Friday, September 28, 2012

Why Capacitor stores Energy? Why Capacitor Blocks DC?

Capacitor Analogy

In the hydraulic analogy, a capacitor is analogous to a rubber membrane sealed inside a pipe. It is possible to push water towards the membrane, but as the membrane stretches it will push back against the flow more and more. 
This animation illustrates a membrane being repeatedly stretched and un-stretched by the flow of water, which is analogous to a capacitor being repeatedly charged and discharged by the flow of current.

When water is forced into one pipe, equal water is simultaneously forced out the other pipe, yet no water can penetrate the rubber diaphragm.
Energy is stored by the stretching of the rubber. As more current flows "through" the capacitor, the back-pressure (voltage) becomes greater, thus current "leads" voltage in a capacitor. 

As the back-pressure from the stretched rubber approaches the applied pressure, the current becomes less and less.

Thus capacitors "filter out" constant pressure differences and slowly-varying, low-frequency pressure differences, while allowing rapid changes in pressure to pass through.

In the Case of DC, the Capacitor will become stretched and then it cannot move. So it blocks the DC.

The water flow shown in the Animation is equivalent to the AC current. 


To multiply throughput of a radio link, multiple antennas (and multiple RF chains accordingly) are put at both the transmitter and the receiver. This system is referred to as Multiple Input Multiple Output (MIMO). A MIMO system with similar count of antennas at both the transmitter and the receiver in a point-to-point (PTP) link is able to multiply the system throughput linearly with every additional antenna. For example, a 2x2 MIMO will double the throughput.

                                             Fig: Multiple Input Multiple Output (MIMO), 2x2
                                            Two antennas at both the transmitter and the receiver.

         MIMO often employs Spatial Multiplexing (SM) to enable signal (coded and modulated data stream) to be transmitted across different spatial domains. Meanwhile, Mobile WiMAX supports multiple MIMO which will maximize spectral efficiency (increase throughput) without shrinking the coverage area. The dynamic switching between these modes based on channel conditions is called Adaptive MIMO Switching (AMS). If combined with AAS (Adaptive Antenna System), MIMO can further boost WiMAX performance.  

Wednesday, September 26, 2012

A simple analogy for the Band Theory in Semiconductor Physics

Band Gaps are some set of Rules 

In a regular periodic crystal lattice, electrons as the carriers of electrical current are not allowed to move around freely. Instead, they have to obey certain rules enforced by quantum mechanics. 

As a consequence, electrons have to occupy so-called “energy bands” which are separated from each other by small or large “band gaps”

Band Diagram

 What is Conduction Band and Valance Band

This situation can be compared to a two storey building consisting of a ground floor and a first floor. 

Energy Band analogy
In the language of solid state physics, these two storey's are called “valence band” and “conduction band”, respectively. 

Both floors are covered by a well-ordered array of quadratic tiles, representing the periodic lattice of atoms in a semiconductor crystal. 

The movement of electrons in a crystal is then analogous to the movement of inhabitants in our building, whose most important purpose it is to transport “charge” from one end of the building to the other end. (Inhabitant=The person who is staying in the House)

The inhabitants of our “semiconductor house” have to obey one additional important rule: at no time more than one inhabitant is allowed to occupy the space of a given tile! 

In the same way, electrons in a solid crystal have to obey the quantum-mechanical “exclusion principle” formulated by the famous physicist Wolfgang Pauli.

Now that the blueprints of our semiconductor building and the basic rules for its inhabitants have been defined, let us start to occupy this building with people. 

Analogy for the Band Concept 

At first, all inhabitants can be accommodated on the ground floor, where they can move around more or less freely and transport their cargo across the building. This leads to a steady increase of the amount of cargo transported through the building, until the occupancy of the ground floor has increased so much that the inhabitants start to hinder one another on their way. Eventually, the stream of cargo will come to a complete stop, once all tiles in the ground floor are occupied by an inhabitant, so that nobody is able to move any more. 

Further inhabitants can only occupy the first floor, where they again have sufficient room to move about. As a consequence, the overall cargo stream through the building will again start to increase, reach a maximum, and eventually come to an end when also the first floor is fully occupied. 

A. What is a Conductor?

Crystals in which energy bands are only partially filled will belong to the group of electrical conductors, since their electrons can move more or less freely through the crystal lattice. 

If, on the other hand, all energy bands are fully occupied or completely empty, no electrical current can pass through the crystal at all and we are dealing with an electrical insulator. 
Which situation will be encountered for a given solid crystal depends on how many electrons per atom are available to occupy the energy bands of the crystal. 

