Republic of the Philippines

Biliran Province State University

(formerly NAVAL STATE UNIVERSITY)
School of Engineering
ISO 9001:2015 CERTIFIED

MODULE 5

BASIC TRANSISTOR
APLLICATION, AMPLIFIER,
SWITCH
Electronic Circuits: Devices and Analysis (Lecture)

(EE 243)

Prepared By:
Engr. Dante Añasco

Instructor
Introduction

Transistor was called as “Transit Resistor” or “Transfer Resistor” initially. These are the
elemental unit of electronic circuits. They are so heavily used that you cannot imagine a
PCB without this component. This article will discuss about what is Transistor, its
configuration, classification, working principle, applications, advantages and
disadvantages.

What is Transistor?
Transistors are electronic devices which forms the basic and major component of any
electronic circuits. Earlier Transistors were made of Germanium which were temperature
sensitive and slowly were replaced with silicon. Silicon Transistors are cheaper to
manufacture. They are elementary units of microchips and computers.

Applications of Transistor
The applications include:

 Transistors are used in oscillators and modulators as amplifiers.

 They are used in digital circuits as switches.
 Transistors are used in Radio-frequency circuits for wireless systems.
 Transistor switches are used in Burglar alarms, industrial control circuits, memories
and microprocessors.
 They are used in Sub Wordline Driver (SWD) to produce low frequency currents.
 MOSFET’s are used in Chopper circuits.
 JFET, MOSFET can act as a passive element like Resistor.

Advantages of Transistor
The advantages are:

 Transistors are compact in size.

 They provide high voltage gain and requires less supply voltage.
 They do not require heating power as they do not have filament.
 Transistors have higher life expectancy than vacuum tubes.
 Controlling high power circuits is easier.
 Circuit design is simpler.
Disadvantages of Transistor
The disadvantages are:

 Leakage current is amplified in the Common Emitter circuit.

 They have lower Power dissipation below 300W.
 Transistors except FET (Field Effect Transistors) have low input impedance.
 They are temperature dependent.

Types of Transistors

There are a varieties and different types of transistors available in today's market
including Bipolar, Darlington, IGBT, and MOSFET Transistors.

 Bipolar Transistor - A Bipolar Junction Transistor (BJT) is a three-terminal

electronic device made of doped semiconductor material and may be used in
amplifying or switching applications. Bipolar transistors are so named because
their operation involves both electrons and holes. A bipolar transistor will have
terminals that are labeled: emitter, collector, base. A small current at the base
terminal (passing from the base to the emitter) can modify or switch a much larger
current between the collector and emitter terminals.
 Darlington Transistor - The Darlington Transistor is actually two bipolar
transistors, connected in such a way that the current amplified by the first transistor
is amplified even further by the second one. This model offers a higher common-
emitter current gain than if both types of transistors are separated and can even
take up less space because both transistors can share a collector.
 IGBT Transistor - An Insulated Gate Bipolar Transistor (IGBT) is a three-terminal
power semiconductor device typically used as an electronic switch. IGBT's are
types of transistors that are capable of switching electric power in many modern
appliances such as electric cars, trains, variable speed refrigerators, air-
conditioners and even stereo systems with switching amplifiers.
 MOSFET Transistor - A Metal-Oxide-Semiconductor Field-Effect Transistor
(MOFET) is used in integrated circuits to control the conductivity of a channel.
MOSFETs are highly dependent on negative and positive charges. They have
many purposes, including limiting a device's power levels, storing data, and being
used as a switch for a wide variety of electronic devices.
Transistor Amplifier

A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias
voltage applied to the emitter base junction, makes it remain in forward biased condition.
This forward bias is maintained regardless of the polarity of the signal. The below figure
shows how a transistor looks like when connected as an amplifier.

he low resistance in input circuit, lets any small change in input signal to result in an
appreciable change in the output. The emitter current caused by the input signal
contributes the collector current, which when flows through the load resistor R L, results
in a large voltage drop across it. Thus a small input voltage results in a large output
voltage, which shows that the transistor works as an amplifier.
Example
Let there be a change of 0.1v in the input voltage being applied, which further produces
a change of 1mA in the emitter current. This emitter current will obviously produce a
change in collector current, which would also be 1mA.
A load resistance of 5kΩ placed in the collector would produce a voltage of
5 kΩ × 1 mA = 5V
Hence it is observed that a change of 0.1v in the input gives a change of 5v in the output,
which means the voltage level of the signal is amplified.

