BIPOLAR TRANSISTORS

Bipolar Transistor Basics

In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short.

Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:

• 1. Active Region   -   the transistor operates as an amplifier and Ic = β.Ib
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• 2. Saturation   -   the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
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• 3. Cut-off   -   the transistor is "fully-OFF" operating as a switch and Ic = 0

Typical Bipolar Transistor

The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, PNP and NPN, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.

The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.

Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.

Bipolar Transistor Construction

The construction and circuit symbols for both the PNP and NPN bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of "conventional current flow" between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol.

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.

• 1. Common Base Configuration   -   has Voltage Gain but no Current Gain.
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• 2. Common Emitter Configuration   -   has both Current and Voltage Gain.
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• 3. Common Collector Configuration   -   has Current Gain but no Voltage Gain.

The Common Base (CB) Configuration

As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.

The Common Base Transistor Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av) for a common base configuration is therefore given as:

Common Base Voltage Gain

Where: Ic/Ie is the current gain, alpha (α) and RL/Rin is the resistance gain.

The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequency response.

The Common Emitter (CE) Configuration

In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.

The Common Emitter Amplifier Circuit

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, (β). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.

Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180^o out-of-phase with the input voltage signal.

The Common Collector (CC) Configuration

In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.

The Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

The Common Collector Current Gain

This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.

Bipolar Transistor Summary

Then to summarise, the behaviour of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to input impedance, output impedance and gain whether this is voltage gain, current gain or power gain and this is summarised in the table below.

Bipolar Transistor Characteristics

The static characteristics for a Bipolar Transistor can be divided into the following three main groups.

Input Characteristics:-	Common Base  -	ΔVEB / ΔIE
	Common Emitter  -	ΔVBE / ΔIB
Output Characteristics:-	Common Base  -	ΔVC / ΔIC
	Common Emitter  -	ΔVC / ΔIC
Transfer Characteristics:-	Common Base  -	ΔIC / ΔIE
	Common Emitter  -	ΔIC / ΔIB

with the characteristics of the different transistor configurations given in the following table:

Characteristic	Common Base	Common Emitter	Common Collector
Input Impedance	Low	Medium	High
Output Impedance	Very High	High	Low
Phase Angle	0o	180o	0o
Voltage Gain	High	Medium	Low
Current Gain	Low	Medium	High
Power Gain	Low	Very High	Medium

In the next tutorial about Bipolar Transistors, we will look at the NPN Transistor in more detail when used in the common emitter configuration as an amplifier as this is the most widely used configuration due to its flexibility and high gain. We will also plot the output characteristics curves commonly associated with amplifier circuits as a function of the collector current to the base current.

Electromagnetism

Electromagnetism is produced when an electrical current flows through a simple conductor such as a piece of wire or cable. A small magnetic field is created around the conductor with the direction of this magnetic field with regards to its "North" and "South" poles being determined by the direction of the current flowing through the conductor. Magnetism plays an important role in Electrical and Electronic Engineering because without it components such as relays, solenoids, inductors, chokes, coils, loudspeakers, motors, generators, transformers, and electricity meters etc, would not work if magnetism did not exist. Then every coil of wire uses the effect of electromagnetism when an electrical current flows through it. But before we can look at Magnetism and especially Electromagnetism in more detail we need to remember back to our physics classes of how magnets and magnetism works.

The Nature of Magnetism

Magnets can be found in a natural state in the form of a magnetic ore, with the two main types being Magnetite also called "iron oxide", ( FE₃O₄ ) and Lodestone, also called "leading stone". If these two natural magnets are suspended from a piece of string, they will take up a position inline with the earths magnetic field always pointing north. A good example of this effect is the needle of a compass. For most practical applications these natural occurring magnets can be disregarded as their magnetism is very low and because nowadays, man-made artificial magnets can be produced in many different shapes, sizes and magnetic strengths.

There are basically two forms of magnetism, "Permanent Magnets" and "Temporary Magnets", with the type being used dependant upon its application. There are many different types of materials available to make magnets such as iron, nickel, nickel alloys, chromium and cobalt and in their natural state some of these elements such as nickel and cobalt show very poor magnetic quantities on their own. However, when mixed or "alloyed" together with other materials such as iron or aluminium peroxide they become very strong magnets producing unusual names such as "alcomax", "hycomax", "alni" and "alnico".

Magnetic material in the non-magnetic state has its molecular structure in the form of loose magnetic chains or individual tiny magnets loosely arranged in a random pattern. The overall effect of this type of arrangement results in zero or very weak magnetism as this haphazard arrangement of each molecular magnet tends to neutralise its neighbour. When the material is Magnetised this random arrangement of the molecules changes and the tiny unaligned and random molecular magnets become "lined-up" in such a way that they produce a series magnetic arrangement. This idea of the molecular alignment of ferromagnetic materials is known as Weber's Theory and is illustrated below.

Magnetic Molecule Alignment of a Piece of Iron and a Magnet

Weber's theory is based on the fact that all atoms have magnetic properties due to the spinning action of the atoms electrons. Groups of atoms join together so that their magnetic fields are all rotating in the same direction. Magnetic materials are composed of groups of tiny magnets at a molecular level around the atoms, and a magnetised material will have most of its tiny magnets lined up in one direction only to produce a north pole in one direction and a south pole in the other direction. Likewise, a material that has its tiny molecular magnets pointing in all directions will have its molecular magnets neutralised by its neighbouring magnet, thereby neutralising any magnetic effect. These areas of molecular magnets are called "domains".