For example, almost all metals are very good electrical conductors due to a half-filled conduction band, whereas metal oxides very often are good insulators with a completely filled valence band and an empty conduction band. 

B. What is a Semi-Conductor?

Semiconductors are solids which are able to pass an electrical current much better than insulators, but at the same time not as efficiently as an electrical conductor. Obviously, semiconductors are solids in which for one reason or the other a few of the many tiles on the ground floor remain empty or a few of the conduction band tiles are occupied, or both.

C. What is an Insulator?

Insulator, where all tiles in the ground floor are occupied by exactly one inhabitant and all tiles in the first floor are empty. Thus, no charge transport can occur. 

Doping and the Band Theory

In the analogue of our semiconductor building, doping can be achieved by adding special tiles with the following properties. 

1. Acceptor Ion and Hole Theory

As a first example, so-called “acceptor tiles” can be added to the ground floor. These acceptor tiles have the unpleasant property of swallowing exactly one inhabitant of the fully occupied ground floor, thus creating a “hole” in the overall occupancy. 

Holes and Electrons in an Atom

This allows the other inhabitants of the ground floor to move again. The hole created by the acceptor-tiles also will move at the same time, however in the opposite direction as compared to the inhabitants. In the same way, acceptor atoms incorporated into a semiconductor crystal will create a hole in the occupancy of the valence band, which will act as a “missing electron” and, thus, as a positively charged particle in electrical transport. 

Valance Band with Holes

Therefore, doping of a semiconductor crystal with acceptor atoms is referred to as “p-type” doping (“p” as in positive). 

2. Donor Ion and Electron Theory

The second possibility to induce controlled electrical conduction in an insulator is the doping with donor impurities. 

In our semiconductor building, such “donor tiles” bring along one additional inhabitant, who has to occupy a free tile in the first floor, since all tiles of the ground floor are already occupied. Accordingly, donor atoms added to a semiconductor crystal will provide additional electrons in the conduction band, which contribute to electronic charge transport in the expected way (“n-type” doping by additional negatively charged electrons).

Conduction Band with Extra Electrons

3. Effect of Thermal Energy and Light Energy

There is yet another way to produce additional holes in the valence band or electrons in the conduction band of a semiconductor without doping, namely by providing external energy in the form of heat or light. 

We all know from our own experience that it takes energy to walk up the stairs from the ground floor to the first floor. 

 The same holds for the electrons in a semiconductor: electrons in the conduction band (first floor) have a higher energy than electrons in the valence band (ground floor). This difference in energy is determined by the band gap of the semiconductor. 

Since electrons are lazy, they prefer to stay on the ground floor. In order to move up to the first floor, they have to be stimulated by an external influence. One possibility is provided by the thermal movement of the atoms. At low temperatures, atoms are frozen at their lattice sites, but at higher temperatures they start to wiggle more and more and to push the electrons around. 

In the analogue of our semiconductor building, the thermal motion of the atoms can be visualized by a staircase leading from the ground floor to the first floor. The thermal motion of the atoms will push the electrons upwards step by step. The larger the band gap of the semiconductor, the longer the staircase and the smaller the number of electrons which actually make it all the way up to the first floor. 

However, in every well-planned building, there is also another possibility to reach the upper floors more easily: an elevator. In semiconductors, the job of the elevator is done by the elementary particles of light, the photons. 

If such a flash of light hits a semiconductor, it can directly elevate an electron from the valence band up to the conduction band. The stronger the light beam that falls onto the semiconductor, the more often the photon elevator will make the trip between the two floors, each time taking an electron with it. 


But also the other direction of electron transport is possible: electrons in the conduction band can return to the valence band, if there is a hole to accommodate the returning electron. This process is called “recombination”. 

Recombination in a Diode

To do this, the electrons can either take the staircase down, giving their energy back to the atoms, or they can take the photon elevator. Then, each time the elevator doors open in the valence band and an electron recombines with a hole, an elementary flash of light is emitted by the semiconductor. 

The energy of the emitted photon is the same as the band gap of the semiconductor. Semiconductors with a small band gap emit red photons, whereas semiconductors with a large band gap emit blue photons. 


What are the major differences between AM and FM?

Origin: AM method of audio transmission was first successfully carried out in the mid 1870s. FM radio was developed in the United states mainly by Edwin Armstrong in the 1930s.
Modulating differences: In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.
Importance: It is used in both analog and digital communication and telemetry. It is used in both analog and digital communication and telemetry.
Pros and cons: AM has poorer sound quality compared to FM, but is cheaper and can be transmitted over long distances. It has a smaller bandwidth so it can have more stations available in any frequency range. FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has greater sound quality due to higher bandwidth.
Stands for: AM stands for Amplitude Modulation FM stands for Frequency Modulation
Range: AM radio ranges from 535 to 1705 kilohertz (OR) Up to 1200 Bits per second FM radio ranges in a higher spectrum from 88 to 108 megahertz. (OR) 1200 to 2400 bits per second
Bandwidth Requirements: Twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz

What is the difference between Amplitude modulation and frequency modulation?

