Thursday, December 20, 2012

Band Stop Filter

It is also called band-elimination, band-reject, or notch filters which passes all frequencies above and below a particular range set by the component.

Twin-T Band-stop filter

The low-pass filter section is comprised of R1, R2, and C1 in a “T” configuration. The high-pass filter section is comprised of C2, C3, and R3 in a “T” configuration .Hence it is commonly known as a “Twin-T” filter

  • A band-stop filter screens out frequencies with in a certain range and thus provides easy way to frequency  outside that range. Also known as band-elimination, band-reject, or notch filters.
  • It  can be built by placing a low-pass filter in parallel with a high-pass filter. Both the low-pass and high-pass filter sections are of the “T” configuration, giving the name “Twin-T” to the band-stop combination.
  • The frequency of maximum attenuation is called the notch frequency.


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    Band Pass Filter

    It passes frequency with in the certain range.Circuits can be designed to accomplish this task by combining the properties of low-pass and high-pass into a single filter. The result is a band-pass filter.


    1. Capacitive band-pass filter.

    This construction allows the frequency that are neither too high nor too low.The response of a capacitive bandpass filter peaks within a narrow frequency range.

    2.Inductive band-pass filter.

    Band-pass filters can also be constructed using inductors. But, the capacitive band pass filter gives more advantage.

    Here, the high-pass section comes “first” in this design instead of the low-pass section makes no difference in its overall operation. It will still filter out all frequencies too high or too low.

  • A band-pass filter works to screen out frequencies that are too low or too high, giving easy passage only to frequencies within a certain range.
  • Band-pass filters are built by stacking a low-pass filter on the end of a high-pass filter, or vice versa.


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    High Pass Filter

    A high-pass filter performs  the opposite function  of a low-pass filter.It offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal.



    1.Capacitive high-pass filter.

    The capacitor's impedance increases with decreasing frequency. This high impedance in series leads to block  the low-frequency signals from getting to load.
    The response of the capacitive high-pass filter increases with frequency.

    2.Inductive high-pass filter.

    The inductor's impedance decreases with decreasing frequency. This low impedance in parallel tends to short out low-frequency signals from getting to the load resistor. As a result, most of the voltage gets dropped across series resistor R1.
    The response of the inductive high-pass filter increases with frequency.

    By Conclusion,
    The capacitive design is the simplest since it requires only one component above and below the load.And, again, the reactive purity of capacitors over inductors tends to favor their use in filter design, especially with high-pass filters where high frequencies commonly cause inductors to behave strangely due to the skin effect and electromagnetic core losses.


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    Wednesday, December 19, 2012

    Low Pass Filter

    A low-pass filter is a circuit that allows low-frequency very easily and does not allows high-frequency easily. There are two basic kinds of circuits

    1.Capacitive low-pass filter

    The capacitor's impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals.It drops most of the voltage across series resistor R1

    The response of a capacitive low-pass filter falls off with increasing frequency.

    2.Inductive low-pass filter

    The inductor's impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load.

    The response of an inductive low-pass filter falls off with increasing frequency.

    By Conclusion,

    In this case,the  inductive low-pass filter is very simple in structure. The capacitive version of this filter is not that much more complex, with only a resistor and capacitor needed for operation. Due to its complexity, capacitive filter designs are generally preferred over inductive because capacitors tend to be “purer” reactive components than inductors and therefore are more predictable in their behavior. By “pure” I mean that capacitors exhibit little resistive effects than inductors, making them almost 100% reactive. Inductors, on the other hand, typically exhibit significant dissipative (resistor-like) effects, both in the long lengths of wire used to make them, and in the magnetic losses of the core material. Capacitors also tend to participate less in “coupling” effects with other components than inductors, and are less expensive.

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    what is filter?

    Sometimes, Some kind of operations requires certain range of frequencies to perform its operation more efficiently.Thus a circuit is designed to perform such operation is called filter circuit.It is mostly needed in stereo systems,where certain range of audio frequency is required to be amplified.Also equalizers are used that allow the amplitude of several  frequency ranges to be adjusted to suit the listener's taste and acoustic properties of the listening area.Another one,Cross network which block certain ranges of frequencies from reaching speakers.                     

    A tweeter (high-frequency speaker) is inefficient at reproducing low-frequency signals such as drum beats, so a crossover circuit is connected between the tweeter and the stereo's output terminals to block low-frequency signals, only passing high-frequency signals to the speaker's connection terminals. This gives better audio system efficiency and thus better performance. Both equalizers and crossover networks are examples of filters, designed to accomplish filtering of certain frequencies.

