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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 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:
1. Active Region - the transistor operates as an amplifier and Ic = β.Ib
2. Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0
Typical Bipolar Transistor
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, NPN and PNP, 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 ) respectively.
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 NPN and PNP, 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 NPN and PNP 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
1. Common Base Configuration - has Voltage Gain but no Current Gain.
2. Common Emitter Configuration - has both Current and Voltage Gain.
3. 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 photo-diode. 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
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 response.
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 resulting in the output signal being 180 out-of-phase with the input
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 inphase. 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.
Common Base Common Emitter -
ΔVEB / ΔIE
ΔVBE / ΔIB
Common Base Common Emitter -
ΔVC / ΔIC
ΔVC / ΔIC
Transfer Characteristics:- Common Base Common Emitter -
ΔIC / ΔIE
ΔIC / ΔIB
with the characteristics of the different transistor configurations given in the following table:
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.
The NPN Transistor
In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. An NPN
(Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used
transistor type being the NPN Transistor. We also learnt that the transistor junctions can be biased in one of three
different ways - Common Base, Common Emitter and Common Collector. In this tutorial we will look more closely
at the "Common Emitter" configuration using NPN Transistors with an example of the construction of a NPN
transistor along with the transistors current flow characteristics is given below.
An NPN Transistor Configuration
Note: Conventional current flow.
We know that the transistor is a "current" operated device (Beta model) and that a large current ( Ic ) flows freely
through the device between the collector and the emitter terminals when the transistor is switched "fully-ON".
However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the
same time thus allowing the Base to act as a sort of current control input. The transistor current in an NPN transistor
is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or
nowadays Beta, ( β ). The value of β can be large up to 200 for standard transistors, and it is this large ratio between
Ic and Ib that makes the NPN transistor a useful amplifying device when used in its active region as Ib provides the
input and Ic provides the output. Note that Beta has no units as it is a ratio.
Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ),
and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current Ie is the
product of a very small base current plus a very large collector current, the value of alpha α, is very close to unity,
and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999
α and β Relationship in a NPN Transistor
By combining the two parameters α and β we can produce two mathematical expressions that gives the relationship
between the different currents flowing in the transistor.
The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low
power type bipolar transistors. The value of Beta for most standard NPN transistors can be found in the
manufactures datasheets but generally range between 50 - 200.
The equation above for Beta can also be re-arranged to make Ic as the subject, and with a zero base current ( Ib =
0 ) the resultant collector current Ic will also be zero, ( β x 0 ). Also when the base current is high the corresponding
collector current will also be high resulting in the base current controlling the collector current. One of the most
important properties of the Bipolar Junction Transistor is that a small base current can control a much larger
collector current. Consider the following example.
An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib required to switch a
resistive load of 4mA.
Therefore, β = 200, Ic = 4mA and Ib = 20µA.
One other point to remember about NPN Transistors. The collector voltage, ( Vc ) must be greater and positive with
respect to the emitter voltage, ( Ve ) to allow current to flow through the transistor between the collector-emitter
junctions. Also, there is a voltage drop between the Base and the Emitter terminal of about 0.7v (one diode volt drop)
for silicon devices as the input characteristics of an NPN Transistor are of a forward biased diode. Then the base
voltage, ( Vbe ) of a NPN transistor must be greater than this 0.7V otherwise the transistor will not conduct with the
base current given as.
Where: Ib is the base current, Vb is the base bias voltage, Vbe is the base-emitter volt drop (0.7v) and Rb is the
base input resistor. Increasing Ib, Vbe slowly increases to 0.7V but Ic rises exponentially.
An NPN Transistor has a DC base bias voltage, Vb of 10v and an input base resistor, Rb of 100kΩ. What will be the
value of the base current into the transistor.
Therefore, Ib = 93µA.
The Common Emitter Configuration.
As well as being used as a semiconductor switch to turn load currents "ON" or "OFF" by controlling the Base signal to
the transistor in ether its saturation or cut-off regions, NPN Transistors can also be used in its active region to
produce a circuit which will amplify any small AC signal applied to its Base terminal with the Emitter grounded. If a
suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within
its linear active region, an inverting amplifier circuit called a single stage common emitter amplifier is produced.
One such Common Emitter Amplifier configuration of an NPN transistor is called a Class A Amplifier. A "Class A
Amplifier" operation is one where the transistors Base terminal is biased in such a way as to forward bias the Baseemitter junction. The result is that the transistor is always operating halfway between its cut-off and saturation regions,
thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input
signal superimposed upon this DC biasing voltage. Without this "Bias Voltage" only one half of the input waveform
would be amplified. This common emitter amplifier configuration using an NPN transistor has many applications but is
commonly used in audio circuits such as pre-amplifier and power amplifier stages.
With reference to the common emitter configuration shown below, a family of curves known as the Output
Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values
of Base current, (Ib) are applied to the transistor for transistors with the same β value. A DC "Load Line" can also be
drawn onto the output characteristics curves to show all the possible operating points when different values of base
current are applied. It is necessary to set the initial value of Vce correctly to allow the output voltage to vary both up
and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point, Qpoint for short and this is shown below.
Single Stage Common Emitter Amplifier Circuit
Output Characteristics Curves for a Typical Bipolar Transistor
The most important factor to notice is the effect of Vce upon the collector current Ic when Vce is greater than about
1.0 volts. We can see that Ic is largely unaffected by changes in Vce above this value and instead it is almost entirely
controlled by the base current, Ib. When this happens we can say then that the output circuit represents that of a
"Constant Current Source". It can also be seen from the common emitter circuit above that the emitter current Ie is
the sum of the collector current, Ic and the base current, Ib, added together so we can also say that " Ie = Ic + Ib "
for the common emitter configuration.
By using the output characteristics curves in our example above and also Ohm´s Law, the current flowing through the
load resistor, (RL), is equal to the collector current, Ic entering the transistor which inturn corresponds to the supply
voltage, (Vcc) minus the voltage drop between the collector and the emitter terminals, (Vce) and is given as:
Also, a straight line representing the Load Line of the transistor can be drawn directly onto the graph of curves above
from the point of "Saturation" ( A ) when Vce = 0 to the point of "Cut-off" ( B ) when Ic = 0 thus giving us the
"Operating" or Q-point of the transistor. These two points are joined together by a straight line and any position along
this straight line represents the "Active Region" of the transistor. The actual position of the load line on the
characteristics curves can be calculated as follows:
Then, the collector or output characteristics curves for Common Emitter NPN Transistors can be used to predict
the Collector current, Ic, when given Vce and the Base current, Ib. A Load Line can also be constructed onto the
curves to determine a suitable Operating or Q-point which can be set by adjustment of the base current. The slope of
this load line is equal to the reciprocal of the load resistance which is given as: -1/RL
In the next tutorial about Bipolar Transistors, we will look at the opposite or compliment form of the NPN
Transistor called the PNP Transistor and show that the PNP Transistor has very similar characteristics to their
NPN transistor except that the polarities (or biasing) of the current and voltage directions are reversed.
The PNP Transistor
The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial.
Basically, in this type of transistor constr ...