1. In case of frequency modulation the change in amplitude may be due to noise. If we make use of amplitude limiters in FM receivers then we can completely vanish this noise effect.
2. FM waves are waves having constant amplitude. These are independent of the modulation. So, due to this the power transmission of these waves is also constant. The power transmission of FM waves is better than that of the AM signals.
3. In FM signals, all the transmitted power can be used, but in AM wave the transmission carriers contain most of the power. So, complete use of power is not possible.
4. In FM wave’s noise can be controlled by increasing the deviation up to some amount. This is impossible in case of AM waves.
5. VHF and UHF are the bands of FM broadcasting. In these bands noise effect is very less. But on the other hand bands of AM broadcasting such as MF and HF has higher effects.
6. Co-channel interference can be reduced by using some space wave in FM broadcasting.

The major disadvantages of FM are:

1. Complex apparatus is used to transmit and receive the FM wave.
2. FM waves needs 10 times larger channel width than that of the AM waves.
3. The reception area of FM waves is less than that of AM waves. Due to this wide area communication using the Fm waves is not possible.

Monday, September 24, 2012


As the 2 stroke engine animation below shows, a two-stroke engine in its purest form is extremely simple in construction and operation, as it only has three primary moving parts such as the piston, connecting rod, and crankshaft.
The subject of the 2 stroke engine animation is known as a case-reed type because induction is controlled by a reed valve mounted in the side of the crankcase.

2 stroke engine animation running

As the piston moves upward, a vacuum is created beneath the piston in the enclosed volume of the Tcrankcase. Air flows through the reed valve and carburetor to fill the vacuum created in the crankcase. The intake phase is completed when the piston reaches the top of the stroke.

2 stroke engine diagram transfer ports closed

During the down stroke, the falling piston creates a positive pressure in the crankcase which causes the reed valve to close. The mixture in the crankcase is compressed until the piston uncovers the transfer port openings, at which point the mixture flows up into the cylinder. The engine depicted in the 2 stroke engine animation and diagrams is known as a loop-scavenged two-stroke because the incoming mixture describes a circular path. 
2 stroke engine diagram piston at top dead center

Mixture transfer continues until the piston once again rises high enough to shut off the transfer ports . Let's fast-forward about 25 degrees of crank rotation to the point where the exhaust port is covered by the piston. The trapped mixture is now compressed by the upward moving piston  at the same time that a new charge is being drawn into the crankcase down below.

2 stroke engine diagram transfer ports open

 If you watch the 2 stroke engine animation you will see this event is timed such that the burning mixture reaches peak pressure slightly after top dead center. The expanding mixture drives the piston downward until it begins to uncover the exhaust port. The majority of the pressure in the cylinder is released within a few degrees of crank rotation after the port begins to open:

2 stroke engine diagram compression

Residual exhaust gases are pushed out the exhaust port by the new mixture entering the cylinder from the transfer ports. In the 2 stroke engine animation you can see the gases moving out of the exhaust at the same time new mixture is entering the cylinder. That completes the chain of events for the basic two-stroke cycle.

2 stroke engine diagram exhaust

The 2 stroke engine animation demonstration has an added device commonly known as an expansion chamber attached to the exhaust port. The expansion chamber (an improperly named device) utilizes sonic energy contained in the initial sharp pulse of exhaust gas exiting the cylinder to supercharge the cylinder with fresh mixture. This device is also known as a tuned exhaust.

2 stroke engine diagram pressure pulse

The sonic compression wave resulting from this abrupt release of cylinder pressure travels down the exhaust pipe until it reaches the beginning of the divergent cone, or diffuser, of the expansion chamber. From the perspective of the sound waves reaching this junction, the diffuser appears almost like an open-ended tube in that part of the energy of the pulse is reflected back up the pipe, except with an inverted sign; a rarefaction, or vacuum pulse is returned.

Watch the 2 stroke engine animation closely to see the waves reflected back up the pipe. The angle of the walls of the cone determine the magnitude of the returned negative pressure, and the length of the cone defines the duration of the returning waves

2 stroke engine diagram returned negative pressure

The negative pressure assists the mixture coming up through the transfer ports, and actually draws some of the mixture out into the exhaust header. The original pressure pulse is still making its way down the expansion chamber, although a considerable portion of its energy was given up in creating the negative pressure waves.

