Infosys Technologies Limited (BSE: 500209, NASDAQ: INFY) is a multinational information technology services company headquartered in Bangalore, India. It is one of India's largest IT companies with 103,905 professionals (including subsidiaries) as of Jun 30, 2009.It has offices in 22 countries and development centers in India, China, Australia, UK, Canada and Japan.
History
Infosys was founded on July 2, 1981 in Pune by N. R. Narayana Murthy and six others: Nandan Nilekani, N. S. Raghavan, Kris Gopalakrishnan, S. D. Shibulal, K. Dinesh and Ashok Arora,[4] with N. S. Raghavan officially being the first employee of the company. Murthy started the company by borrowing INR 10,000 from his wife Sudha Murthy. The company was incorporated as "Infosys Consultants Pvt Ltd.", with Raghavan's house in Model Colony, north-central Pune as the registered office.
In 1982, Infosys opened an office in Bangalore which soon became its headquarters.
Infosys went public in 1993. Interestingly, Infosys IPO was undersubscribed but it was "bailed out" by US investment banker Morgan Stanley which picked up 13% of equity at the offer price of Rs. 95 per share. The share price surged to Rs. 8,100 by 1999 making it the costliest share on the market at the time.[6] At that time, Infosys was among the 20 biggest companies by market capitalization on the Nasdaq well ahead of Adobe Systems, Novell and Lycos.
According to Forbes magazine, since listing on the Bombay Stock Exchange till the year 2000, Infosys' sales and earnings compounded at more than 70% a year.In the year 2000, President of the United States Bill Clinton complimented India on its achievements in high technology areas citing the example of Infosys.
In 2001, it was rated Best Employer in India by Business Today. Infosys won the Global MAKE (Most Admired Knowledge Enterprises) award, for the years 2003, 2004 and 2005, being the only Indian company to win this award and is inducted into the Global Hall of Fame for the same.
Infosys was rated best employer to work for in 2000, 2001, and 2002 by Hewitt Associates. In 2007, Infosys received over 1.3 million applications and hired fewer than 3% of applicants.
BusinessWeek reported that Infosys, along with Wipro and Tata accounted for nearly 80% of the [H-1B] visa petitions approved in 2007 for the top 10 participants in the program.
In April 2009, Forbes rated Infosys among the 5 best performing companies in the software and services sector in the world.
In 2009, Infosys was considered one of the BusinessWeek's 50 Most Innovative Companies.
From December 2008 till April 2009, Infosys fired over 2500 employees for poor performance. The company has been hit hard by lower income from a crisis hit European and North American market. On April 15, 2009 Infosys reported its first ever sequential fall in its revenue in a decade during the March 2009 quarter.
Timeline
1981: Infosys is established by N. R. Narayana Murthy and six engineers in Pune, India, with an initial capital of US$ 250. Signs up its first client, Data Basics Corporation, in New York
1983: Moved its headquarters to Bangalore, the capital of Karnataka
1987: Opens first international office in Boston, US
1992: Opened its first overseas sales office in Boston.
1993: Became a public limited company in India with an initial public offering of Rs. 13 crores.
1996: First office in Europe in Milton Keynes, UK
1997: Office in Toronto, Canada
1999: First Indian company to be listed on Nasdaq on March 11.[17]
1999: Attained a SEI-CMM Level 5 ranking
2000: Opened offices in France and Hong Kong
2001: Opened offices in United Arab Emirates and Argentina
2002: Opened new offices in Netherlands, Singapore and Switzerland.
2002: Business World named Infosys "India's Most Respected Company".[18]
2002: Started Progeon, its BPO (business process outsourcing) subsidiary[19]
2003: Acquired 100% equity of Expert Information Services Pty Limited, Australia (Expert) and changed the name to Infosys Australia Pty Limited.
2004: Set up Infosys Consulting Inc., U.S. consulting subsidiary in California, U.S.
2006: Became the first Indian company to ring the NASDAQ Stock Market Opening Bell
2006: August 20, N. R. Narayana Murthy retired from his position as the executive chairman. [20]
2006: Acquired the 23% stake Citibank had in its BPO offshoot Progeon, making it a wholly owned subsidiary of Infosys and changed the name to Infosys BPO Ltd.[21]
2006: December, became the first Indian company to make it to Nasdaq-100[22]
2007: April 13, Nandan Nilekani stepped down as CEO and made way for Kris Gopalakrishnan to occupy his chair effective June 2007
2007: July 25, Infosys bags multi-million dollar outsourcing contract with Royal Philips Electronics in the area Finance & Accounting services strengthening its European operations.
2007: September, Infosys establishes a wholly-owned Latin American subsidiary, Infosys Technologies S. de R. L. de C. V., and opens its first software development center in Latin America in the city of Monterrey, Mexico.
2008: Agreed to buy British consultancy Axon Group for 407 million pounds ($753 million), but HCL Technologies outbid Infosys for 441 million pounds[23]. However, Infosys gained Rs. 180 million from the failed Axon bid.[24]
During the 14-year period from its IPO in 1993 to 2007, the price of an Infosys share increased three thousandfold. This excludes the dividends that the company has paid out over that duration.