Performance of Amplifier

As the common emitter mode of connection is mostly adopted, let us first understand a
few important terms with reference to this mode of connection.
Input Resistance
As the input circuit is forward biased, the input resistance will be low. The input resistance
is the opposition offered by the base-emitter junction to the signal flow.
By definition, it is the ratio of small change in base-emitter voltage (ΔVBE) to the resulting
change in base current (ΔIB) at constant collector-emitter voltage.
Input resistance, Ri=ΔVBEΔIBRi=ΔVBEΔIB
Where Ri = input resistance, VBE = base-emitter voltage, and IB = base current.

Output Resistance
The output resistance of a transistor amplifier is very high. The collector current changes
very slightly with the change in collector-emitter voltage.
By definition, it is the ratio of change in collector-emitter voltage (ΔVCE) to the resulting
change in collector current (ΔIC) at constant base current.
Output resistance = Ro=ΔVCEΔICRo=ΔVCEΔIC
Where Ro = Output resistance, VCE = Collector-emitter voltage, and IC = Collector-emitter
voltage.

Effective Collector Load

The load is connected at the collector of a transistor and for a single-stage amplifier, the
output voltage is taken from the collector of the transistor and for a multi-stage amplifier,
the same is collected from a cascaded stages of transistor circuit.
By definition, it is the total load as seen by the a.c. collector current. In case of single
stage amplifiers, the effective collector load is a parallel combination of R C and Ro.
Effective Collector Load, RAC=RC//RoRAC=RC//Ro
=RC×RoRC+Ro=RAC=RC×RoRC+Ro=RAC
Hence for a single stage amplifier, effective load is equal to collector load R C.
In a multi-stage amplifier (i.e. having more than one amplification stage), the input
resistance Ri of the next stage also comes into picture.
Effective collector load becomes parallel combination of RC, Ro and Ri i.e,
Effective Collector Load, RAC=RC//Ro//RiRAC=RC//Ro//Ri
RC//Ri=RCRiRC+RiRC//Ri=RCRiRC+Ri
As input resistance Ri is quite small, therefore effective load is reduced.

Current Gain
The gain in terms of current when the changes in input and output currents are observed,
is called as Current gain. By definition, it is the ratio of change in collector current (ΔI C)
to the change in base current (ΔIB).
Current gain, β=ΔICΔIBβ=ΔICΔIB
The value of β ranges from 20 to 500. The current gain indicates that input current
becomes β times in the collector current.

Voltage Gain
The gain in terms of voltage when the changes in input and output currents are observed,
is called as Voltage gain. By definition, it is the ratio of change in output voltage (ΔVCE)
to the change in input voltage (ΔVBE).
Voltage gain, AV=ΔVCEΔVBEAV=ΔVCEΔVBE
=Changeinoutputcurrent×effectiveloadChangeininputcurrent×inputresi
stance=Changeinoutputcurrent×effectiveloadChangeininputcurrent×inputresistance
=ΔIC×RACΔIB×Ri=ΔICΔIB×RACRi=β×RACRi=ΔIC×RACΔIB×Ri=ΔICΔIB×RACRi=
β×RACRi
For a single stage, RAC = RC.
However, for Multistage,
RAC=RC×RiRC+RiRAC=RC×RiRC+Ri
Where Ri is the input resistance of the next stage.

