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
The construction and terminal voltages for an NPN transistor are
shown above. The voltage between the Base and Emitter ( VBE ), is
positive at the Base and negative at the Emitter because for an NPN transistor,
the Base terminal is always positive with respect to the Emitter. Also the
Collector supply voltage is positive with respect to the Emitter ( VCE ). So
for an NPN transistor to conduct the Collector is always more positive with
respect to both the Base and the Emitter.
NPN Transistor Connections
Then the voltage sources are connected to an NPN transistor as
shown. The Collector is connected to the supply voltage VCC via the
load resistor, RL which
also acts to limit the maximum current flowing through the device. The Base
supply voltage VB is
connected to the Base resistor RB, which again is used to limit the maximum
Base current.
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.
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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.
Example No1
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.
Example No2
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 Base-emitter 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, Q-point
for short and this is shown below.
Single Stage Common Emitter Amplifier
Circuit
Output Characteristics Curves of 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 (CE) 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 Dynamic 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
Then we can define a NPN
Transistor as being normally "OFF" but a small input
current and a small positive voltage at its Base (B) relative to its Emitter (E) will
turn it "ON" allowing a much large Collector-Emitter current to flow.
NPN transistors conduct when Vc is much greater than Ve.
In the next tutorial about Bipolar Transistors, we will look at the opposite or complementary 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.