Any magnetic material will produce a magnetic field itself which depends on the degree of alignment of magnetic domains in the material set up by orbital and spinning electrons. This degree of alignment can be specified by a quantity known as magnetisation, M. In an unmagnetised material, M = 0, but some of the domains remain aligned over small regions in the material once the magnetic field is removed. The effect of applying a magnetising force to the material is to align some of the domains to produce a non-zero magnetisation value.

Once the magnetising force has been removed, the magnetism within the material will either remain or decay away quiet quickly depending on the magnetic material being used. This ability of a material to retain its magnetism is called Retentivity and materials which are required to retain their magnetism will have a high retentivity and are used to make permanent magnets, while those materials required to loose their magnetism quickly such as soft iron cores for relays and solenoids will have a very low retentivity.

Magnetic Flux

All magnets, no matter what their shape, have two regions called magnetic poles with the magnetism both in and around a magnetic circuit producing a definite chain of organised and balanced pattern of invisible lines of flux around it. These lines of flux are collectively referred to as the "magnetic field" of the magnet. The shape of this magnetic field is more intense in some parts than others with the area of the magnet that has the greatest magnetism being called "poles". At each end of a magnet is a pole.

These lines of flux (called a vector field) can not be seen by the naked eye, but they can be seen visually by using iron fillings sprinkled onto a sheet of paper or by using a small compass to trace them out. Magnetic poles are always present in pairs, there is always a region of the magnet called the North-pole and there is always an opposite region called the South-pole. Magnetic fields are always shown visually as lines of force that give a definite pole at each end of the material where the flux lines are more dense and concentrated. The lines which go to make up a magnetic field showing the direction and intensity are called Lines of Force or more commonly "Magnetic Flux" and are given the Greek symbol, Phi ( Φ ) as shown below.

Lines of Force from a Bar Magnets Magnetic Field

As shown above, the magnetic field is strongest near to the poles of the magnet were the lines of flux are more closely spaced. The general direction for the magnetic flux flow is from the North ( N ) to the South ( S ) pole. In addition, these magnetic lines form closed loops that leave at the north pole of the magnet and enter at the south pole. Magnetic poles are always in pairs. However, magnetic flux does not actually flow from the north to the south pole or flow anywhere for that matter as magnetic flux is a static region around a magnet in which the magnetic force exists. In other words magnetic flux does not flow or move it is just there and is not influenced by gravity. Some important facts emerge when plotting lines of force:

• 1.  -  Lines of force NEVER cross.
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• 2.  -  Lines of force are CONTINUOUS.
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• 3.  -  Lines of force always form individual CLOSED LOOPS around the magnet.
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• 4.  -  Lines of force have a definite DIRECTION from North to South.
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• 5.  -  Lines of force that are close together indicate a STRONG magnetic field.
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• 6.  -  Lines of force that are farther apart indicate a WEAK magnetic field.

Magnetic forces attract and repel like electric forces and when two lines of force are brought close together the interaction between the two magnetic fields causes one of two things to occur:

• 1.  -  When adjacent poles are the same, (north-north or south-south) they REPEL each other.
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• 2.  -  When adjacent poles are not the same, (north-south or south-north) they ATTRACT each other.

It can be remembered by the famous expression that "opposites attract" and this interaction of magnetic fields is easily demonstrated with iron fillings. The effect upon the magnetic fields of the various combinations of poles as like poles repel and unlike poles attract can be seen below.

Magnetic Field of Like and Unlike Poles

When plotting magnetic field lines with a compass it will be seen that the lines of force are produced in such a way as to give a definite pole at each end of the magnet where the lines of force leave the North pole and re-enter at the South pole. Magnetism can be destroyed by heating or hammering the magnetic material, but cannot be destroyed or isolated by simply breaking the magnet into two pieces. If you take a bar magnet and break it into two pieces, each piece will have its own North pole and a South pole. If you take one of those pieces and break it into two again, each of the smaller pieces will have a North pole and a South pole. No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole. In order to make use of magnetism in electrical or electronic calculations, it is necessary to define the various aspects of magnetism.

The Magnitude of Magnetism

We now know that the lines of force or more commonly the magnetic flux around a magnetic material is given the Greek symbol, Phi, ( Φ ) with the unit of flux being the Weber, ( Wb ) after Wilhelm Eduard Weber. But the number of lines of force within a given unit area is called the "Flux Density" and since flux ( Φ ) is measured in ( Wb ) and area ( A ) in metres squared, ( m² ), flux density is therefore measured in Webers/Metre² or ( Wb/m² ) and is given the symbol B.

However, when referring to flux density in magnetism, flux density is given the unit of the Tesla after Nikola Tesla so therefore one Wb/m² is equal to one Tesla, 1Wb/m² = 1T. Flux density is proportional to the lines of force and inversely proportional to area so we can define Flux Density as:

Magnetic Flux Density

The symbol for magnetic flux density is B and the unit of magnetic flux density is the Tesla, T.

It is important to remember that all calculations for flux density are done in the same units, e.g., flux in webers, area in m2 and flux density in Teslas.