Power Gain
The gain in terms of power when the changes in input and output currents are observed,
is called as Power gain.
By definition, it is the ratio of output signal power to the input signal power.
Power gain, AP=(ΔIC)2×RAC(ΔIB)2×RiAP=(ΔIC)2×RAC(ΔIB)2×Ri
=(ΔICΔIB)×ΔIC×RACΔIB×Ri=(ΔICΔIB)×ΔIC×RACΔIB×Ri
= Current gain × Voltage gain
Hence these are all the important terms which refer the performance of amplifiers.

Working of Transistor as a Switch

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. We will learn
different operating modes (Active, Saturation and Cut-off) of a Transistor, how a
transistor works as a switch (both NPN and PNP) and some practical application
circuits using transistor as a switch.

Transistors is a three-layer, three-terminal semiconductor device, which is often used in

signal amplification and switching operations. As one of the significant electronic devices,
transistor has found use in enormous range of applications such as embedded systems,
digital circuits and control systems.

You can find Transistors in both digital and analog domains as they are extensively used
for different application usage like switching circuits, amplifier circuits, power supply
circuits, digital logic circuits, voltage regulators, oscillator circuits and so on.

This article mainly concentrates on the switching action of the transistor and gives a brief
explanation of transistor as a switch.

A Brief Note on BJT

There are two main families of Transistors: Bipolar Junction Transistors (BJT) and Field
Effect Transistors (FETs). The Bipolar Junction Transistor or simply BJT is a three-layer,
three terminal and two junction semiconductor device. It consists of two PN Junctions
coupled back-to-back with a common middle layer.

Whenever we say the term ‘transistor’, it often refers to BJT. It is a current controlled
device, where the output current is controlled by the input current. The name bipolar
indicates that two types of charge carriers i.e., Electrons and Holes conduct current in the
BJT, where holes are positive charge carriers and electrons are negative charge carriers.
The transistor has three regions, namely base, emitter and collector. The emitter is a
heavily doped terminal and emits electrons into the base. Base terminal is lightly doped
and passes the emitter-injected electrons on to the collector. The collector terminal is
moderately doped and collects electrons from base. This collector is large when
compared to the other two regions so it can dissipate more heat.

BJTs are of two types: NPN and PNP. Both these function in the same way but they differ
in terms of biasing and power supply polarity. In PNP transistor, N-type material is
sandwiched between two P-type materials whereas in case of NPN transistor P-type
material is sandwiched between two N-type materials.

These two transistors can be configured into different types like common emitter, common
collector and common base configurations.

If you are looking for working of MOSFET as a Switch, then first learn the basics
of MOSFET.

Operating Modes of Transistors

Depending on the biasing conditions like forward or reverse, transistors have three major
modes of operation namely cutoff, active and saturation regions.

Active Mode
In this mode, the transistor is generally used as a current amplifier. In active mode, two
junctions are differently biased that means emitter-base junction is forward biased
whereas collector-base junction is reverse biased. In this mode, current flows between
emitter and collector and the amount of current flow is proportional to the base current.

Cutoff Mode

In this mode, both collector base junction and emitter base junction are reverse biased.
As both the PN Junctions are reverse biased, there is almost no current flow except small
leakage currents (usually in the order of few nano amps or pico amps). BJT in this mode
is switched OFF and is essentially an open circuit.

Cutoff Region is primarily used in switching and digital logic circuits.

Saturation Mode

In this mode of operation, both the emitter-base and collector-base junctions are forward
biased. Current flows freely from collector to emitter with almost zero resistance. In this
mode, the transistor is fully switched ON and is essentially a close circuit.