    Practical Application:

    Conditioning of non-sinusoidal voltage waveforms in POWER CIRCUIT.
    some devices are sensitive to the harmonics in the power supply and hence requires power conditioning.
    If there is a harmonics in the power supply,then the filter can be constructed that allows only the fundamental frequency by blocking the harmonics.

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    Accelerometers for the measurement of acceleration, shock or vibration come in many types using different principles of operation.

    Inside a piezoelectric version, the sensing element is a crystal which has the property of emitting a charge when subjected to a compressive force.
    In the accelerometer, this crystal is bonded to a mass such that when the accelerometer is subjected to a 'g' force, the mass compresses the crystal which emits a signal. This signal value can be related to the imposed 'g' force.

    The sensing element is housed in a suitable sensor body to withstand the environmental conditions of the particular application. Body are usually made in stainless steel with welding of the various parts to prevent the ingress of dust, water, etc.
    Electrical connection can be via a sealed cable or a plug/socket arrangement.
    Many present accelerometers have internal electronic circuitry to give outputs which can be directed used by the associated acquisition or control systems.

    Mechanical fixing of the sensor is important in order to achieve true transfer of the vibration or acceleration. Many fixing methods are used including beeswax, hard glues, threaded stud (male or female), magnetic mounts.
    Accelerometers are used in many scientific and industrial applications such as predictive maintenance, aerospace, automotive, medical, process control, etc.


    Differences between alternating currents (AC) and direct currents (DC). You can read a brief synopsis of these two electrical systems and then begin your exploration of these inventions by clicking on either AC or DC in the electrical system interactive. By clicking on AC, DC, wire, battery, AC generator, or light bulb you will learn about alternating and direct currents, what exactly an electric current is, and two ways that the currents can be produced.

    Alternating Current (AC)

    Alternating current (AC) electricity is the type of electricity commonly used in homes and businesses throughout the world. While direct current (DC) electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country.

    AC electricity is created by an AC electric generator, which determines the frequency. What is special about AC electricity is that the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.

    Direct Current (DC) Electricity.

    Direct current or DC electricity is the continuous movement of electrons from an area of negative (−) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire.
    A DC circuit is necessary to allow the current or steam of electrons to flow. Such a circuit consists of a source of electrical energy (such as a battery) and a conducting wire running from the positive end of the source to the negative terminal. Electrical devices may be included in the circuit. DC electricity in a circuit consists of voltage, current and resistance. The flow of DC electricity is similar to the flow of water through a hose.

    Difference between AC and DC electricity

    Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire.
    Electrons move direct with DC electricity

    With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) terminal will cause the negative charged electrons to rush through the wire toward the positive charged side. The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal.
    Electrons alternate directions in AC electricity

    With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position.
    AC movement of electrons in wire

    The charge at the ends of the wire alternates between negative (−) and positive (+). If the charge is negative (−), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction.

    Rate of change

    AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. This is called the frequency and is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).

    Light bulbs in Both AC and DC

    Many electrical devices—like light bulbs—only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity.

    Tuesday, December 18, 2012

    Direct and Indirect Semiconductors

    Band Gap

    The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band,

    However, the top of the valence band and the bottom of the conduction band are not generally at the same value of the electron momentum. 

    Direct Band Gap

    In a direct band gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum, as in the schematic below.

    Direct Band Gap

    When an electron sitting at the bottom of the Conduction Band recombines with a hole sitting at the top of the Valance Band, there will be no change in momentum values. 

    Energy is conserved by means of emitting a photon, such transitions are called as radiative transitions.

    Direct band gap semiconductors are capable of photon emission, by radiative recombination,but indirect semiconductors have a low probability of radiative recombination.

    Indirect Band Gap

    In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum when comparing with the minimum in the conduction band.

    Indirect Band Gap

    For an indirect-band gap material; the minimum of the CB and maximum of the VB lie at different k-values. 

    When an e- and hole recombine in an indirect-band gap s/c, phonons must be involved to conserve momentum.

    In indirect band gap semiconductors may have iso electronic impurity states or Defect state which are direct, and therefore the recombination from these states may also be radiative.


    • Atoms vibrate about their mean position at a finite temperature.These vibrations produce vibrational waves inside the crystal. 
    • Phonons are the quanta of these vibrational waves. 
    • Phonons travel with a velocity of sound . 
    • Their wavelength is determined by the crystal lattice constant. Phonons can only exist inside the crystal.
    So in order to have efficient LED’s and LASER’s, one should choose materials having direct band gaps such as compound Semi Coductors of GaAs, AlGaAs, etc…

    The below table show the properties of various Semiconductor materials
    Direct / Indirect Bandgap
    Band Gap Energy at 300 K (eV)


    C (diamond)


    Groups III-V compounds
    Groups IV-IV compounds
    Groups II-VI compounds


    Monday, December 17, 2012

    DC and AC Electric motors

    DC motors

    A simple DC motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes that make moving contact with a split ring. The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create a torque on the coil.

