Chapter 3:
Bipolar Junction Transistors
1
Transistor Construction
There are two types of transistors:
• pnp
• npn
pnp
The terminals are labeled:
• E - Emitter
• B - Base
• C - Collector
npn
2
Transistor Operation
With the external sources, VEE and VCC, connected as shown
below:
•
•
The emitter-base junction is forward biased
The base-collector junction is reverse biased
3
Currents in a Transistor
Emitter current is the sum of the collector and
base currents:
I
E
 I
C
 I
B
The collector current is comprised of two
currents:
I C  I Cmajority
 I COminority
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 various levels of
output voltage (VCB).
6
Common-Base Amplifier
Output Characteristics
This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
various levels of input
current (IE).
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
Approximations
Emitter and collector currents:
IC
 IE
Base-emitter voltage:
V BE  0.7
9
Alpha (a)
Alpha (a) relates the DC currents IC and IE :
α dc 
IC
IE
Ideally: a = 1
In reality: a is between 0.9 and 0.998
Alpha (a) in the AC mode:
α ac 
ΔI C
ΔI E
10
Transistor Amplification
Currents and Voltages:
IE  Ii 
I
I
C
L
V
L
 I
 I
Vi

Ri
200mV
20 Ω
Voltage Gain:
Av 
 10mA
E
i
 I
L
 10 mA
R  ( 10 ma )( 5 kΩ )  50 V
11
VL
Vi

50V
200mV
 250
Common–Emitter Configuration
The emitter is common to both input
(base-emitter) and output (collectoremitter).
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
IC = a IE
IE = IC + IB
Actual Currents
IC = a IE + ICBO
where ICBO = minority collector
current
This is usually so small that it can be
ignored, except in high power
transistors and in high temperature
environments.
When IB = 0 A the transistor is in cutoff, but there is some minority
current flowing called ICEO.
I CEO 
I CBO
1a
I B  0 μA
14
Beta ()
 represents the amplification factor of a transistor. ( is sometimes
referred to as hfe, a term used in transistor modeling calculations)
In DC mode:
β dc 
IC
IB
In AC mode:
 ac 
 IC
 IB
V C E  cons tan t
15
Beta ()
Determining  from a Graph
β AC 

(3.2 mA  2.2 mA)
(30 μA  20 μA)
1 mA
10 μA
V C E  7.5
 100
β DC 
2.7 mA
25  A
V C E  7.5
 108
Note: AC = DC
16
Beta ()
Relationship between amplification factors  and a
α 
β
β1
β 
α
α1
Relationship Between Currents
I C  βI B
I E  (β  1)I B
17
Common–Collector Configuration
The input is on the base
and the output is on the
emitter.
18
Common–Collector Configuration
The characteristics are similar
to those of the commonemitter configuration, except
the vertical axis is IE.
19
Limitations of Operation 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
the saturation region.
The transistor operates in the active
region between saturation and cutoff.
20
Power Dissipation
Common-base:
PCmax
 V CB I C
Common-emitter:
PCmax
 V CE I C
Common-collector:
PCmax
 V CE I E
21
Transistor Specification Sheet
more…
22
Transistor Testing
•
Curve Tracer
Provides a graph of the characteristic curves.
•
DMM
Some DMMs measure bDC or hfe.
•
Ohmmeter
23
Transistor Terminal Identification
24