Bipolar Transistor Basics
In the Diode tutorials we
saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple
PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal
diodes back-to-back, this will give us two PN-junctions connected together in series that share 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 Bipolar Junction Transistor, or BJT for short.
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 these two
states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then
bipolar transistors have the ability to operate within three different regions:
- • Active Region - the transistor operates as an amplifier and Ic = β.Ib
- • Saturation - the transistor is "Fully-ON" operating as a switch and Ic = I(saturation)
- • Cut-off - the transistor is "Fully-OFF" operating as a switch and Ic = 0
The word Transistor is an acronym, and is a combination of the words
Transfer Varistor used to describe their mode of operation way back in
their early days of development. There are two basic types of bipolar transistor construction, PNP
and NPN, which basically describes the physical arrangement of the P-type and N-type semiconductor
materials from which they are made.
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 known
and labelled as the Emitter ( E ), the Base
( B ) and the Collector ( C )
Bipolar Transistors are current regulating devices that control the amount of current flowing through them
in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle
of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their
biasing and the polarity of the power supply for each type.
Bipolar Transistor Construction
The construction and circuit symbols for both the PNP and
NPN bipolar transistor are given above 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 arrow 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 output. Each method of
connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary
with each circuit arrangement.
- • Common Base Configuration - has Voltage Gain but no Current Gain.
- • Common Emitter Configuration - has both Current and Voltage Gain.
- • Common Collector Configuration - has Current Gain but no Voltage Gain.
The Common Base (CB) Configuration
As its name suggests, in the Common Base or grounded base configuration, the
BASE connection is common to both the input signal AND the output signal with the input
signal being applied between the base and the emitter terminals. The corresponding output signal is taken from
between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed
reference voltage point.
The input current flowing into the emitter is quite large as its the sum of both the base
current and collector current respectively therefore, the collector current output is less than the emitter current
input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base
configuration "attenuates" the input signal.
The Common Base Transistor Circuit
This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal
voltages Vin and Vout are "in-phase".
This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output
characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated
Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more
importantly "load" resistance ( RL ) to "input" resistance
( Rin ) giving it a value of "Resistance Gain". Then the voltage gain
( Av ) for a common base configuration is therefore given as:
Common Base Voltage Gain
Where: Ic/Ie is the current gain, alpha ( α )
and RL/Rin is the resistance gain.
The common base circuit is generally only used in single stage amplifier circuits such as microphone
pre-amplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency
The Common Emitter (CE) Configuration
In the Common Emitter or grounded emitter configuration, the input signal is
applied between the base, while the output is taken from between the collector and the emitter as shown. This type
of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal"
method of bipolar transistor connection.
The common emitter amplifier configuration produces the highest current and power gain of all the
three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a
forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.
The Common Emitter Amplifier Circuit
In this type of configuration, the current flowing out of the transistor must be equal to the
currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib.
Also, as the load resistance ( RL ) is connected in series with the collector, the
current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib
and is given the Greek symbol of Beta, ( β ). As the
emitter current for a common emitter configuration is defined as Ie = Ic + Ib,
the ratio of Ic/Ie is called Alpha, given the Greek symbol of
α. Note: that the value of Alpha will always be less than unity.
Since the electrical relationship between these three currents, Ib,
Ic and Ie is determined by the physical construction of the transistor
itself, any small change in the base current ( Ib ), will result in a much larger change
in the collector current ( Ic ). Then, small changes in current flowing in the base will
thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20
and 200 for most general purpose transistors.
By combining the expressions for both Alpha, α
and Beta, β the mathematical relationship between these parameters
and therefore the current gain of the transistor can be given as:
Where: "Ic" is the current flowing into the collector terminal,
"Ib" is the current flowing into the base terminal and "Ie"
is the current flowing out of the emitter terminal.
Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current
and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration
is an inverting amplifier circuit. This means that the resulting output signal is 180o "out-of-phase"
with the input voltage signal.
The Common Collector (CC) Configuration
In the Common Collector or grounded collector configuration, the collector is now
common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter
load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit.
The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance,
in the region of hundreds of thousands of Ohms while having a relatively low output impedance.
The Common Collector Transistor Circuit
The common emitter configuration has a current gain approximately equal to the
β value of the transistor itself. In the common collector configuration the load resistance
is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is
the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration
also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit
is given as:
The Common Collector Current Gain
This type of bipolar transistor configuration is a non-inverting circuit in that the signal
voltages of Vin and Vout are "in-phase".
It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives
both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing
good current amplification with very little voltage gain.
Bipolar Transistor Summary
Then to summarise, the behaviour of the bipolar transistor in each one of the above circuit configurations is very different
and produces different circuit characteristics with regards to input impedance, output impedance and gain whether this is voltage gain, current
gain or power gain and this is summarised in the table below.
Bipolar Transistor Characteristics
The static characteristics for a Bipolar Transistor can be divided into the following three main groups.
|Input Characteristics:-|| Common Base - ||ΔVEB / ΔIE|
| || Common Emitter - ||ΔVBE / ΔIB|
|Output Characteristics:-|| Common Base - ||ΔVC / ΔIC|
| || Common Emitter - ||ΔVC / ΔIC|
|Transfer Characteristics:-|| Common Base - ||ΔIC / ΔIE|
| || Common Emitter - ||ΔIC / ΔIB|
with the characteristics of the different transistor configurations given in the following table:
|Output Impedance||Very High||High||Low|
|Power Gain||Low||Very High||Medium|
In the next tutorial about Bipolar Transistors, we will look at
the NPN Transistor in more detail when used in the common emitter configuration
as an amplifier as this is the most widely used configuration due to its flexibility and high gain.
We will also plot the output characteristics curves commonly associated with amplifier circuits as
a function of the collector current to the base current.