    AC motors

    With AC currents, we can reverse field directions without having to use brushes. This is good news, because we can avoid the arcing, the ozone production and the ohmic loss of energy that brushes can entail. Further, because brushes make contact between moving surfaces, they wear out.

    The first thing to do in an AC motor is to create a rotating field. 'Ordinary' AC from a 2 or 3 pin socket is single phase AC--it has a single sinusoidal potential difference generated between only two wires--the active and neutral. (Note that the Earth wire doesn't carry a current except in the event of electrical faults.) With single phase AC, one can produce a rotating field by generating two currents that are out of phase using for example a capacitor. In the example shown, the two currents are 90° out of phase, so the vertical component of the magnetic field is sinusoidal, while the horizontal is cosusoidal, as shown. This gives a field rotating counterclockwise

    From simple AC theory, neither coils nor capacitors have the voltage in phase with the current. In a capacitor, the voltage is a maximum when the charge has finished flowing onto the capacitor, and is about to start flowing off. Thus the voltage is behind the current. In a purely inductive coil, the voltage drop is greatest when the current is changing most rapidly, which is also when the current is zero. The voltage (drop) is ahead of the current. In motor coils, the phase angle is rather less than 90 degee, because electrical energy is being converted to mechanical energy.

    Thursday, December 13, 2012

    Properties of Silicon

    Atomic Number: 14
    Symbol: Si
    Atomic Weight: 28.0855
    Discovery: Jons Jacob Berzelius 1824 (Sweden)

    The melting point of silicon is 1410°C, boiling point is 2355°C, specific gravity is 2.33 (25°C), with a valence of 4. 

    Crystalline silicon has a metallic grayish color. Silicon is relatively inert, Silicon transmits over 95% of all infrared wavelengths (1.3-6.7 mm). Silicon doped with gallium, arsenic, boron, etc. is used to produce transistors, solar cells, rectifiers, and other important solid-state electronic devices. 
    Silicon's range from liquids to hard solids and have many useful properties, including use as adhesives, sealants, and insulators. Sand and clay are used to make building materials. 
    Silica is used to make glass, which has many useful mechanical, electrical, optical, and thermal properties.

    It commonly occurs as the oxide and silicates, including sand, quartz, amethyst, agate, flint, jasper, opal, and citrine. Silicate minerals include granite, hornblende, feldspar, mica, clay, and asbestos.

    Silicon may be prepared by heating silica and carbon in an electric furnace, using carbon electrodes. Amorphous silicon may be prepared as a brown powder, which can then be melted or vaporized. 

    The Czochralski process is used to produce single crystals of silicon for solid-state and semiconductor devices. 
    Hyperpure silicon may be prepared by a vacuum float zone process and by thermal decompositions of ultra-pure trichlorosilane in an atmosphere of hydrogen.

      • Silicon is the eighth most abundant element in the universe.
      • Silicon crystals for electronics must have a purity of one billion atoms for every non-silicon atom (99.9999999% pure).
      • The most common form of silicon in the Earth's crust is silicon dioxide in the form of sand or quartz.
      • Silicon, like water, expands as it changes from liquid to solid.
      • Silicon oxide crystals in the form of quartz are piezoelectric. The resonance frequency of quartz is used in many precision timepieces.


        Wednesday, December 12, 2012

        What is Gunn Effect in Gunn Diode?

        In some materials (III-V compounds such as GaAs and InP), after an electric field in the material reaches a threshold level, the mobility of electrons decrease as the electric field is increased, thereby producing negative resistance.

        The I-V curves of a Gunn diode will help explain the effect. For low voltages (up to 1 volt perhaps), the Gunn diode behaves nearly as a linear resistor. Then at some point the current stops increasing with increasing voltage. This is known as the threshold voltage. Above this point the diode has negative resistance (curve slopes downward), which mean that it is just itching to oscillate! The operating point is usually about 4X the threshold voltage. 

        Characteristics of Gunn Diode

        A two-terminal device made from such a material can produce microwave oscillations, the frequency of which is primarily determined by the characteristics of the specimen of the material and not by any external circuit. The Gunn Effect was discovered by J. B. Gunn of IBM in 1963.
        J. B Gunn

        What is Tunnel Diode?

        A tunnel diode or Esaki diode is a type of semiconductor diode that is capable of very fast operation, well into the microwave frequency region, by using the quantum mechanical effect called "Tunneling". 

