BJT Biotech 106

BJT
Bipolar Junction Transistor

Dr. Saurabh Ratra
Assistant Professor, DEEIT
Bipolar Junction Transistor (BJT)
• A bipolar junction transistor is a three-terminal semiconductor

device that consists of two p-n junctions which are able to amplify
or magnify a signal.
• It is a current controlled device.
• The three terminals of the BJT are the base, the collector, and the
emitter.
• A signal of a small amplitude applied to the base is available in the
amplified form at the collector of the transistor.
• This is the amplification provided by the BJT.
• It does require an external source of DC power supply to carry out
the amplification process
Symbolic Representation
Construction
• BJT is a semiconductor device that is constructed with 3 doped
semiconductor Regions i.e. Base, Collector & Emitter separated by
2 p-n Junctions.
• Bipolar transistors are manufactured in two types, PNP and NPN,
and are available as separate components, usually in large quantities.
• The prime use or function of this type of transistor is to amplify
current. This makes them useful as switches or amplifiers.
• They have a wide application in electronic devices like mobile
phones, televisions, radio transmitters, and industrial control.
PNP Transistor
• In PNP BJT, the n-type semiconductor is sandwiched between the two p-
type semiconductors. The two p-type semiconductors act as emitter and
collector respectively while the n-type semiconductor acts as a base.
• The current enters the transistor through the emitter such that the emitter-
base junction is forward biased and the collector-base junction is reverse
biased.
NPN Transistor
• In NPN BJT, p-type semiconductor is sandwiched between the two
n-type semiconductors. The two n-type semiconductors act as
emitter and collector respectively while the p-type semiconductor
acts as a base.
• Current entering the emitter, base, and collector has the sign
convention of positive while the current that leaves the transistor has
the sign convention of negative.
Configuration of Bipolar Junction Transistors
• Common Emitter Configuration – has both voltage and current

gain
• The common Collector Configuration – has no voltage gain but
has a current gain
• The common base configuration – has no current gain but has a
voltage gain
Characteristics of Common Base (CB) Configuration
• The emitter to base voltage VEB can be varied by adjusting the

potentiometer R1. A series resistor RS is inserted in the emitter circuit to
limit the emitter current IE
Input Characteristic
• The curve plotted between emitter current I E and the emitter-base

voltage VEB at constant collector base voltage V CB is called input
characteristic curve.
Input Characteristic
Conclusions
• For a specific value of VCB, the curve is a diode characteristic in the
forward region. The PN emitter junction is forward biased.
• When the value of the voltage base current increases the value of
emitter current increases slightly. The junction behaves like a better
diode.
• The emitter current IE increases with the small increase in emitter-
base voltage VEB. It shows that input resistance is small.
Output Characteristic conclusions
• The active region of the collector-base junction is reverse biased, the collector
current IC is almost equal to the emitter current IE. The transistor is always
operated in this region.
• The curve of the active regions is almost flat. The large charges in VCB
produce only a tiny change in IC .The circuit has very high output resistance ro.
Transistor Load Line Analysis
• The load line analysis of transistor means for the given value of
collector-emitter voltage we find the value of collector current.
• This can be done by plotting the output characteristic and then
determine the collector current IC with respect to collector-emitter
voltage VCE.
DC Load Line
• The DC load represents the desirable combinations of the collector

current and the collector-emitter voltage.
• It is drawn when no signal is given to the input, and the transistor
becomes bias.
• By applying Kirchhoff’s voltage law to the collector circuit, we get,
• The above equation shows that the VCC and RC are the constant
value, and it is the first-degree equation which is represented by the
straight line on the output characteristic. This load line is known as a
DC load line.
• The collector-emitter voltage VCE is maximum when the collector
current IC = 0 then from the equation (1) we get,
• The collector current IC becomes maximum when the collector-
emitter voltage VCE = 0 then from the equation (1) we get
• This gives the second point on the collector current axis as shown in
the figure above.
• By adding the points A and B, the DC load line is drawn. With the
help of load line, any value of collector current can be determined.

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BJT

Bipolar Junction Transistor

• A bipolar junction transistor is a three-terminal semiconductor

• Common Emitter Configuration – has both voltage and current

• The emitter to base voltage VEB can be varied by adjusting the

• The curve plotted between emitter current I E and the emitter-base

• The DC load represents the desirable combinations of the collector

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