BJTs INTRODUCTION - What Are They?
• Bipolar junction transistors (BJTs) are essentially two individual signal diodes that are joined back-to-back to
produce two PN-junctions connected together in series with a common P or N terminal.
• The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of
a BJT
What Are Transistors?
• Transistors are three terminal active devices made from different semiconductor materials that can act as
either an insulator or a conductor by the application of a small signal voltage.
• The transistor’s ability to change between two states enables it to have two basic functions:
◦Switching (digital electronics)
◦Amplification (analogue electronics)
• Bipolar transistors have the ability to operate within three different regions:
Active Region - The transistor operates as an amplifier:
Saturation Region - The transistor is ‘fully ON’ operating as a switch and:
Cut-Off Region - The transistor is ‘fully OFF’ operating as a switch and:
• The word ‘Transistor’ is a combination of the two words ‘transfer’ and ‘varistor’, which describes their mode of
operation way back in their early days of electronics development
• There are two basic types of bipolar transistor construction
◦PNP
◦NPN
• These describe the physical arrangement of the P-type and N-type semiconductor materials from which they
are made
Basic Construction
• The bipolar transistor basic construction consists of two PN-junctions producing three connecting terminals
with each terminal being given a name to identify it from the other two. These three terminals are labelled as:
The Emitter (E)
The Base (B)
The Collector (C)
• Bipolar transistors are current regulating devices that control the amount of current flowing through them
from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base
terminal, thus acting like a current-controlled switch
• As a small current flowing into the base terminal controls a much larger collected current forming the basis of
transistor action
• The principle of operation of the two transistor types (PNP and NPN) is exactly the same with only one
difference being in their biasing and the polarity of the power supply for each type
, Construction
• The construction and circuit symbols for both
PNP and NPN bipolar transistors are given
here with the arrow in the circuit symbol
always showing the direction of ‘conventional
current flow’ between the base terminal and
its emitter terminal.
• The direction of the array always points from
the positive P-type region to the negative N-
type region for both transistor types, exactly
the same as for the standard diode symbol
Bipolar Transistor Configurations
• As the bipolar transistor is a three terminal device, there are basically three possible ways to connect it within
an electronic circuit with one terminal being common to both the input and the output
• Each method of connection responds differently to its input signal within a circuit as the static characteristics
of the transistor vary with each circuit arrangement
COMMON BASE - Has voltage gain but no current gain
COMMON EMITTER - Has both current and voltage gain
COMMON COLLECTOR - Has current gain but no voltage gain
• Bipolar junction transistors (BJTs) are essentially two individual signal diodes that are joined back-to-back to
produce two PN-junctions connected together in series with a common P or N terminal.
• The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of
a BJT
What Are Transistors?
• Transistors are three terminal active devices made from different semiconductor materials that can act as
either an insulator or a conductor by the application of a small signal voltage.
• The transistor’s ability to change between two states enables it to have two basic functions:
◦Switching (digital electronics)
◦Amplification (analogue electronics)
• Bipolar transistors have the ability to operate within three different regions:
Active Region - The transistor operates as an amplifier:
Saturation Region - The transistor is ‘fully ON’ operating as a switch and:
Cut-Off Region - The transistor is ‘fully OFF’ operating as a switch and:
• The word ‘Transistor’ is a combination of the two words ‘transfer’ and ‘varistor’, which describes their mode of
operation way back in their early days of electronics development
• There are two basic types of bipolar transistor construction
◦PNP
◦NPN
• These describe the physical arrangement of the P-type and N-type semiconductor materials from which they
are made
Basic Construction
• The bipolar transistor basic construction consists of two PN-junctions producing three connecting terminals
with each terminal being given a name to identify it from the other two. These three terminals are labelled as:
The Emitter (E)
The Base (B)
The Collector (C)
• Bipolar transistors are current regulating devices that control the amount of current flowing through them
from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base
terminal, thus acting like a current-controlled switch
• As a small current flowing into the base terminal controls a much larger collected current forming the basis of
transistor action
• The principle of operation of the two transistor types (PNP and NPN) is exactly the same with only one
difference being in their biasing and the polarity of the power supply for each type
, Construction
• The construction and circuit symbols for both
PNP and NPN bipolar transistors are given
here with the arrow in the circuit symbol
always showing the direction of ‘conventional
current flow’ between the base terminal and
its emitter terminal.
• The direction of the array always points from
the positive P-type region to the negative N-
type region for both transistor types, exactly
the same as for the standard diode symbol
Bipolar Transistor Configurations
• As the bipolar transistor is a three terminal device, there are basically three possible ways to connect it within
an electronic circuit with one terminal being common to both the input and the output
• Each method of connection responds differently to its input signal within a circuit as the static characteristics
of the transistor vary with each circuit arrangement
COMMON BASE - Has voltage gain but no current gain
COMMON EMITTER - Has both current and voltage gain
COMMON COLLECTOR - Has current gain but no voltage gain