        A tunnel diode is a high conductivity two terminal P-N Junction diode doped heavily about 1000 times higher than a conventional junction diode. Tunnel diodes are useful in many circuit applications in microwave amplification, microwave oscillation and binary memory.  

        The tunnel diode exhibits a special characteristic know as negative resistance. This feature makes it useful in oscillator and microwave amplifier applications. Tunnel diodes are constructed with germanium or gallium arsenide by doping the p and n regions much more heavily than in a covenional rectifier diode. 

        This heavy doping allows conduction for all reverse voltages so that there is no breakdown effect as with the conventional rectifier diode.

        Working of Tunnel Diode

        When a small forward bias voltage is applied across a tunnel diode, it begins to conduct current. As the voltage is raised, the current increases and attains a peak value known as peak current. If the current is increased a little more, the current actually begins to decreases until it reaches a low point called the valley current. If the voltage is increased further yet, the current begins to increase again, the time without decreasing into another “valley”. The region on the graph where the current is decreasing while applied voltage is increasing is known as the region of the negative resistance.

        It has negative resistance in the shaded voltage region, between v1 and v2.

        Mechanics behind working

        According to classical mechanics theory, a particle must have an energy at least equal to the the height of a potential-energy barrier if it has to move from one side of the barrier to the other. In other words, energy has to be supplied from some external source so that the electrons on N side of junction climb over the junction barrier to reach the P-side. 

        However if the barrier is thin such as in tunnel diode ,the Schrodinger equation(Quantum Mechanics) indicates that there is a large probability that an electron will penetrate through the barrier. This will happen without any loss of energy on the part of electron. This quantum mechanical behavior is referred to as tunneling and the high-impurity P-N junction devices are called tunnel-diodes. The tunneling phenomenon is a majority carrier effect.

        Why tunneling? 

        It is that the reduced depletion layer can form result in carriers “punching through” the junction with the velocity of light even when they do not possess enough energy to overcome the potential barrier. The result is that large forward current is produced at relatively low forward voltage (less than 100mv) such a mechanism of conduction in which charge carriers (possessing very little energy) punch through a barrier directly instead of climbing over it is called tunnelling. That’s why such diodes are known as tunnel diodes. Because of heavy doping the tunnel diode can conduct in reverse as well as in formed direction but it is usually used in forward biased mode. 

        Reverse Bias 

        In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.  

        Applications of Tunnel Diode 

        • The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. 
        • Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. 
        • The tunnel diode can also be used as low-noise microwave amplifier.
        • Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications 


        Saturday, December 08, 2012


        A smart card, typically a type of chip card, is a plastic card that contains an embedded computer chip–either a memory or microprocessor type–that stores and transacts data. This data is usually associated with either value, information, or both and is stored and processed within the card's chip. The card data is transacted via a reader that is part of a computing system. Systems that are enhanced with smart cards are in use today throughout several key applications, including healthcare, banking, entertainment, and transportation. All applications can benefit from the added features and security that smart cards provide.


        • SIM Cards and Telecommunication 
        • Loyalty and Stored Value 
        • Securing Digital Content and Physical Assets 
        • E-Commerce 
        • Bank Issued Smart Cards and Healthcare Informatics

        Types of Smart Card

        Types of smart cards

        1. Contact Cards

        These are the most common type of smart card. Electrical contacts located on the outside of the card connect to a card reader when the card is inserted. This connector is bonded to the encapsulated chip in the card.

        Smart card module

        2. Contactless Cards

        A second card type is the contactless smart card, in which the card communicates with and is powered by the reader through RF induction technology (at data rates of 106–848 kbit/s). These cards require only proximity to an antenna to communicate. Like smart cards with contacts, contactless cards do not have an internal power source. Instead, they use an inductor to capture some of the incident radio-frequency interrogation signal, rectify it, and use it to power the card's electronics.

        3. Multifunction Cards

        These cards have on-card dynamic data processing capabilities. Multifunction smart cards allocate card memory into independent sections or files assigned to a specific function or application. Within the card is a microprocessor or micro controller chip that manages this memory allocation and file access. This type of chip is similar to those found inside all personal computers and when implanted in a smart card, manages data in organized file structures, via a card operating system (COS). Unlike other operating systems, this software controls access to the on-card user memory. This capability permits different and multiple functions and/or different applications to reside on the card, allowing businesses to issue and maintain a diversity of ‘products’ through the card. One example of this is a debit card that also enables building access on a college campus. Multifunction cards benefit issuers by enabling them to market their products and services via state-of-the-art transaction and encryption technology. Specifically, the technology enables secure identification of users and permits information updates without replacement of the installed base of cards, simplifying program changes and reducing costs. For the card user, multifunction means greater convenience and security, and ultimately, consolidation of multiple cards down to a select few that serve many purposes.