Bipolar Transistor Tutorial, The BJT Transistor
Bipolar Transistor Tutorial, The BJT Transistor
Bipolar Transistor Tutorial, The BJT Transistor
Bipolar Transistor
In the diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material 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 di erent 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 “ampli cation” (analogue electronics). Then bipolar transistors have the ability to
operate within three di erent regions:
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The word Transistor is a combination of the two words Transfer 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 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 ) respectively.
Bipolar Transistors are current regulating devices that control the amount of current owing through them in
A Typical
proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch.
Bipolar Transistor
The principle of operation of the two transistor types PNP and NPN, is exactly the same the only di erence being in
their biasing and the polarity of the power supply for each type.
Common Base Con guration – has Voltage Gain but no Current Gain.
Common Emitter Con guration – has both Current and Voltage Gain.
Common Collector Con guration – has Current Gain but no Voltage Gain.
The input current owing 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 con guration “attenuates” the input signal.
This type of ampli er con guration is a non-inverting voltage ampli er 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 input characteristics
represent that of a forward biased diode while the output characteristics represent that of an illuminated photo-diode.
Also this type of bipolar transistor con guration 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 con guration 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 ampli er circuits such as microphone pre-ampli er or radio frequency
( Rf ) ampli ers due to its very good high frequency response.
The common emitter ampli er con guration produces the highest current and power gain of all the three bipolar transistor
con gurations. 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.
In this type of con guration, the current owing out of the transistor must be equal to the currents owing into the transistor as the
emitter current is given as Ie = Ic + Ib.
As the load resistance ( RL ) is connected in series with the collector, the current gain of the common emitter transistor con guration is
quite large as it is the ratio of Ic/Ib. A transistors current gain is given the Greek symbol of Beta, ( β ).
As the emitter current for a common emitter con guration is de ned 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 owing 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. So if a transistor has a Beta value of say 100, then one electron will ow
from the base terminal for every 100 electrons owing between the emitter-collector terminal.
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 owing into the collector terminal, “Ib” is the current owing into the base terminal and “Ie” is the current
owing out of the emitter terminal.
Then to summarise a little. This type of bipolar transistor con guration has a greater input impedance, current and power gain than that
of the common base con guration but its voltage gain is much lower. The common emitter con guration is an inverting ampli er circuit.
This means that the resulting output signal is 180o “out-of-phase” with the input voltage signal.
The common collector, or emitter follower con guration 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 emitter con guration has a current gain approximately equal to the β value of the transistor itself. In the common collector
con guration 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
con guration also has both the collector current and the input current of the base owing through it. Then the current gain of the circuit
is given as:
This type of bipolar transistor con guration 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 con guration) therefore, providing good current ampli cation
with very little voltage gain.
We can now summarise the various relationships between the transistors individual DC currents owing through each leg and its DC
current gains given above in the following table.
with the generalised characteristics of the di erent transistor con gurations 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
con guration as an ampli er as this is the most widely used con guration due to its exibility and high gain. We will also plot the output
characteristics curves commonly associated with ampli er circuits as a function of the collector current to the base current.
172 Comments
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S Sunidhi Tiwari
Excellent..
S Shivkumar Jadhao
Thanks gives all information about bipolar transistor
c christopher
i love it
V Venky
Good
D DJ
This site is superb…All the information is there at one place
Easy to understand
Thanks
R RISHIKESH
I like your website.
This is very helpful in electronics elds.
H Helal
Lecture was good. But still have confusions on practical use of these.
Wayne Storr
??? no it doesn’t
P Pavankumarkv
Electronics basics in history feature
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