Example No1

The amount of flux present in a round magnetic bar was measured at 0.013 webers. If the material has a diameter of 12cm, calculate the flux density.

The cross sectional area of the magnetic material in m2 is given as:

The magnetic flux is given as 0.013 webers, therefore the flux density can be calculated as:

So the flux density is calculated as 1.15 Teslas.

When dealing with magnetism in electrical circuits it must be remembered that one Tesla is the density of a magnetic field such that a conductor carrying 1 ampere at right angles to the magnetic field experiences a force of one newton-metre length on it and this will be demonstrated in the next tutorial about Electromagnetism.

RF Unit Converter

RF Unit Converter Introduction

This tool is a unit converter for voltage and power. There is an input for impedance that allows the relationship between power and voltage.

Ohm

The unit for resistance or impedance. Resistance can define the relationship between voltage and current and voltage and power. Based upon ohms law the voltage and current relatinship is:

and power is

Vpeak – Peak Voltage

Peak voltage of an AC signal is the peak amplitude.

Vrms – RMS Voltage

The Root Mean Square Voltage or Vrms is:

uV – Microvolt (RMS)

This value is the RMS voltage in microvolts.

uV EMF

This value is microvolt with no termination or load.

uV PD

This value is microvolt with with a load. When a signal has a matched load then half of the voltage is droped across the load. This value is the same value as uV, The unit is just explicitly defined as having the load.

dBuV – dB Microvolts RMS

This unit is the decible of RMS microvolt.

dBuV EMF – dB Microvolts EMF

This unit is the decible of EMF microvolt.

dBuV PD – dB Microvolts PD

This unit is the decible of EMF microvolt.

W – Watts

Watts is a unit of power.

mW – Miliwatts

One thousandth of a watt.

uW – Microwatts

One millionth of a watt.

dBm

dBm is a power measurement and is the decibal of the power in mW.

dBuW

dBuW is a power measurement and is the decibal of the power in uW.

dBuW EMF

dBuW EMF is a power measurement and is the decibal of the power in uW. In a system with no termination or load.

dBuW PD

dBuW PD is a power measurement and is the decibal of the power in uW. In a system with a matched load.

dBpW

dBpW is a power measurement and is the decibal of the power in pW.

dBpW EMF

dBpW EMF is a power measurement and is the decibal of the power in pW. In a system with no termination or load.

dBpW PD

dBpW PD is a power measurement and is the decibal of the power in pW. In a system with a matched load.

Microchip PIC Microcontrollers Now with Configurable Logic

Microchip Technology has introduced new PIC10F, PIC12F, and PIC16F microcontrollers that boast configurable logic cells (CLCs), enabling Engineers to have peripherals talk to each other in new and interesting ways. These new microcontrollers are available in 6- to 20-pin packages.

Microchip PIC with new Blocks
Microchip PIC with new Blocks
The PIC10F(LF)32X and PIC1XF(LF)150X microcontrollers include new peripherals such as Configurable Logic Cells (CLCs), Complementary Waveform Generators (CWGs) and Numerically Controlled Oscillators (NCOs).

The CLC peripherals on the PIC10F(LF)32X and PIC1XF(LF)150X microcontrollers provide software control of configurable logic between peripherals and I/Os.

The CWG peripheral controls peripherals to generate complementary waveforms with dead-band control and auto shutdown.

The NCO peripheral provides linear frequency control at high resolution, targeting applications such as lighting ballast, tone generation and other resonant control circuits.

Microchip Configurable Logic Cell (CLC)

 

Microchip PIC with new Blocks

 

PIC Microcontrollers with CLC Features:
Features of these new PIC10 microcontrollers include:

    * PIC10F, PIC12F, and PIC16F cores at 16MHz
    * 1.75KBytes to 14KBytes Flash
    * 64Bytes to 512Bytes RAM
    * Watchdog
    * Power-On Reset
    * Two 8-bit timers
    * One 16-bit timer
    * Four 10-bit PWM
    * Up to 12-channels 10-bit ADC
    * one 5-bit DAC on some devices
    * Up to two comparators
    * EUSART
    * SPI
    * I2C
    * Configurable Logic Cell
    * Complementary Waveform Generator
    * Numerically Controlled Oscillators

Microchip Configurable Logic Cell (CLC)
Microchip Configurable Logic Cell (CLC)
Low Power performance for these parts are spec'ed at an Active Current of 30uA/MHz and sleep current of 20nA.

A free CLC Configuration Tool is available from Microchip.

Microchip PIC Microcontroller Packaging
The PIC10F(LF)320 and PIC10F(LF)322 microcontrollers are available in a 6-pin SOT-23 package, as well as 8-pin PDIP and 2 mm x 3 mm DFN packages.

The PIC12F(LF)1501 microcontrollers will be available in 8-pin PDIP, SOIC, MSOP and 2 mm x 3 mm DFN packages.

The PIC16F(LF)1503 microcontrollers will be in 14-pin PDIP, SOIC and TSSOP packages, as well as a 3 mm x 3 mm QFN package.

The PIC16F(LF)1507 microcontroller is available in 20-pin SSOP, PDIP, SOIC, and 4 mm x 4 mm QFN packages, as will the PIC16F(LF)1508/9 MCUs, when available.

