
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Silicon)
(for
V
0.7
BE
V =
9
9

10. Ideally: α
α
α
α = 1
Alpha (
Alpha (α
α
α
α
α
α
α
α)
)
Alpha (α
α
α
α) is the ratio of IC to IE :
E
I
C
I
α =
dc
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Ideally: α
α
α
α = 1
In reality: α
α
α
α is between 0.9 and 0.998
Alpha (α
α
α
α) in the AC mode
AC mode:
E
I
C
I
α
∆
∆
ac =
10
10

11. Transistor Amplification
Transistor Amplification
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Gain:
V
50
kΩ
5
ma
10
mA
10
10mA
20Ω
200mV
=
=
=
=
≅
≅
=
=
=
=
)
)(
(
R
L
I
L
V
i
I
L
I
E
I
C
I
i
R
i
V
i
I
E
I
Currents and Voltages:
11
11
250
200mV
50V
=
=
=
i
V
L
V
v
A

12. Common
Common–
–Emitter Configuration
Emitter Configuration
The emitter is common to both input
(base-emitter) and output (collector-
emitter).
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The input is on the base and the
output is on the collector.
12
12

13. Common
Common-
-Emitter Characteristics
Emitter Characteristics
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Collector Characteristics Base Characteristics
13
13

14. Common
Common-
-Emitter Amplifier Currents
Emitter Amplifier Currents
Ideal Currents
Ideal Currents
IE = IC + IB IC = α
α
α
α IE
Actual Currents
Actual Currents
IC = α
α
α
α IE + ICBO where ICBO = minority collector current
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
IC = α
α
α
α IE + ICBO
When IB = 0 µ
µ
µ
µA the transistor is in cutoff, but there is some minority
current flowing called ICEO.
µA
0
=
−
= B
I
CBO
CEO
α
I
I
1
where ICBO = minority collector current
14
14
ICBO is usually so small that it can be ignored, except in high
power transistors and in high temperature environments.

15. Beta (
Beta (β
β
β
β
β
β
β
β)
)
In DC mode:
β
β
β
β represents the amplification factor of a transistor. (β
β
β
β is
sometimes referred to as hfe, a term used in transistor modeling
calculations)
C
I
β =
dc
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
In AC mode:
B
I
β =
dc
constant
ac =
∆
∆
= CE
V
B
C
I
I
β
15
15

16. Determining β
β
β
β from a Graph
Beta (
Beta (β
β
β
β
β
β
β
β)
)
100
µA
10
mA
1
µA)
20
µA
(30
mA)
2.2
mA
(3.2
β
7.5
V
AC
CE
=
=
=
=
=
=
=
=
−
−
−
−
−
−
−
−
=
=
=
=
=
=
=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
108
A
25
mA
2.7
β 7.5
V
DC CE
=
=
=
=
µ
µ
µ
µ
=
=
=
= =
=
=
=
100
=
=
=
=
16
16

17. Relationship between amplification factors β
β
β
β and α
α
α
α
1
β
β
α
+
+
+
+
=
=
=
=
1
α
α
β
−
−
−
−
=
=
=
=
Beta (
Beta (β
β
β
β
β
β
β
β)
)
Relationship Between Currents
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Relationship Between Currents
B
C βI
I =
=
=
= B
E 1)I
(β
I +
+
+
+
=
=
=
=
17
17

18. Common
Common–
–Collector Configuration
Collector Configuration
The input is on the
base and the output is
on the emitter.
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Robert L. Boylestad and Louis Nashelsky
18
18

19. Common
Common–
–Collector Configuration
Collector Configuration
The characteristics are
similar to those of the
common-emitter
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
common-emitter
configuration, except the
vertical axis is IE.
19
19

20. VCE is at maximum and IC is at
minimum (ICmax= ICEO) in the cutoff
region.
IC is at maximum and VCE is at
minimum (VCE max = VCEsat = VCEO) in
Operating Limits for Each Configuration
Operating Limits for Each Configuration
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
minimum (VCE max = VCEsat = VCEO) in
the saturation region.
The transistor operates in the active
region between saturation and cutoff.
20
20

21. Power Dissipation
Power Dissipation
C
CB
Cmax I
V
P =
=
=
=
C
CE
Cmax I
V
P =
=
=
=
Common-base:
Common-emitter:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-collector:
E
CE
Cmax I
V
P =
=
=
=
21
21

22. Transistor Specification Sheet
Transistor Specification Sheet
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22
22

23. Transistor Specification Sheet
Transistor Specification Sheet
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23

24. Transistor Testing
Transistor Testing
•
• Curve Tracer
Curve Tracer
Provides a graph of the characteristic curves.
•
• DMM
DMM
Some DMMs measure β
β
β
βDC or hFE.
•
• Ohmmeter
Ohmmeter
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Electronic Devices and Circuit Theory, 10/e
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24

25. Transistor Terminal Identification
Transistor Terminal Identification
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25