Saturation Region is also primarily used in switching and digital logic circuits.
The below figure shows the output characteristics of a BJT. In the below figure, the cutoff
region has the operating conditions when the output collector current is zero, zero base
input current and maximum collector voltage. These parameters cause a large depletion
layer, which further doesn’t allow current to flow through the transistor. Therefore, the
transistor is completely in OFF condition.
Similarly, in the saturation region, a transistor is biased in such a way that maximum base
current is applied that results in maximum collector current and minimum collector-emitter
voltage. This causes the depletion layer to become small and to allow maximum current
flow through the transistor. Therefore, the transistor is fully in ON condition.

Hence, from the above discussion, we can say that transistors can be made to work as
ON/OFF solid-state switch by operating transistor in cutoff and saturation regions. This
type of switching application is used for controlling LEDs, motors, lamps, solenoids, etc.

Transistor as a Switch

A transistor can be used for switching operation for opening or closing of a circuit. This
type solid state switching offers significant reliability and lower cost when compared to
conventional relays.

Both NPN and PNP transistors can be used as switches. Some of the applications use a
power transistor as switching device, at that time it may necessary to use another signal
level transistor to drive the high-power transistor.

NPN Transistor as a Switch

Based on the voltage applied at the base terminal of a transistor switching operation is
performed. When a sufficient voltage (VIN > 0.7 V) is applied between the base and
emitter, collector to emitter voltage is approximately equal to 0. Therefore, the transistor
acts as a short circuit. The collector current VCC / RC flows through the transistor.

Similarly, when no voltage or zero voltage is applied at the input, transistor operates in
cutoff region and acts as an open circuit. In this type of switching connection, load (here
an LED is used as a load) is connected to the switching output with a reference point.
Thus, when the transistor is switched ON, current will flow from source to ground through
the load.
Example of NPN Transistor as a Switch

Consider the below example, where base resistance RB = 50 KΩ, collector resistance
RC = 0.7 KΩ, VCC is 5V and the beta value is 125. At the base, an input signal varying
between 0V and 5V is given. We are going to see the output at the collector by varying
the VI at two states that is 0 and 5V as shown in figure.
IC = VCC / RC, when VCE = 0

IC = 5V / 0.7 KΩ

IC = 7.1 mA

Base Current IB = IC / β

IB = 7.1 mA / 125

IB = 56.8 µA

From the above calculations, the maximum or peak value of the collector current in the
circuit is 7.1mA when VCE is equal to zero. And the corresponding base current for this
collector current is 56.8 µA.

So, it is clear that when the base current is increased beyond the 56.8 micro ampere,
then the transistor comes into the saturation mode.
Consider the case when zero volt is applied at the input. This causes the base current to
be zero and as the emitter is grounded, emitter base junction is not forward biased.
Therefore, the transistor is in OFF condition and the collector output voltage is equal to
5V.

When VI = 0V, IB = 0 and IC =0,

VC = VCC – (IC * RC)

= 5V – 0

= 5V

Consider that input voltage applied is 5 volts, then the base current can be determined
by applying Kirchhoff’s voltage law.

When VI = 5V,

IB = (VI – VBE) / RB

For silicon transistor, VBE = 0.7 V

Thus, IB = (5V – 0.7V) / 50 KΩ

= 86 µA, which is greater than 56.8 µA

Therefore, as the base current is greater than 56.8 micro ampere current, the transistor
will be driven to saturation i.e., it is fully ON, when 5V is applied at the input. Thus, the
output at the collector becomes approximately zero.

PNP Transistor as a Switch

PNP transistor works same as NPN for a switching operation, but the current flows from
the base. This type of switching is used for negative ground configurations. For the PNP
transistor, the base terminal is always negatively biased with respect to the emitter.
In this switching, base current flows when the base voltage is more negative. Simply, a
low voltage or more negative voltage makes the transistor to short circuit otherwise, it will
be open circuit.

In this connection, load is connected to the transistor switching output with a reference
point. When the transistor is turned ON, current flows from the source through transistor
to the load and finally to the ground.