Pricing and Availability
Pricing starts at $0.37 each, in 10,000-unit quantities. Samples and volume-production quantities of the PIC10F(LF)320, PIC10F(LF)322 and PIC16F(LF)1507 microcontrollers are available today.  The PIC1XF(LF)1501/3/8/9 microcontrollers are expected to be available for sampling and purchase within the next six months.

About Microchip Technology
Microchip Technology Inc. (NASDAQ:  MCHP) is a leading provider of microcontroller, analog and Flash-IP solutions.  The Microchip name and logo, HI-TECH C, MPLAB, and PIC are registered trademarks of Microchip Technology Inc. in the U.S.A., and other countries. mTouch, PICDEM, PICkit, and REAL ICE, are trademarks of Microchip Technology Inc. in the U.S.A., and other countries.  All other trademarks mentioned herein are the property of their respective companies.

Atmel AVR XMEGA-B1 Microcontroller is Lower Power with Innovative LCD

Atmel has introduced the Atmel XMEGA-B1 family of microcontrollers that is lower power, with support for LCD and high accuracy ADC.

Atmel XMEGA-B1 Block Diagram
Atmel XMEGA-B1 Block Diagram
The Atmel XMEGA microcontroller family has an 8-bit datapath with a 16-bit ALU. The Atmel XMEGA-B1 is manufactured in Atmel's proprietary low-leakage 0.25µ process. This microcontroller requires only one 1.6V-3.6V power supply.

The Atmel XMEGA-B1 is spec'ed at 100nA RAM retention mode, which retains RAM and can wakeup from any interrupt in 5µsec.

Features of the new Atmel XMEGA-B1 include:

    * 8/16-bit XMEGA core
    * Up to 128KBytes Flash
    * Up to 8KBytes boot Flash
    * Up to 46KBytes SRAM
    * Up to 4KBytes EEPROM
    * 4x40 Segment LCD controller with built-in ASCII character mapping
    * Watchdog
    * Real-time Clock (RTC)
    * 16-bit Timers
    * 32 PWM outputs
    * QTouch Capacitive Touch Peripheral
    * Check Sum module
    * AES/DES Crypto
    * 4-channel Direct Memory Access (DMA) Controller
    * Up to two 12-bit 2MSPS ADC
    * Up to 4-channels 12-bit 1MSPS DAC
    * USART
    * SPI
    * I2C
    * USB 2.0

Atmel XMEGA-B1 Block Diagram

 

The ultra-low-power LCD controller in the Atmel XMEGA-B1 microcontrollers supports up to 4x40 segments and has built-in ASCII character mapping. The LCD module draws only 3µA and includes LCD buffers and power supply. A SWAP mode allows flexible pinout configurations. Modes include programmable segment blinking scrolling text.

The Atmel AVR XMEGA-B1 family boasts high-precision analog peripherals, including two 12-bit analog-to-digital converters (ADCs) with programmable gain. The ADCs operate down to 1.6V operating voltage, and have a combined sample rate up to 4MSPS. Two 12-bit digital-to-analog converters (DACs) have high current outputs, while built-in current outputs enable embedded applications to remove external resistors or other constant current sources.

Pricing and Availability
"The new Atmel AVR XMEGA devices with USB are available now. Pricing starts at U.S. $2.00 each in 10,000-piece quantities."

About Atmel
"Atmel Corporation is a worldwide leader in the design and manufacture of microcontrollers, capacitive touch solutions, advanced logic, mixed-signal, nonvolatile memory and radio frequency (RF) components. Leveraging one of the industry's broadest intellectual property (IP) technology portfolios, Atmel is able to provide the electronics industry with complete system solutions focused on industrial, consumer, communications, computing and automotive markets."

Development of two laboratory experiments for teaching electrodynamic forces in an advanced course in electromechanical systems

Abstract In this paper, two experiments on electrodynamic forces designed for students on an advanced course in electromechanical systems are proposed. Details are first given for an experiment involving a pendulum system whose damping is controlled by the electrodynamic forces that are induced in a conducting plate. The pendulum system is modelled using Maxwell's equations and experimental results obtained from the system are compared with those estimated from the model developed. A second experiment is also presented, consisting of a magnetic levitation system where electrodynamic forces are induced in a moving conducting plate by two permanent magnets, resulting in lifting and drag forces on them. These two components of the electrodynamic forces, the lifting and drag forces, are analysed based on the magnetic field distribution, also verifying their dependence on plate speed and lifting height.


Keywords continuum electromechnics; electrical education; electrodynamics teaching; electromechanical systems; magnetic levitation

(ProQuest: ... denotes formulae omitted.)

In any advanced course in electromechanical systems, students usually study the distributed parameters modeling of electromagnetic systems, including electrodynamic forces on conducting materials.1-4 In this context, we propose in this paper two experiments where the electrodynamic forces significantly affect the systems' dynamics. The first experiment takes a pendulum system made of a conducting plate, which is subjected to a uniform and constant magnetic field. Due to the electrodynamic forces induced in the conducting plate, the pendulum damping can be controlled. The system is first modelled by the use of Maxwell's equations,5 assuming a set of assumptions coming from the system's geometry and its materials. Experimental results obtained with the pendulum system are compared with results estimated from the model to show the consistency and validity of the model assumptions and thus validate the model.

