The document discusses the bipolar junction transistor (BJT), including its history, operation, and applications. A BJT has three terminals - the base, collector, and emitter - and allows current to flow between the collector and emitter when a small base current is applied. It can be used to amplify signals and operate in different modes like active, cut-off, and saturation depending on terminal voltages. Common configurations include common base, common emitter, and common collector, which differ in their input/output impedances, current/voltage gain, and other properties. The BJT was a pivotal invention that enabled solid-state electronics and the information age.

1. Walter Brattain
William Shockley
John Bardeen

4. THE WORD TRANSISTOR IS A CONTRACTION
OF “CURRENT- TRANSFERRING RESISTOR.”
A BIPOLAR transistor has two P-N junctions.
The Bipolar
Junction Transistor

7. If CPU will have to control the motor:
The CPU can’t be connected to the motor directly
because it can’ handle the power rating of the
motor (normally high)

9. If a microphone is to be connected to
the speaker:

10. • For this condition, we can use BJT to amplify
our voice. As we speak into microphone(input),
we can hear our voice in a louder sound in the
speaker (output). This is called amplification.

11. Trivia about BJT
• The invention of the BJT in 1948 at the Bell
Telephone Laboratories ushered in the era of
solid-state circuits, which led to electronics
changing the way we work, play, and indeed,
live.
• The invention of the BJT also eventually led to
the dominance of information technology and
the emergence of the knowledge-based
economy.

12. • Solid-state electronics are those circuits or
devices built entirely from solid materials and
in which the electrons, or other charge
carriers, are confined entirely within the solid
material.
• Common solid-state devices
include transistors, microprocessor chips,
and DRAM.

13. • A bipolar junction transistor is a threeterminal device that acts like a currentcontrolled switch (because we use current as
input, and amplified current as output).
• It has three terminals, the base, the collector
and the emitter.

14. • If we put a small current into one of the
terminals, called the base, then the switch is
“on”—current may flow between the other
two terminals, called the emitter and the
collector. If no current is put into the base,
then the switch is “off”—no current flows
between the emitter and the collector

15. The circuit symbol for the
npn transistor is shown in
Figure BJT-1(d). Notice that
the symbol contains a
subtle arrow in the
direction of positive
current flow. This also
reminds us that the baseto-emitter junction is a pn
junction, the same as a
diode whose symbol has
an arrow pointing in the
same direction.

16. NPN versus PNP
Generally, PNP and NPN transistors can perform the same functions. The
differences are the polarities of the voltages and the directions of the
resulting currents. In most applications, an PN device can be replaced with a
PNP device or vice versa, the power-supply polarity can be reversed, and the
circuit will work in the same way—as long as the new device has the
appropriate specifications.

17. Die of a KSY34 high-frequency NPN
transistor, base and emitter connected
via bonded wires

18. Doping level and Channel Width
• Base – most lightly doped and its length is 1/150 of the
length of the whole transistor. Usually called or labelled as
the “control” because the flow of current through the
transistor depends critically on what happens at this
electrode.
• Emitter – most highly doped because it has the
responsibility of emitting the carriers that will circulate
during operation.
• Collector – has the widest channel because it has the
responsibility of collecting the carriers that the emitter is
emitting.

19. Biasing (provided with a certain voltage, or
made to carry a certain current)
• We usually consider NPN as an example
because it is more commonly used.
• The normal method of biasing an NPN
transistor is to have the collector voltage
positive with respect to the emitter.
• Typical dc voltages for a transistor power
supply range between 3 V and about 50 V. A
typical voltage is 12 V.

20. (1.) Zero Bias
• Condition where the base of a transistor is at the
same voltage as the emitter. (no potential
difference).
• This prevents current from flowing between the
emitter and collector, unless a signal is injected at
the base to change the situation.
• Such a signal must, at least momentarily, attain a
positive voltage equal to or greater than the
forward breakover voltage of the E-B junction.

21. (2.)Reverse Bias
• Condition where a battery is connected
between the base and the emitter in the
circuit with the polarity such that VB becomes
negative with respect to the emitter.

22. (2.)Reverse Bias
• No current flows through the E-B junction in
this situation (as long as the new battery
voltage is not so great that avalanche
breakdown occurs). A signal might be injected
at the base to cause a flow of current, but
such a signal must attain, at least
momentarily, a positive voltage high enough
to overcome both the reverse bias and the
forward breakover voltage of the junction.

23. (3.) Forward Bias
• A condition where VB is made positive with
respect to the emitter, starting at small
voltages and gradually increasing.
• If the forward bias is less than the forward
breakover voltage, no current will flow.
• But as the base voltage VB reaches the
breakover point, the E-B junction will start to
conduct.

24. (3.) Forward Bias
• The base-collector (B-C) junction of a bipolar
transistor is normally reverse-biased. It will
remain reverse-biased as long as VB is less than
the supply voltage (in this case 12 V). In practical
transistor circuits, it is common for VB to be set at
a fraction of the supply voltage. Despite the
reverse bias of the B-C junction, a significant
emitter-collector current, called collector current
and denoted Ic, will flow once the E-B junction
conducts.

25. (3.) Forward Bias
• In a real transistor circuit, the meter reading
will jump when the forward breakover voltage
of the E-B junction is reached. Then even a
small rise in VB, attended by a rise in IB, will
cause a large increase in Ic.

26. (3.) Forward Bias
• If VB continues to rise, a point will eventually
be reached where the Ic versus VB curve levels
off. The transistor is then said to be saturated
or in saturation. It is wide open, conducting as
much as it can.

28. BJT’s Modes of Operation
• The two junctions of BJT can be either forward
or reverse-biased.
• The BJT can operate in different modes
depending on the junction bias.
• The BJT operates in active mode for amplifier
circuits.
• Switching applications utilize both the cutoff
and saturation modes.

29. BJT’s Modes of Operation
• 1. Active Mode
• 2. Cut-off Mode
• 3. Saturation Mode

30. BJT’s Modes of Operation
Mode
Emitter-Base
Junction
Collector-Base
Junction
Active
Forward
Reverse
Cut-off
Reverse
Reverse
Saturation
Forward
Forward

31. BJT’s Modes of Operation

32. Transistor Circuit Configurations

33. Common Base Configuration
• 1. Input signal to the
emitter and output is
taken from the
collector.
• 2. The input circuit is
very low impedance,
usually 1 to 50 ohms.

34. Common Emitter Configuration
• Input – base
• Output – collector
• Input circuit is low
impedance. (25 to 5k
ohms)

35. Common Collector Configuration
• Input- base
• Output – emitter
• Input circuit is very high
impedance and output
impedance is LOW.

36. Comparison : Summary
CB
CE
CC
INPUT
IMPEDANCE
LOW
MODERATE
HIGH
OUTPUT
IMPEDANCE
HIGH
MODERATE
LOW
CURRENT
LOW = 1
GAIN
VOLTAGE GAIN HIGH
MODERATE
HIGH
MODERATE
LOW
POWER GAIN
MODERATE
HIGH
LOW
PHASE SHIFT
NONE
180 DEG
NONE