2. Transistors
A transistor is a semiconductor device used
to amplify or switch electronic signals
and electrical power
It is composed of semiconductor material
usually with at least three terminals for
connection to an external circuit
2
3. Contd.
A voltage or current applied to one pair of the
transistor's terminals controls the current
Because the controlled (output) power can be
higher than the controlling (input) power, a
transistor can amplify a signal
Today, some transistors are packaged individually,
but many more are found embedded in integrated
circuits.
3
4. Contd.
The transistor is the fundamental building
block of modern electronic devices, and is
ubiquitous in modern electronic systems
The essential usefulness of a transistor
comes from its ability to use a small signal
applied between one pair of its terminals to
control a much larger signal at another pair of
terminals. This property is called gain
4
5. As an amplifier
It can produce a stronger output signal, a
voltage or current, which is proportional to a
weaker input signal; that is, it can act as
an amplifier
Alternatively, the transistor can be used to
turn current on or off in a circuit as an
electrically controlled switch, where the
amount of current is determined by other
circuit elements.
5
7. Types of transistors
Following are the two types of transistors,
they have significant difference in the circuit in
which they are used:
1.Bipolar transistor
2.Field effective transistor
7
8. Bipolar transistor
A bipolar transistor has terminals
labeled base, collector, and emitter
A small current at the base terminal (that is,
flowing between the base and the emitter) can
control or switch a much larger current
between the collector and emitter terminals
8
10. Bipolar junction transistor
It has three pins: Base (b), collector (c) and
emitter (e)
And it comes in two versions: NPN and PNP
10
11. Starting
The invention of the bipolar transistor in 1948
ushered in a revolution in electronics
Technical feats previously requiring relatively
large, mechanically fragile, power-hungry vacuum
tubes were suddenly achievable with tiny,
mechanically rugged, power-thrifty specks of
crystalline silicon
This revolution made possible the design and
manufacture of lightweight, inexpensive electronic
devices that we now take for granted
11
12. BJT
A bipolar transistor consists of a three-layer
“sandwich” of doped (extrinsic) semiconductor
materials, either P-N-P or N-P-N
12
13. Difference
The functional difference between a PNP
transistor and an NPN transistor is the proper
biasing (polarity) of the junctions when
operating.
For any given state of operation, the current
directions and voltage polarities for each kind
of transistor are exactly opposite each other
13
15. Regulators
Bipolar transistors work as current-controlled
current regulators
In other words, transistors restrict the amount
of current passing
The main current that is controlled goes from
collector to emitter, or from emitter to collector,
depending on the type of transistor it is (PNP
or NPN, respectively)
15
16. Contd.
The small current that controls the main
current goes from base to emitter, or from
emitter to base, once again depending on the
kind of transistor it is (PNP or NPN,
respectively)
16
17. Representation
According to the standards of semiconductor
symbology, the arrow always
points against the direction of electron flow
17
18. Calling them bi-polar
Bipolar transistors are called bipolar because
the main flow of electrons through them takes
place in two types of semiconductor material:
P and N, as the main current goes from
emitter to collector (or vice versa)
In other words, two types of charge carriers—
electrons and holes—comprise this main
current through the transistor.
18
19. Base current
Controlling current is usually referred to
simply as the base current because it is the
only current that goes through the base wire
of the transistor
19
20. Controlled current
The large, controlled current is referred to as
the collector current because it is the only
current that goes through the collector wire
20
22. Types of bi-polar junction
Semiconductor offer less resistance to flow
current in one direction and high resistance is
another direction and we can call transistor as
the device mode of the semiconductor
The bipolar junction transistors consist of two
types of transistors. Which, given us
1.Point contact
2.Junction transistor
22
23. Junction transistors
By comparing two transistors the junction
transistors are used more than point type
transistors
Further the junction transistors are classified
into two types
There are three electrodes for each junction
transistor they are emitter, collector, and base
23
24. PNP junction transistor
In PNP transistor, the emitter is more positive with
base and also with respect to the collector
The PNP transistor is a three terminal device
which is made from the semiconductor material
The three terminals are collector, base, and the
emitter and the transistor are used for switching
and amplifying applications
24
25. Working
Generally the collector terminal is connected to
the positive terminal and the emitter to a negative
supply with resistor, either the emitter or collector
circuit
To the base terminal the voltage is applied and it
operates transistor as an ON/OFF state
The transistor is in OFF state when the base
voltage is same as the emitter voltage.