In a second experiment, two permanent magnets are located above a conducting plate which moves with a speed that can be controlled. The relative motion between the magnets and plate induce electric currents in the conducting sheet, producing a repulsive force that causes the lifting of the permanent magnets. The magnetic field distribution in the system is studied first to explain the appearance of drag and lifting forces. Following this, the co-dependence of these forces with speed of the conducting plate and the lifting height is discussed taking into account the experimental results obtained.

Experiment 1

Experimental setup

Figure 1(a) shows the experimental apparatus where a rectangular conducting plate made of aluminum is suspended by a pendulum with length r, and located in a gap (see this detail in Fig. 1(b)) where a constant magnetic flux density B0 of 70.2 mT is imposed by a magnetic circuit. Magnetic drag forces appearing in the conducting plate cause damping when it swings in the perpendicular direction to B0 (see Fig. 1(a)). The potential and kinetic energies of the pendulum are dissipated in the form of power losses in the aluminium plate.

The magnetic circuit shown in Fig. 1(b) is composed of two parts with an E-form. Also, one coil with 400 turns and located in the central leg was used to energise the magnetic circuit. The pendulum position was measured using an ultrasonic sensor.

Experiment 1 - distributed parameter modelling

Figure 2 is a schematic diagram of the experimental system shown in Fig. 1 but seen from the upper side. The conducting sheet shown in Fig. 2 has the shape of a long plate with uniform thickness d and conductivity σ, moving in the positive x direction with constant speed vx. This experiment considers a uniform and constant magnetic field B = -B0ey located in a rectangular region on the conducting plate with area lz0 which is imposed by an electromagnet not represented in the figure.

With the assumptions of a symmetric system and neglecting border effects, the system is studied in two dimensions only, -x and -y directions, with the z component of B assumed to be zero. The system's magnetic flux density becomes, thus, defined by:

... (1)

with the conducting plate moving with a linear speed defined by:

... (2)

The electric field E induced in the conducting plate is given by:

... (3)

which results in an induced density current according to Ohm's law J = σE of:

... (4)

Using (4) with the following two Maxwell equations

... (5)

and

... (6)

the equation establishing field B in the conducting plate becomes defined as:

... (7)

The -x and -y components of (7) can be obtained considering the earlier assumptions used in the definition of the magnetic flux density in (1), resulting in:

... (8)

... (9)

Considering that the thickness of the conducting plate is very small when compared with the distance between the magnetic poles (d

- the -x component of B inside the plate can be ignored, B^sub x^ [congruent with] 0;

- and it can be considered that the vertical component B^sub y^ only varies with the x direction, B^sub y^(x,y) [congruent with] B^sub y^(x).

Using these approximations (9) can be rewritten as:

... (10)

The solution of (10) can assume the form given by (11). In this equation, parameter R^sub m^ = σμv^sub x^l is usually called the magnetic Reynolds number, and parameters C1 and C2 are constants to be determined from two boundary conditions.

... (11)

First boundary condition

As indicated in Fig. 3, the first boundary condition is obtained by integrating Ampere's Law over the ABCD way yielding:

... (12)

where Jz is the -z component of the eddy current density induced in the conducting plate in z direction, and linked by the contour ABCD. Due to the plate's extreme thinness, we assume that the magnetic field component Hy does not vary appreciably over its width d and, from our previous assumptions, that the magnetic field component Hx becomes zero. Therefore (12) results in:

... (13)

where J^sub z^ = σv^sub x^B^sub y^.

Second boundary condition

This condition is obtained using the magnetic flux conservation through the plate surface that is located between the magnetic poles.B^sub 0^ being the magnetic flux density, imposed by the magnetic circuit it can be written:

... (14)

Resolving (12) and (13) for the constants C1 and C2, we obtain the analytic expression for the flux density distribution inside the conducting plate given by:

... (15)

The drag force appearing in the conducting plate can thus be determined by integrating the force density (16) through plate volume, yielding the result shown in (17). This result shows that the drag force depends on the integral of B^sub y^^sup 2^, being thus proportional to the power dissipated by the eddy currents.

... (16)

... (17)

Drag force

Because the aluminium plate is very thin and the induced currents are not considered to be high enough, in order for their magnetic field to distort the applied one B0, it has been assumed that the value of the flux density in the plate is constant:

... (18)

Substituting eqn (18) for eqn (17), the drag force becomes given by:

... (19)

The pendulum linear velocity v^sub x^ can be approximated by expression (20) for small pendulum angular displacements.

... (20)

The motion equation of the pendulum given by (21) results then in the approximated (22), where θ is the angular position, m is the pendulum mass, r its radius and I is the moment of inertia of the pendulum, being equal to I = mr^sub 2^.

... (21)

... (22)

Substituting (19) and (20) in eqn (22) and rearranging the equation terms, this results in the second-order position equation (23) for the pendulum system where the parameter Kθ becomes given by (24).

... (23)

... (24)

The roots of the characteristic equation for the differential equation (23) are:

... (25)

Where, because the pendulum is a damped oscillating system, relation (26) must be satisfied.

