There are two types of transistors, PNP and NPN. A transistor has three terminals - emitter, base, and collector. In an NPN transistor, the emitter-base junction is forward biased and the base-collector junction is reverse biased. There are three common transistor configurations - common-base, common-emitter, and common-collector. The common-emitter configuration is most widely used. A transistor can be used to amplify signals and its gain is determined by its beta value. Transistors have defined operating regions and limits that depend on the configuration.

1. Chapter 3:
Bipolar Junction Transistors

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

3. 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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4. Currents in a Transistor
Emitter current is the sum of the collector and
base currents:
IE = IC + IB
The collector current is comprised of two
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currents:
IC = IC +
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
minority
ICO
majority
4

5. Common-Base Configuration
The base is common to both input (emitter–base) and
output (collector–base) of the transistor.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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6. Common-Base Amplifier
Input Characteristics
This curve shows the relationship
between of input current (IE) to input
voltage (VBE) for three output voltage
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(VCB) levels.
Electronic Devices and Circuit Theory, 10/e
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7. Common-Base Amplifier
Output Characteristics
This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
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various levels of input
current (IE).
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Robert L. Boylestad and Louis Nashelsky
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8. Operating Regions
• Active – Operating range of the
amplifier.
• Cutoff – The amplifier is basically
off. There is voltage, but little
current.
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• Saturation – The amplifier is full on.
There is current, but little voltage.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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9. Approximations
E
I
Emitter and collector currents:
I ≅
C
Base-emitter voltage:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
VBE = 0.7 V (for Silicon)
9

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

11. Transistor Amplification
50V
= = =
Vi
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Currents and Voltages:
Vi
= = = =
10 mA
≅
≅ =
R ( )( )
L
I
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Gain:
10mA
200mV
20
10 ma 5 k 50 V
= = =
L
V
i
I
L
I
E
I
C
I
Ri
IE Ii
11
250
200mV
VL
Av

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

15. Beta (β)
β represents the amplification factor of a transistor. (β is
sometimes referred to as hfe, a term used in transistor modeling
calculations)
In DC mode:
C I
βdc =
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In AC mode:
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Robert L. Boylestad and Louis Nashelsky
B I
=
C
I
ac VCE
=constant
B
I
β
15

16. Determining β from a Graph
Beta (β)
(3.2 mA −
2.2 mA)
(30 μA 20 μA)
1 mA
10 μA
100
β
V 7.5
AC
CE
=
=
−
=
=
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2.7 mA
= =
β DC V 7.5 CE
25 A
108
=
μ
Electronic Devices and Circuit Theory, 10/e
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17. Beta (β)
Relationship between amplification factors β and α
β
β 1
α
+
=
α
α 1
β
−
=
Relationship Between Currents
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IC = βIB IE = (β + 1)IB
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18. Common––Collector Configuration
The input is on the
base and the output is
on the emitter.
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19. Common––Collector Configuration
The characteristics are
similar to those of the
common-emitter
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configuration, except the
vertical axis is IE.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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20. Operating L Limits for Each Configuration
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
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the saturation region.
The transistor operates in the active
region between saturation and cutoff.
Electronic Devices and Circuit Theory, 10/e
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21. Power Dissipation
PCmax = VCBIC
PCmax = VCEIC
Common-base:
Common-emitter:
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Common-collector:
PCmax = VCEIE
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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22. Transistor Specification Sheet
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Electronic Devices and Circuit Theory, 10/e
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23. Transistor Specification Sheet
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Electronic Devices and Circuit Theory, 10/e
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24. Transistor Testing
• Curve Tracer
Provides a graph of the characteristic curves.
• DMM
Some DMMs measure bDC or hFE.
• Ohmmeter
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Electronic Devices and Circuit Theory, 10/e
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25. Transistor Terminal Identification
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