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Chapter 3:
Bipolar Junction Transistors
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Construction
There are two types of transistors:
pnp
npn
The terminals are labeled:
E - Emitter
B - Base
C - Collector
pnp
npn
2
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor OperationWith the external sources, VEE and VCC, connected as shown:
The emitter-base junction is forward biased
The base-collector junction is reverse biased
3
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Currents in a Transistor
The collector current is comprised of two
currents:
BICIEI
minorityCOI
majorityCICI
Emitter current is the sum of the collector and
base currents:
4
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Base Configuration
The base is common to both input (emitterbase) and
output (collectorbase) of the transistor.
5
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Base Amplifier
Input Characteristics
This curve shows the relationship
between of input current (IE) to input
voltage (VBE) for three output voltage
(VCB) levels.
6
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
various levels of input
current (IE).
Common-Base Amplifier
Output Characteristics
7
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Operating Regions
Active Operating range of the
amplifier.
Cutoff The amplifier is basically
off. There is voltage, but little
current.
Saturation The amplifier is full on.
There is current, but little voltage.
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
EI
CI
Silicon)(for V 0.7BEV
Approximations
Emitter and collector currents:
Base-emitter voltage:
9
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Ideally: a = 1
In reality: a is between 0.9 and 0.998
Alpha (a)
Alpha (a) is the ratio of IC to IE :
EI
CI dc
Alpha (a) in the AC mode:
EI
CI
ac
10
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Amplification
Voltage Gain:
V 50k 5ma 10
mA 10
10mA20
200mV
))((RLI
LV
iI
LI
EI
CI
iR
iV
iIEI
Currents and Voltages:
11
250200mV
50V
iV
LV
vA
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
CommonEmitter Configuration
The emitter is common to both input
(base-emitter) and output (collector-
emitter).
The input is on the base and the
output is on the collector.
12
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Characteristics
Collector Characteristics Base Characteristics
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Amplifier Currents
Ideal Currents
IE = IC + IB IC = a IE
Actual Currents
IC = a IE + ICBO
When IB = 0 A the transistor is in cutoff, but there is some minority
current flowing called ICEO.
A 0
BI
CBOCEO
II
1
where ICBO = minority collector current
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ICBO is usually so small that it can be ignored, except in high
power transistors and in high temperature environments.
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Beta ()
In DC mode:
In AC mode:
represents the amplification factor of a transistor. ( is sometimes referred to as hfe, a term used in transistor modeling
calculations)
B
C
I
I dc
constantac
CEVB
C
I
I
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Determining from a Graph
Beta ()
108
A 25
mA 2.7 7.5VDC CE
100
A 10
mA 1
A) 20 A (30
mA) 2.2mA (3.2
7.5V
AC
CE
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Relationship between amplification factors and a
1
1
Beta ()
Relationship Between Currents
BC II BE 1)I(I
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
CommonCollector Configuration
The input is on the
base and the output is
on the emitter.
18
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
CommonCollector Configuration
The characteristics are
similar to those of the
common-emitter
configuration, except the
vertical axis is IE.
19
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
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
the saturation region.
The transistor operates in the active
region between saturation and cutoff.
Operating Limits for Each Configuration
20
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Power Dissipation
Common-collector:
CCBCmax IVP
CCECmax IVP
ECECmax IVP
Common-base:
Common-emitter:
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Specification Sheet
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Specification Sheet
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Testing
Curve Tracer
Provides a graph of the characteristic curves.
DMM
Some DMMs measure DC or hFE.
Ohmmeter
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Copyright 2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Terminal Identification
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