... (26)

The main dimensions of the pendulum system are: z^sub 0^ = 4 cm, d = 1 mm, l = 4 cm and r = 10 cm. The aluminium conductivity is equal to σ = 3 × 10^sup 7^ S*m^sup -1^ resulting in a mass to the conducting plate equal to m = 24.3 g. Substituting these parameter values in (26), this becomes satisfied, presenting a value of about 6 × 10^sup -4^, which is less than 1 as required for an oscillatory damping system.

Experiences in teaching electric drives based on basic modelling simulations and industrial a.c. drive measurements

Abstract The objective of this paper is to compare laboratory tests of industrial electric a.c. drives and simulation results obtained from the basic model introduced to our students in practical sessions. This basic model takes into account the electrical, electronic and control subsystems which comprise a simplifi ed electric a.c. drive. The proposed tests allow us to analyse the electric drive in four-quadrant operation, which are included in this paper. The main advantage of this approach is that students can verify the differences between real and simulated results and, in this way, they can also progress, strengthen and expand their theoretical knowledge. The proposed methodology has been implemented during the past few years in two optional subjects related to electric and electronic fi elds. Academic results and student reaction are also discussed.

Keywords drives; electric machines education; power electronics education; PWM inverters

In universities all over the world, courses in electric machines and drives are suffering from lack of student interest1 and, in some cases, are being cancelled from the curriculum. However, nowadays, industry is demanding more and more trained engineers in this fi eld. From this point of view, universities have to modernise and improve the electrical machines and drives curriculum, meeting these industry demands, since power electronics and electric machines/ drives courses have in many cases not changed in several decades.2 With the aim of attracting more students, a modernisation of both course and the supporting laboratory is thus essential. In this way, laboratory experiments must be updated to fulfi l these requirements, though continuing the conventional way of providing practical experience to electrical and electronic engineering students, through the use of extensive laboratorybased systems. Engineering students must become acquainted with the equipment and techniques used in professional environments. When selecting the practical resources to be used in an engineering course, a trade-off must be found between their industrial signifi cance and the educational requirements.3

One essential element of power engineering education must be a renewed emphasis on the laboratory.4-6. Any new laboratory should address the areas of electrical machines, power electronics and their control, maintaining the conventional way of providing practical experience to electrical and electronic engineering students. In this context, it is necessary to point out that more than 75% of all generated power is processed by power electronics. The extensive usage of switching converter circuits in electronic products and systems makes the fundamental understanding of power electronics a necessity for students and electronic engineers.7 However, these laboratories are costly to build and diffi cult to maintain, being usually designed for their use along the years. Indeed, some authors claim that virtual instruction environments overcome this problem adding safety and security,8,9 and the use of computers do much to alleviate the students' frustration and give them an early sense of satisfaction and accomplishment10. Additionally, during the past decade, Internet technologies have become the basis of computer-assisted education. Web-based systems take advantage of giving access to many resources using a standard, universal, and well-known user interface11. In this context, computer games have also been integrated in a basic automatic control course at the undergraduate university level.12

In our opinion, it would be desirable if engineering education could combine both aspects: computer simulations and laboratory experiments. From this point of view, the main contribution of this paper is focused on describing our satisfactory experience when both simulations and laboratory experiments are proposed as practical sessions in different subjects. In our case, electric drives are considered in the present work as topic of interest, since they appear as a preferred objective in different electrical and electronics undergraduate subjects.

Industrial Engineering Degree

According to the current undergraduate curriculum of the Industrial Engineering degree13, the Electrical Engineering and Electronic Technology Departments of the Technical University of Cartagena (Spain), teach the subjects shown in Table 1, where ECTS represents the European Credit Transfer System equivalent14. This undergraduate curriculum was elaborated from a generalist education point of view, with emphasis on industrial engineering fundamentals and practices.

In this curriculum there is only one compulsory subject directly related to the fi eld of electronics - Power Electronics - , in which the goals are limited and have to be focused on basic concepts. The optional subjects give us the opportunity to extend the basic concepts, but those subjects are normally assigned to different departments. For this reason, a good coordination between these departments would be desirable in order to optimise the contents, avoiding overlap and offering a more effi cient education. In our case, Extension of Electric Machines and Power Electronics optional subjects - both given during the 5th-year course - have been coordinated during the past few years. They involve different areas including electric, electronics, semiconductors, and control, and, from our experience, students do not easily assimilate these interdisciplinary concepts. Moreover, they normally do not relate them, tending to study them as separate - even isolated - disciplines. With the aim of mitigating this problem, different laboratory tests of adjustablespeed drives have been proposed from these optional subjects to relate and strengthen power electronics, electric machine control and inverter performance concepts.

On the other hand, the average ratio between the ECTS credits of subjects related to electronics and the total undergraduate ECTS credits for the Industrial Engineering curriculum approved and given in different Spanish universities is around 40% - nowadays, in Spain, this curriculum includes the equivalent Electrical, Mechanical, Power and Electronics Engineering degree. However, in our case, the percentage of credits (20%) is signifi cantly lower than the average value, which justifi es our interest and necessity to minimise this defi ciency by means of the different proposed laboratory experiments.15

Resource description

In this section, a brief description of the hardware and software is provided. In this case, all the hardware system equipment is common in the industrial environment, and thus, less costly than their educational counterparts. Specifi cally, these items are the following:

* Variable speed a.c. drive.16 Unidrive UNI1403 - 1.5kW - from Control Technniques �. In our case, two a.c. drives have been used: one works in motor mode, and the other one in regenerative braking mode.