2 stroke engine diagram mixture extraction

This pulse is timed to reach the exhaust port after the transfer ports close, but before the exhaust port closes. The returning compression wave pushes the mixture drawn into the header by the negative pressure wave back into the cylinder, thus supercharging a bigger charge than normal the engine. The straight section of pipe between the two cones exists to ensure that the positive waves reaches the exhaust port at the correct time.

2 stroke engine diagram pressure wave supercharging

Since this device uses sonic energy to achieve supercharging, it is regulated by the speed of sound in the hot exhaust gas, the dimensions of the different sections of the exhaust system, and the port durations of the engine. Because of this, it is only effective for a very narrow RPM range.

Basics of Physics


Animation on centripetal acceleration


Animation on projectile motion


Animation on projectile range


Animation on newton's cannon


Animation on kepler's law


Animation on Doppler Effect


Animation on Shock Waves

All The Best.........

Saturday, September 22, 2012


How LCD Projectors Work

LCD projectors employ a three-panel LCD (Liquid Crystal Display) system, referred to as 3LCD. LCD projectors crisply reproduce bright, naturally colored images that are easy on the eyes. LCD projectors are also capable of detailed shadow reproduction that is ideal for demanding business and home theater applications.
The white light from the projector lamp is split into red, green, and blue components using two dichroic mirrors, special mirrors that only transmit light of a specified wavelength. Each red, green and blue beam then passes through a dedicated LCD panel made up of thousands of miniscule pixels. An electrical current turns the panel's pixels on or off to create the grayscale equivalent of that color channel. The three colors are then recombined in a prism and projected through the projector lens and onto the screen.

By using a combination of three LCDs to produce a final image, LCD projectors are capable of billions of colors and smooth grayscale gradations. The resolution of the image is determined by the number of pixels in the LCD panels used. Currently LCD panels offer resolutions as high as true HD (1920 x 1080) for home theater applications. New panels promise resolutions as high as 4K (3840 x 2160).

How does DLP technology work?

Digital Light Processing is a proprietary system developed by Texas Instruments, and works differently to LCD projection. Most DLP projectors have a single chip instead of glass panels through which light is passed, and this chip has a reflective surface composed of thousands of tiny mirrors which correspond to individual pixels. These mirrors can move back and forth when light is beamed onto the chip to direct the light from individual pixels either towards the projector lens or away from it. In order to define colours, DLP projectors have a colour wheel that consists of red, green and blue filters. This wheel spins between the light source and the DLP chip and alternates the colour of the light hitting the chip between red, green and blue. The mirrors tilt away from or into the lens path depending on how much of each color is required for each pixel at any given moment.
            The various advantages and disadvantages of LCD and DLP projectors mean that each is suited to different applications. Lighter, less bulky DLP projectors are favored by presenters on the road. DLP projectors are also very popular with home theatre enthusiasts due to the higher colour saturation, better contrast and image stability. Entry level DLP home theater projectors are also very affordable.
LCD projectors are often more affordable, making them attractive for education organizations. Their higher light output make them well suited for classrooms and larger conference facilities, as does their increased image sharpness which makes them good for displaying data-rich presentations such as spreadsheets and graphs.
In terms of market share, LCD projection technology is currently leading DLP technology due to the larger number of projectors using the LCD system. Sony and Epson are the largest LCD manufacturers, along with Hitachi and Sanyo. Optoma, InFocus and BenQ, on the other hand, use DLP technology.

Thursday, September 20, 2012

Thermal runaway in a BJT

We know that in a transistor , power is dissipated in the collector and hence it is made physically larger than the emitter and base region. As the power is dissipated , there is a chance for the collector base junction temperature to be raised. As the temperature at collector base junction increases, the reverse leakage current ICBO increases. This is because ICBO arises due to the flow of minority carriers which are thermally generated across reverse biased collector-base junction (reverse biased pn junction) . As the temperature increases, thermal generation increases, ICBO increases..

IC  = αIE + ICBO
 So, as ICBO increases,  Iincreases. Power dissipated =I2 * R

So, as collector current increases, power dissipated increases which in turn increases the collector base junction temperature. So the process is cumulative leading eventually to the destruction of the transistor.
Thermal runaway can be prevented by using a heat sink.


Cryptography or cryptology is  from the Greek words κρυπτός, "hidden, secret"; and γράφειν, graphein, "writing", or -λογία, -logia, "study", respectively) is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation.Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.
Cryptography prior to the modern age was effectively synonymous with encryption, the conversion of information from a readable state to apparent nonsense. The originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby precluding unwanted persons to do the same.