Download Chapter 5

Electronics
Principles & Applications
Fifth Edition
Charles A. Schuler
Chapter 5
Junction Transistors
©1999
Glencoe/McGraw-Hill
INTRODUCTION
• Amplification
• Transistors
• Characteristic Curves
• Transistor Testing
• Other Transistor Types
In
Gain =
Amplifier
Out
In
Out
NPN Transistor Structure
The collector is lightly doped.
N
C
The base is thin and
is lightly doped.
P
B
The emitter is heavily doped.
N
E
NPN Transistor Bias
No current flows.
The C-B junction
is reverse biased.
N
C
P
B
N
E
NPN Transistor Bias
The B-E junction
is forward biased.
Current flows.
N
C
P
B
N
E
NPN Transistor Bias
IC
Current flows
everywhere.
Most of the emitter carriers
diffuse through the thin base
When
both
junctions
region since they are attracted
biased....
by are
the collector.
Note that IB is smaller
than IE or IC.
N
C
P
B
N
E
IB
IE
Note: when the
switch opens, all
currents go to zero.
Although IB is smaller
it controls IE and IC.
Gain is something small
controlling something large
(IB is small).
IC
N
C
P
B
N
E
IB
IE
Transistor structure and bias quiz
The heaviest doping is found in the
___________ region.
emitter
The thinnest of all three regions is called
the ____________.
base
The collector-base junction is ___________
biased.
reverse
The base-emitter junction is ____________
biased.
forward
The majority of the emitter carriers flow to
the ___________.
collector
IC = 99 mA
The current gain from
base to collector
is called b.
IB = 1 mA
b =
99
ICmA
1IBmA
= 99
IE = 100 mA
C
P
B
N
E
IC = 99 mA
Kirchhoff’s
current law:
IB = 1 mA
C
P
B
N
E
IE = I B + I C
= 1 mA + 99 mA
= 100 mA
IE = 100 mA
IC = 99 mA
In a PNP transistor,
holes flow from
emitter to collector.
IB = 1 mA
Notice the PNP
bias voltages.
C
B
E
IE = 100 mA
Transistor currents quiz
b is the ratio of collector current to ______
current.
base
The sum of the base and collector currents
is the __________ current.
emitter
In NPN transistors, the flow from emitter to
collector is composed of _______. electrons
In PNP transistors, the flow from emitter to
collector is composed of _______. holes
Both NPN and PNP transistors show
__________ gain.
current
NPN schematic symbol
Collector
Base
Emitter
Memory aid: NPN
means Not Pointing iN.
PNP schematic symbol
Collector
Base
Emitter
IC
This circuit is used to
collect IC versus
VCE data for
several values of IB.
C
IB
B
E
VCE
When graphed, the data provide an
NPN collector family of curves.
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
8 10 12 14 16 18
VCE in Volts
0 mA
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
8 10 12 14 16 18
0 mA
VCE in Volts
b =
14
mA
6ICmA
40
IB mA
100
mA
150
== 140
This type of gain
is called bdc or hFE.
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
bac =
2.5DImA
C
20DImA
B
8 10 12 14 16 18
0 mA
VCE in Volts
= 125
Another type of gain
is called bac or hfe.
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
IB
C
8 10 12 14 16 18
VCE in Volts
0 mA
With these values of IB:
The C-E model is a resistor.
E
100 mA
14
12
10
IC in mA 8
6
4
2
0 2 4 6
IB
80 mA
60 mA
40 mA
20 mA
8 10 12 14 16 18
VCE in Volts
0 mA
When IB >> 100 mA
VCE @ 0
The model is a closed switch.
100 mA
14
12
10
IC in mA 8
6
4
2
0 2 4 6
IB
80 mA
60 mA
40 mA
20 mA
8 10 12 14 16 18
VCE in Volts
0 mA
When IB = 0
IC = 0
The model is an open switch.
Transistor operating conditions quiz
When IB is large and VCE @ 0, the transistor
acts as a ___________ switch. closed
When IB = 0 and IC = 0, the transistor
acts as an ___________ switch. open
When IB > 0 and VCE > 0, the transistor
acts as a ___________.
resistor
Two current gain measures are bdc and
__________.
bac
The symbol hfe is the same as _________.
bac
NPN
V
mA
E
B
C
E-B junction is forward biased by the ohmmeter.
NPN
V
mA
E
B
C
The C-E resistance is very high.
NPN
V
mA
E
B
C
100 kW
The meter reading is < 100 kW due to gain.
Current In
The BJT is
a current
amplifier.
Current
Amplifier
Current Out
Voltage In
The JFET is
a voltage
controlled
amplifier.
Voltage
Amplifier
Current Out
Source
Gate
Drain
P
N-channel
P-type substrate
Structure of an
N-channel JFET
The channel has carriers so it
conducts from source to drain.
Drain
Gate
Source
Source
Gate
Drain
P
N-channel
P-type substrate
A negative gate voltage
can push the carriers from
the channel and turn
the JFET off.
Drain
Gate
Source
0V
-1 V
-2 V
ID in mA
-3 V
VGS
-4 V
0
-5 V
VDS in Volts
This is known as a depletion-mode device.
N-channel JFET drain family of characteristic curves
It’s possible to make enhancement
type field effect transistors as well.
Metal
oxide
insulator
Drain
n
Gate
VGG
D
VDD
p
G
n
S
N-channel
MOSFET
Source
Gate bias enhances the channel and turns the device on.
5V
4V
3V
ID in mA
2V
VGS
1V
0
VDS in Volts
0V
Drain
Enhancement mode MOSFET drain
family of characteristic curves Gate
Source
Emitter voltage
The unijunction transistor fires
when its emitter voltage reaches VP.
VP
Then, the emitter voltage
drops due to its negative
resistance characteristic.
Base 2
Emitter current
Emitter
The UJT is not useful as an amplifier.
It is used in timing and control applications.
Base 1
Other transistor types quiz
BJTs are __________ -controlled
amplifiers.
current
FETs are __________ -controlled
amplifiers.
voltage
JFETs operate in the _________ mode.
depletion
MOSFETs operate in the __________
mode.
enhancement
UJTs are not useful as __________.
amplifiers
REVIEW
• Amplification
• Transistors
• Characteristic Curves
• Transistor Testing
• Other Transistor Types