Induction electric machine.17 This induction electric machine is a three-phase motor, 4 pole, with squirrel-cage rotor from Lucas-N�lle� laboratory technology.

Electrical measurements.18 Two-channel oscilloscopes and digital voltmeters and ammeters have been used to measure and store the real voltage and current evolution.

In reference to the software packages, an evaluation version of Orcad-PSpice Computer � has been used for the simulation stage.19 In this case, the circuits have been simplifi ed according to our educational and curriculum objectives: for example, power converters are simulated as d.c.-controlled-transformers. In the same way, this freeware distribution allows students to simulate any equivalent circuit as well as share their experiences and contributions.


Laboratory tests: results

From these equipments and software packages, different laboratory experiments have been proposed. Laboratory tests of industrial electric a.c. drives have been implemented inside the Extension of Electric Machines subject, being their equivalent circuits simulated inside the Power Electronics subject. The main aim of these simulations is to show clearly how converters work, avoiding complex control systems according to our curriculum objectives. From this point of view, the proposed real and simulated laboratory experiments include: One-leg switch-mode inverter (PWM), averaged one-leg switch-mode inverter (simplifi cation), threephase inverter (PWM), averaged three-phase inverter, averaged three-phase inverter with 3rd harmonic injection, and regenerative braking process - including energy balance.

A project-based competitive learning scheme to teach mobile communications

Abstract This paper presents a project-based learning experience to teach mobile communications courses, carried out at Alcal� University, Madrid, Spain. The experience consists of a competition among teams formed by students, who act as consultant companies working for imaginary operators. The objective of the teams is to obtain the best network design and the most economical budget for an imaginary operator in the city of Alcal� de Henares. The proposed learning scheme uses existing concepts of different fields, such as the project-based or competitive games methodologies, and applies them to the teaching of telecommunications subjects. Details of the practical application of the methodology and results obtained at Alcal� University are discussed in this paper.

Keywords competitive games; electrical engineering course; mobile communications; project-based learning
Mobile communications are nowadays one of the fastest growing technologies in the communications business,1,2 representing close to 2% of GDP in the majority of developed countries.3 Following this trend, most universities and technical schools offer mobile telecommunications courses to prepare engineers in these technologies. 4,5 Specifically, in most institutions where electrical engineering is taught, basic courses about the GSM (2nd Generation Mobile Communications), and the UMTS (3rd Generation) systems are offered.
These courses, usually called Mobile Communications, Wireless Communications or Communications Networks, may vary among different universities, but generally all of them give a good overview of the technical and economical foundations of this field. Teaching the GSM and UMTS systems in technical schools is usually carried out by following two steps. First, the students receive theoretical classes comprising a description of how the system and its components work; afterwards, they take some laboratory classes where practical aspects of the GSM and UMTS6 systems implementation are revised.

In the last few years, the teaching methodology known as project-based learning has arisen as one of the most promising and used techniques in higher education. Specifically, different works have been published recently discussing several applications of project- and problem-based learning in electrical engineering.7-11 These works have shown the applicability of the project-based learning scheme in very different aspects of electrical engineering teaching, and very good results in terms of improvement of teaching quality and students' acceptance have been obtained. In spite of the large amount of works in the literature discussing project-based schemes in electrical engineering, up to now we have not found a direct application of the project-based technique to specific mobile communications courses.

The objective of this paper is to describe a specific implementation of a projectbased learning (PjBL) scheme in a Mobile Communications course taught in a Spanish university. Specifically, we discuss the implementation of the PjBL scheme in the course Mobile Communications taught in the 5th year of the Telecommunications Engineering degree at the Universidad de Alcal� (UAH), Spain. The idea is to include in the course concepts of engineering and economics related to mobile communication systems. Briefly, we have structured the PjBL implementation in the following way: after a first stage in which the basic theoretical notions of the GSM architecture are explained, a project-based learning scheme is organized by dividing the students on the course into groups of 3 to 6 people. Each group represents a Consulting Firm, with its corresponding team leader. The work consists of performing a full technical and economical study for a virtual operator in a specific city, with its specific economic and technical features. This work is planned as realistically as possible, and then it is like a competition among all teams in such a way that the best project will obtain the best qualification. The evaluation is done by considering the technical quality of the solution, the presentation (how well the team is able to show how good their study is) and the efficiency (in terms of network investment and consulting budgets).

The rest of the paper has the following structure: the next section presents the PjBL proposed, describing the organization and structure of the projects given to the students. The following section shows the results of the experience in a graduate course at the University of Alcal� (UAH), Madrid, Spain. The concluding section presents some final remarks.

Description of the proposed project-based methodology

Nowadays, most network deployments take place in GSM and UMTS systems, so a solid foundation in these technologies is a must in a telecommunications engineering degree. On the other hand, most telecommunications engineers have jobs related to techno-economical studies, consulting studies12 and/or project management, whereas only a few of them work on low level technical issues. This fact made us prepare a different type of mobile communications course, with special focus on project management. In this section the basic details of this project-based learning scheme are given.

