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Experiment no.5
§ Transistor as a Switch §
5.1 Objective:
In this Transistor tutorial, we will learn about the working of a
Transistor as a Switch. Switching and Amplification are the two areas
of applications of Transistors and Transistor as a Switch is the basis
for many digital circuits.
5.2 Theory:
“William Shockley” invented the transistor in 1947. A transistor is a
three-terminal semiconductor device which can be used for switching
applications, amplification of weak signals and in quantities of
thousands and millions of transistors are interconnected and
embedded into a tiny integrated circuit/chip, which makes computer
memories. A Transistor switch, which is used for opening or closing of
a circuit that means the transistor, is commonly used as a switch in
the electronic devices only for the low voltage applications because of
its low power consumption. Transistor work as a switch when it is in
cutoff and saturation regions. In this article, we will discuss how to use
a transistor as a switch.
Figure (1)
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5.2.1 Types of BJT transistors:
A transistor consists of two PN junctions; these junctions are formed
by sandwiching either N-type or P-type semiconductor material
between a pair of opposite type of semiconductor materials.
Bipolar junction transistors are classified into two types:
NPN
PNP
Figure (2)
The transistor has three terminals, namely Base, Emitter, and
Collector.
1. The emitter is a heavily doped terminal and it emits the
electrons into the Base region.
2. The Base terminal is lightly doped and passes the emitter-
injected electrons onto the collector.
3. The collector terminal is intermediately doped and collects
electrons from the Base.
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NPN-type transistor is the composition of two N-type doped
semiconductor materials between a P-type doped
semiconductor layer as shown above.
PNP-type transistors are the composition of two P-type doped
semiconductor materials between an N-type doped
semiconductor layers as shown above. The functioning of both
NPN and PNP transistor is same but differ in terms of their
biasing and power supply polarity.
5.2.2 Transistor as a Switch:
If either the circuit uses the BJT transistor as a switch, then the
biasing of the transistor, NPN or PNP is arranged to operate the
transistor at the both sides of the I-V characteristics curves shown
below. A transistor can be operated in three modes, active region,
saturation region and cut-off region:
In the active region, transistor works as an amplifier. The two
operating regions of transistor Saturation Region (fully ON) and
the Cut-off Region (fully OFF) were used to operate a transistor
switch.
Operating Regions: We can observe from the above
characteristics, the pink shaded area at the bottom of the
curves represents the Cut-off region and the blue area to the
left represent the Saturation region of the transistor. These
transistor regions are defined as
Figure (3)
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Cut-off Region: The operating conditions of the transistor are
zero input base current (IB=0), zero output collector
current(Ic=0), and maximum collector voltage (VCE) which
results in a large depletion layer and no current flowing through
the device. Therefore, the transistor is switched to “Fully-OFF”.
So we can define the cut-off region when using a bipolar
transistor as a switch as being, bother the junctions of NPN
transistors are reverse biased, VB< 0.7v and Ic=0. Similarly, for
PNP transistor, the emitter potential must be –ve with respect to
the base of the transistor.
Cut-off Region
Figure (4)
Then we can define the “cut-off region” or “OFF mode” when using
a bipolar transistor as a switch as being, both junctions reverse
biased, IC = 0 and VB < 0.7v. For a PNP transistor, the Emitter
potential must be -ve with respect to the Base.
Saturation Region: In this region, the transistor will be biased
so that the maximum amount of base current (IB) is applied,
resulting in maximum collector current(IC=VCC/RL) and then
resulting in the minimum collector-emitter voltage (VCE ~ 0)
drop. At this condition, the depletion layer becomes as small as
the possible and maximum current flowing through the
transistor. Therefore, the transistor is switched “Fully-ON”.
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Saturation Region
Figure (5)
Saturation Region: The definition of “saturation region” or “ON
mode” when using a bipolar NPN transistor as a switch as being, both
the junctions are forward biased, IC = Maximum and VB > 0.7v. For a
PNP transistor, the Emitter potential must be +ve with respect to the
Base.
5.3 Procedure:
1. Connect the circuit that shown in the figure below:
100Ω
100Ω
5V dc
4V AC
Figure (6)
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2. Set the AC source (4V) and frequency (200 HZ) (Square wave) and
draw it.
3. Set the oscilloscope (first terminal) at the common terminal of
collector and resistance (RL) and (second terminal of OSC) to the
emitter of transistor.
4. Draw the wave shown in the screen of the Oscilloscope (Square
wave).
5.4 Discussion:
1. What is a transistor? What is it Types? Draw it symbols?
2. What are the transistor working areas?
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