Example of PNP Transistor as a Switch

Similar to the NPN transistor switch circuit, PNP circuit input is also base, but the emitter
is connected to constant voltage and the collector is connected to ground through the
load as shown in figure.
In this configuration, base is always biased negatively with respect to the emitter by
connecting the base at negative side and the emitter at the positive side of the input
supply. So, the voltage VBE is negative and the emitter supply voltage with respect to the
Collector is positive (VCE positive).

Therefore, for the conduction of transistor emitter must be more positive with respect to
both collector and base. In other words, base must be more negative with respect to the
emitter.

For calculating the base and collector currents following expressions are used.

IC = IE – IB

IC = β * IB

IB = IC / β

Consider the above example, that the load requires 100 milli ampere current and the
transistor has the beta value of 100. Then the current required for the saturation of the
transistor is
Minimum base current = collector current / β

= 100 mA / 100

= 1mA

Therefore, when the base current is 1 mA, the transistor will be fully ON. But practically
30 percent of more current is required for guaranteed saturation of transistor. So, in this
example the base current required is 1.3mA.

Practical Examples of Transistor as a Switch

Transistor to Switch the LED

As discussed earlier, the transistor can be used as a switch. The schematic below shows
how a transistor is used to switch the Light Emitting Diode (LED).

 When the switch at the base terminal is open, no current flows through the base
so the transistor is in the cutoff state. Therefore, the transistor acts as an open-
circuit and the LED becomes OFF.
 When the switch is closed, base current starts flowing through the transistor and
then drives into saturation, which results in LED to turn ON.
 Resistors are placed to limit the currents through the base and LED. It is also
possible to vary the intensity of LED by varying the resistance in the base current
path.
Transistor to Operate the Relay

It is also possible to control the relay operation using a transistor. With a small circuit
arrangement of a transistor able to energize the coil of the relay so that the external load
connected to it is controlled.

 Consider the below circuit to know the operation of a transistor to energize the
relay coil. The input applied at the base causes to drive the transistor into
saturation region, which further results the circuit becomes short circuit. So, the
relay coil gets energized and relay contacts get operated.
 In inductive loads, particularly switching of motors and inductors, sudden removal
of power can keep a high potential across the coil. This high voltage can cause
considerable damage to the rest circuit. Therefore, we have to use the diode in
parallel with inductive load to protect the circuit from induced voltages of the
inductive load.
Transistor to Drive the Motor
 A transistor can also be used to drive and regulate the speed of the DC motor in a
unidirectional way by switching the transistor in regular intervals of time as shown
in the below figure.
 As mentioned above, the DC motor is also an inductive load so we have to place
a freewheeling diode across it to protect the circuit.
 By switching the transistor in cutoff and saturation regions, we can turn ON and
OFF the motor repeatedly.
 It is also possible to regulate the speed of the motor from standstill to full speed by
switching the transistor at variable frequencies. We can get the switching
frequency from control device or IC like microcontroller.
Transistor Switch

Let's look at the most fundamental transistor-switch circuit: an NPN switch. Here we use an NPN to
control a high-power LED:
Kindly please click the link to watch……

https://www.youtube.com/watch?v=WZD9RZoMhVE
https://www.youtube.com/watch?v=um2HfSDu-LI
https://www.youtube.com/watch?v=UIEGKvCfDOA
https://www.youtube.com/watch?v=h9zL235htnQ

REFFERENCES:

https://www.google.com/search?q=basic+transistor+application&sxsrf=AOaemvJ0q
WVp6ofDNoS_1SFl3aabWHttEQ:1636706791172&source=lnms&tbm=vid&sa=X&ve
d=2ahUKEwj69433t5L0AhW-
sFYBHVvGATYQ_AUoAnoECAEQBA&biw=1242&bih=568&dpr=1.1

https://electricalfundablog.com/transistor-classification-configuration/

https://learn.sparkfun.com/tutorials/transistors/applications-ii-amplifiers

https://learn.sparkfun.com/tutorials/transistors/applications-i-switches