1- Bipolar Junction Transistors-2.ppt

Chapter 3:
Bipolar Junction Transistors

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

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
3

Currents in a Transistor
The collector current is comprised of two
currents:
B
I
C
I
E
I 

minority
CO
I
majority
C
I
C
I 

Emitter current is the sum of the collector and
base currents:
4

Common-Base Configuration
The base is common to both input (emitter–base) and
output (collector–base) of the transistor.
5

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

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

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.
8

E
I
C
I 
Silicon)
(for
V
0.7
BE
V 
Approximations
Emitter and collector currents:
Base-emitter voltage:
9

Ideally: a = 1
In reality: a is between 0.9 and 0.998
Alpha (a)
Alpha (a) is the ratio of IC to IE :
E
I
C
I
α 
dc
Alpha (a) in the AC mode:
E
I
C
I
α
Δ
Δ
ac 
10

Transistor Amplification
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
250
200mV
50V



i
V
L
V
v
A

Common–Emitter 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

Common-Emitter Characteristics
Collector Characteristics Base Characteristics
13

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.
μA
0


 B
I
CBO
CEO
α
I
I
1
where ICBO = minority collector current
14
ICBO is usually so small that it can be ignored, except in high
power transistors and in high temperature environments.

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
β 
dc
constant
ac 


 CE
V
B
C
I
I

15

Determining  from a Graph
Beta ()
108
A
25
mA
2.7
β 7.5
V
DC CE


 
100
μA
10
mA
1
μA)
20
μA
(30
mA)
2.2
mA
(3.2
β
7.5
V
AC
CE






16

Relationship between amplification factors  and a
1
β
β
α


1
α
α
β


Beta ()
Relationship Between Currents
B
C βI
I  B
E 1)I
(β
I 

17

Common–Collector Configuration
The input is on the
base and the output is
on the emitter.
19

Common–Collector Configuration
The characteristics are
similar to those of the
common-emitter
configuration, except the
vertical axis is IE.
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
the saturation region.
The transistor operates in the active
region between saturation and cutoff.
Operating Limits for Each Configuration
21

Power Dissipation
Common-collector:
C
CB
Cmax I
V
P 
C
CE
Cmax I
V
P 
E
CE
Cmax I
V
P 
Common-base:
Common-emitter:
22

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