25
26. Contd.
The transistor mode is in ON state when the
base voltage decreases with respect to the
emitter
By using this property the transistor can act
on both applications like switch and amplifier
26
27. NPN junction transistor
The NPN transistor is exactly opposite to the
PNP transistor
The NPN transistor contains three terminals
which are same as the PNP transistor which
are emitter, collector, and base
27
28. Working
Generally the positive supply is given to the
collector terminal and the negative supply to the
emitter terminal with a resistor either the emitter or
collector or emitter circuit
To the base terminal the voltage is applied and it
operated as an ONN/OFF state of a transistor
The transistor is in OFF state when the base
voltage is same as the emitter.
28
29. Contd.
If the base voltage is increased with respect
to the emitter then the transistor mode is in
ON state
By using this condition the transistor can act
like both applications which are amplifier and
switch
29
31. Biasing
Biasing in electronics means establishing
predetermined voltages or currents at various
points of an electronic circuit
Many electronic devices such
as transistors and vacuum tubes, whose
function is processing time-varying
(AC) signals also require a steady (DC)
current or voltage to operate correctly
31
32. Biasing in BJT
The process of obtaining a certain DC
collector current at a certain DC collector
voltage by setting up the operating point is
called biasing
After establishing the operating point, when
an input signal is applied, the output signal
should not move the transistor either to
saturation or to cut-off
32
33. Types of biasing
Fixed bias.
Collector-to-base bias.
Fixed bias with emitter resistor.
Voltage divider bias or potential divider.
Emitter bias.
33
34. Transistor biasing
Transistor Biasing is the process of setting a
transistors DC operating voltage or current
conditions to the correct level so that any AC
input signal can be amplified correctly by the
transistor.
34
35. BJT transistors
Biasing of the bipolar junction transistor
(BJT) is the process of applying external voltages
to it
In order to use the BJT for any application like
amplification, the two junctions of the transistor
CB and BE should be properly biased according
to the required application
Depending on whether the two junctions of
the transistor are forward or reverse biased, a
transistor is capable of operating in three different
modes
35
36. Types of BJT transistors
Cut off mode
Saturation mode
Active mode
36
37. Cut off mode
The BJT is fully off in this state
In the cutoff mode both the base emitter as
well as collector base junction is reverse
biased
The BJT is equivalent to an open switch in
this mode
37
38. Saturation mode
The transistor is fully on in this state, the CB
as well as BE junctions are forward biased
The BJT operates like a closed switch in the
saturation mode
38
39. Active mode
In order to use the transistor as an amplifier, it
must be operated in the active mode
The BE junction is forward biased whereas
the CB junction is reverse biased
39
40. Advantages of BJT
The bipolar junction transistor (BJT) has a
large gain bandwidth.
The BJT shows better performance at high
frequency.
The BJT has a better voltage gain.
The BJT can be operated in low or high power
applications.
The BJT has high current density.
There is low forward voltage drop.
40
41. Disadvantages of BJT
The bipolar junction transistor (BJT) produces
more noise
The BJT are more effected by radiation.
BJT has a low thermal stability.
The switching frequency of BJT is low.
41
42. Amplifier
Transistor is a device which can act as an
amplifier(also as switch)
The amount of Amplification is measured in
terms of gain
For a transistor to act as a amplifier this gain
has to be constant
If it is not so , it is meaningless to have
amplification
42
43. Contd.
Everyone wants faithful amplification so that
they can use it in speakers, carrier waves for
signal transmission etc
Amplification is done by superimposing the
input base current(signal) on the dc collector
voltage
But sadly transistor cannot superimpose the
input signals on all dc collector voltage
43
45. Description
For any constant input base current, the
collector current should be constant but it is
not so in some regions of the curve
In those regions the transistor either
completely conducts (ON state) or it
completely stops conducting(OFF
state),which is not suitable for amplification
45
46. Q-point
The collector voltage should be selected carefully
This point on the output characteristics curve is called
as Q point
The operating point (quiescent point)(Q point) is
the constant dc voltage or current given to the collector
terminal for its operation
When transistor acts as a amplifier, this point has to be
maintained constant, that’s where biasing helps us, by
providing this constant dc collector voltage
46
47. Getting Q point
Q point lies on the load line
Load line is the line joining the points,
maximum output current at zero collector
voltage and the maximum output voltage at
zero collector current
DC load line is also obtained by the dc
analysis of the transistor
47
48. Contd.
Generally midpoint of this line is
considered as Q-point for amplification so
that, it can amplify upper and lower part of the
input signal
We can see that in those region, output
collector current is constant for unchanged
input base current.
48
49. Contd.
If Q point is near the saturation region(current
rises rapidly) or cut off region(current flow is
very low),then transistors act as ON and OFF
switch respectively
So in general it is the Q point which
determines the actual operation of the
transistor (switch/amplifier)
49
