- BJT stands for bipolar junction transistor. A small base current can control a larger collector current, allowing it to function as a switch. - A transistor can be used to turn a lamp on and off by positioning its collector and emitter where the contacts of a switch were previously located. The base current is provided by connecting a switch between the base and collector. - Transistors operate in three modes: cutoff (non-conducting), saturation (fully conducting), and active. Cutoff and saturation correspond to a switch being open or closed, used to control circuit current.

1. Basic
Electronics
Lecture 5
Course instructor
Rida Shifa

2. BJT
 BJT stands for Bi-polar Junction Transistor
 A transistor’s collector current is
proportionally limited by its base current, it
can be used as a sort of current-controlled
switch
 A relatively small flow of electrons sent
through the base of the transistor has the
ability to exert control over a much larger flow
of electrons through the collector
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3. Circuitry Representation
 Suppose we had a lamp that we wanted to
turn on and off with a switch
 Such a circuit would be extremely simple as in
Figure
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4. Description
 For the sake of illustration, let’s insert a
transistor in place of the switch to show how it
can control the flow of electrons through the
lamp
 Remember that the controlled current through
a transistor must go between collector and
emitter
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5. Contd.
 Since it is the current through the lamp that
we want to control, we must position the
collector and emitter of our transistor where
the two contacts of the switch were
 We must also make sure that the lamp’s
current will move against the direction of the
emitter arrow symbol to ensure that the
transistor’s junction bias will be correct
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6. Selection PNP or NPN
 The choice between NPN and PNP is really
arbitrary
 All that matters is that the proper current
directions are maintained for the sake of
correct junction biasing (electron flow
going against the transistor symbol’s arrow)
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7. For NPN
 Going back to the NPN transistor in our
example circuit, we are faced with the need to
add something more so that we can have
base current
 Without a connection to the base wire of the
transistor, base current will be zero, and the
transistor cannot turn on, resulting in a lamp
that is always off
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8. Contd.
 Remember that for a NPN transistor, base
current must consist of electrons flowing from
emitter to base (against the emitter arrow
symbol, just like the lamp current)
 Perhaps the simplest thing to do would be to
connect a switch between the base and
collector wires of the transistor as in Figure
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9. Figure
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10. As shown in figure (a)
 If the switch is open as in Figure (a), the base
wire of the transistor will be left “floating” (not
connected to anything) and there will be no
current through it
 In this state, the transistor is said to be cutoff
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11. As shown in figure (b)
 If the switch is closed as in Figure above (b),
electrons will be able to flow from the emitter
through to the base of the transistor, through the
switch, up to the left side of the lamp, back to the
positive side of the battery
 This base current will enable a much larger flow of
electrons from the emitter through to the collector,
thus lighting up the lamp
 In this state of maximum circuit current, the
transistor is said to be saturated
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12. Review
 Transistors may be used as switching elements to
control DC power to a load
 The switched (controlled) current goes between emitter
and collector; the controlling current goes between
emitter and base.
 When a transistor has zero current through it, it is said
to be in a state of cutoff (fully non conducting).
 When a transistor has maximum current through it, it is
said to be in a state of saturation (fully conducting)
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13. Explanation of each
 As we all know transistors operate in 3
modes:
1.Cut off
2.Saturation
3.Active mode
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14. Operation as switch(recalling)
 When a transistor is in the fully-off state (like an
open switch), it is said to be cutoff
 Conversely, when it is fully conductive between
emitter and collector (passing as much current
through the collector as the collector power supply
and load will allow), it is said to be saturated
 These are the two modes of operation explored
thus far in using the transistor as a switch
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15. Active mode
 Active region is one in which Base emitter
junction is forward biased and Base Collector
junction will be reverse biased in a transistor
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16. 16

17. Saturation mode
 Saturation region is one in which both Emitter
Base and Base Collector junctions of the
transistor are forward biased
 In this region high currents flows through the
transistor, as both junctions of the transistor
are forward biased and bulk resistance
offered is very much less
 Transistor in saturation region is considered
as on state in digital logic
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18. Contd.
 This is due to the fact that as both junctions of
transistor are forward biased along with
electron current flowing from emitter to base
in active region there will be additional
component of electron current flowing from
collector to base
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19. Contd.
 Small changes in Collector to base forward
voltage leads to large variations in collector
currents
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21. Cut off mode
 In this region both junctions of the transistor are
reverse biased
 Hence transistor in cut off does not conduct any
currents expect for small reverse saturation currents
that flow across junctions
 In cutoff condition emitter current is zero and the
collector current consists of small reverse saturation
currents
 The transistor when used as switch is operated in cutoff
on condition and saturation regions which corresponds
to switch off and on condition respectively
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23. BJT as Amplifier
 As we all know that BJT operates as an
amplifier when it is in the active mode
 We are needed to see the classes of amplifier
as well
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24. Characteristics
 The main operating characteristics of an ideal
amplifier are linearity, signal gain, efficiency
and power output but in real world amplifiers
there is always a trade off between these
different characteristics
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25. Amplifiers in speakers
 Generally, large signal or power amplifiers are
used in the output stages of audio amplifier
systems to drive a loudspeaker load
 A typical loudspeaker has an impedance of
between 4Ω and 8Ω, thus a power amplifier
must be able to supply the high peak currents
required to drive the low impedance speaker
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26. Classes of amplifiers
 One method used to distinguish the electrical
characteristics of different types
of amplifiers is by “class”, and as such
amplifiers are classified according to their
circuit configuration and method of operation
 Amplifier Classes is the term used to
differentiate between the different amplifier
types
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27. Amplifier classes
 Amplifier Classes represent the amount of the
output signal which varies within the amplifier
circuit over one cycle of operation when excited by
a sinusoidal input signal
 The classification of amplifiers range from entirely
linear operation with very low efficiency, to entirely
non-linear operation but with a much higher
efficiency, while others are a compromise between
the two
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28. Classification
 Amplifier classes are mainly lumped into two
basic groups:
1. A, B, AB and C
2. D, E, F, G, S, T
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29. First class of amplifiers
 The first are the classically controlled
conduction angle amplifiers forming the more
common amplifier classes of A, B, AB and C
 These are defined by the length of their
conduction state over some portion of the
output waveform, such that the output stage
transistor operation lies somewhere between
being “fully-ON” and “fully-OFF”
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30. Second class of amplifiers
 The second set of amplifiers are the newer
so-called “switching” amplifier classes of D, E,
F, G, S, T etc
 These use digital circuits and pulse width
modulation (PWM) to constantly switch the
signal between “fully-ON” and “fully-OFF”
driving the output hard into the transistors
saturation and cut-off regions
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31. Common
 The most commonly constructed amplifier
classes are those that are used as audio
amplifiers, mainly class A, B, AB and C
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