The complete course is based on two interrelated blocks:

* A set of theoretical lectures, where the concepts and working of mobile communications are explained;

* The Work Project, which is designed as real consultancy work.

The final mark for the course is the mean of the mark obtained in the final exam of the course (over 10 points), and the mark of the consulting work project (also over 10 points). The main objectives of the project are the following:

* Learning how to work in a real case scenario inside a work team;

* Learning how to assume a specific role and responsibilities in team work;

* Learning how to manage efficiently scarce resources, specifically time and human effort;

* Learning how to apply the knowledge acquired in the lectures to a real case;

* Learning how to survive in a harsh competitive environment;

* Learning how to present the results of the work (how to sell them to the client).

The project has been structured as follows: the students are divided into several groups with 3 to 6 students, are each group is treated as a consulting company that works for an imaginary operator. The task of each group is to perform the complete mobile network deployment in a specific city of Spain, with the corresponding real equipment, and to calculate the values of the cost per minute of the different services offered by the operator.13 In the courses in 2007/2008 and 2008/2009 the city selected was our home town, Alcal� de Henares, where the University of Alcal� is located. In the next subsections we detail the complete methodology used to supervise the projects.

Composition of the teams

As has been mentioned before, each group is composed of 3 to 6 students. The selection of the members of the group is done by the students themselves. The students are previously told that smaller groups will have less complicated projects than bigger groups, because bigger groups can use economy of scale to leverage the work of the project. Each group has to select the name of the consulting firm and the corresponding corporate logo. Furthermore they have to establish a template for all the reports they have to produce, in order to provide a professional image.

A relevant issue at this point is the figure of the team leader. In the real world the team leader in a consulting project is a senior consultant who, obviously, has more responsibility than the rest of the project members. On the other hand, he/she earns more money. In this case, the team leader takes responsibility for the success of the project. If the project ends correctly, the qualification of the team leader will be higher than the rest of team members. On the other hand, if the project fails, the team leader will be consequently penalized. The students with more initiative are encouraged to be team leaders.

Work definition

As outlined above, each team is commissioned with the mobile network deployment of a virtual operator. The input data given to the team is described below.

* The technologies adopted by the operator, GSM or GSM / UMTS.

* The market share of the operator.

* The bandwidth acquired by the operator in each frequency band, GSM 900 MHz, GSM 1800 MHz, UMTS 2000 MHz.

* The total annual minutes billed by the operator.

* The service briefcase for each technology.

The level of difficulty of the work depends on the number of members in each team. We have divided the possible consulting firms to choose from into three categories.

- Small operator. Using only GSM technology with low market power, about 15-20% and with not very favourable frequency bands and limited service briefcase. The network design is simple, but the profitability is low. Therefore in the conclusions of the report they have to make many statements to the regulatory authorities. This kind of operator is adequate for small teams of 3 members.

- Medium operator. GSM and UMTS technology with market power about 30% and reasonable frequency bands and service briefcase. It is a comfortable position for the network design but the level of exigency in the profitability results will be high. Adequate for teams of 4 members.

- Large operator. This will be the case of the dominant operator with a very high market power, more than 40%, with a large briefcase and large spectrum due to its economic power. The network design is more difficult but the profitability is ensured due to economy of scale. However in the report the students have to defend against possible regulatory intrusions to favour the small operators. Adequate for teams of 5 or 6 members.

The result of the project is a final report where the following points must be described:

* An executive summary describing the network design;

* Firm, model, price and description of the commercial equipment used for each network element;

* Location in UTM coordinates of each BTS, BSC and radio link used in the network design;

* Graphical representation of the results, by means of the corresponding maps;

* Consultant economic report;

* Network design economic report;

* Profitability of the proposed solution;

* Diagram with the effort in man/months;

* Conclusions and allegations for the regulatory authorities.

Apart from the final report, each team will have to perform a short presentation to the lectures of the subject and other colleagues, some of them coming from mobile operators such as Vodafone group. The objective of this presentations to force the students to defend their solution against a semi-hostile audience, which represents the client in the real world, and which tries to find any possible error on the design.

Economic budgets

There are three topics in the final report related to economic issues: the consultancy budget, the network investment budget and the profitability study. This section will go deeper into these points.

In the project definition it is specified that each team must act as a consulting company performing a work for a virtual operator. Therefore the members of the team spend some effort, measured in working hours, to complete this work. In the final report they have to provide an economic report with the total amount they will bill to the operator for the work done. This bill may include not only the personal payments but also the corresponding part of the material used by the consulting firm (computers, printers and so on), and the benefit margin.

In the economic budget there will be a specific item related to the things acquired by the team for doing the project. In fact, these things are, in most cases, only information about network equipment and prices, economic issues, network planning procedures or geographical and demographical information about the city of Alcal� de Henares. Information can be obtained on the Internet, from reference books or asking the lecturers directly. However, in this work, the role of the lecturer is to be an external consultant, therefore if the team asks the lecturer something, the lecturer will bill the team for this concept. The amount of virtual money the lecturer will charge depends not only on the type and complexity of the question, but also on other factors such as the type of team asking: a larger team works for a larger operator and will have to pay more. Also, when a team asks for information, it has been imposed that the closer to the deadline of the project, the higher the price the team will have to pay. This service request has to be done to the lecturer by filling in a form using the company's template.