Key industries
Infosys serves various industries through its Industrial Business Units (IBU), such as:
Banking & Capital Markets (BCM)
Communications, Media and Entertainment (CME)
Energy, Utilities and Services (EUS)
Insurance, Healthcare and Life Sciences (IHL)
Manufacturing (MFG)
Retail, Consumer Product Goods and Logistics (RETL)
New Markets and Services (NMS) : Non US and Non European markets, SaaS, Learning Services
India Business Unit (IND)
In addition to these, there are Horizontal Business Units (HBUs)
Consulting (CS)
Enterprise Solutions (ES): ERP, CRM, HCM, SCM, BI/DW, BPM-EAI
Infrastructure Management Services (IMS)
Product Engineering and Validation Services (PEVS)
Systems Integration (SI)
Finacle : Core Banking Product
Initiatives
In 1996, Infosys created the Infosys Foundation in the state of Karnataka, operating in the areas of health care, social rehabilitation and rural uplift, education, arts and culture. Since then, this foundation has spread to the Indian states of Tamil Nadu, Andhra Pradesh, Maharashtra, Kerala, Orissa and Punjab. The Infosys Foundation is headed by Mrs. Sudha Murthy, wife of Chairman Narayana Murthy.
Since 2004, Infosys has embarked on a series of initiatives to consolidate and formalize its academic relationships worldwide under the umbrella of a program called AcE - Academic Entente. Through case study writing, participation in academic conferences and university events, research collaborations, hosting study trips to Infosys Development Centers and running the InStep Global Internship Program, the company communicates with important stakeholders in the academia.
Infosys' Global Internship Program, known as InStep, is one of the key components of the Academic Entente initiative. It offers live projects to interns from the universities around the world. InStep recruits undergraduate, graduate and PhD students from business, technology, and liberal arts universities to take part in an 8 to 24 week internship at one of Infosys' global offices. InStep interns are also provided career opportunities with Infosys.
In 1997 Infosys started the "Catch them Young Programme", to expose the urban youth to the world of Information Technology by conducting a summer vacation programme. The programme is aimed at developing an interest and understanding of computer science and information technology. This programme is targeted at students in Grade IX level.[25]
In 2002, the Wharton Business School of the University of Pennsylvania and Infosys started the Wharton Infosys Business Transformation Award. This technology award recognizes enterprises and individuals who have transformed their businesses and the society leveraging information technology. Past winners include Samsung, Amazon.com, Capital One, RBS and ING Direct.
Infosys also has the largest training center for a private sector organization in Asia. The training center is located in Mysore, Karnataka. It currently accommodates 4,500 trainees each year. In 2009 a new training center has been opened which accommodates 10,000 trainee software professionals. This new center is also located in Mysore.
In 2008, Infosys along with National Institute of Advanced Studies created 'Infosys Mathematics Prize' for excellence in Mathematics research.
Tuesday, October 27, 2009
Thursday, October 15, 2009
HP Pavilion is a line of personal computers produced by Hewlett-Packard and introduced in 1995. The name is applied to both desktops and laptops for the Home and Home Office product range.
When HP merged with Compaq in 2002, it took over Compaq's existing naming rights agreement. As a result, HP sells both HP and Compaq-branded machines. Computers can be ordered either directly from the factory or over the phone, and can be customized through choosing desired specifications. This is known as a CTO (formerly BTO) option.
HP Pavilion is a line of personal computers produced by Hewlett-Packard and introduced in 1995. The name is applied to both desktops and laptops for the Home and Home Office product range.
When HP merged with Compaq in 2002, it took over Compaq's existing naming rights agreement. As a result, HP sells both HP and Compaq-branded machines. Computers can be ordered either directly from the factory or over the phone, and can be customized through choosing desired specifications. This is known as a CTO (formerly BTO) option.
History
.In 1995, The HP Pavilion PC marks the company's highly successful introduction into the home-computing market.
Dave Packard publishes The HP Way, a book that chronicles the rise of HP and gives insight into the business practices, culture and management style that helped make it a success. The industry's first low-cost, high-speed small infrared transceiver allows wireless "point and shoot" data exchange in a wide range of portable computing applications such as phones, computers, printers, cash registers, ATMs, digital cameras and more.[1]
[edit] The First HP Pavilion PC
The HP pavilion 5030 was HP's first multimedia PC designed specifically for the home market, and it went on to become a market leader in consumer PCs. It featured a quad-speed CD-ROM drive, Altec Lansing speakers, software for online service access and Microsoft Windows 95. This entry-level model features an 75 MHz Intel Pentium processor, 8 MB RAM and an 850 MB hard drive.[2]
[edit] Notebooks
HP offers 8 notebook lines under the HP Pavilion name, 2 under HDX, 4 under HP Mini, 1 under TouchSmart, 2 under G series, and 1 under Compaq Presario. These are customizable in the US only. A wide variety of different models with different setups are offered in Canada and elsewhere.
[edit] Current Notebook Models
18.4 inch : HDX18t
17.3 inch : dv7t
17.0 inch : G70t
16.0 inch : HDX16t / dv6t / dv6z / dv6zae (Artist Edition 2) / G60t
15.6 inch : Compaq CQ60
15.4 inch : dv5tse
14.1 inch : dv4tse / dv4t
13.3 inch : dv3t / Voodoo Envy 133
12.1 inch : TouchSmart tx2z / HP Pavilion dv2z
10.1 inch : HP Mini 110 Mi / HP Mini 110 XP / / Mini 1000 Vivienne Tam
[edit] Previous Notebook Models
20.1 inch : HDX9000
17.0 inch : dv7z / dv9000 / dv8000 / zd8000 / zd7000
15.4 inch : dv5 / dv6500tse (Special Edition) / dv6000 / dv5000 / dv4000 / zv6000 / zv5000 / zx5000 / ze5000 / ze4000 / zt3000
15.0 inch : ze2000 / ze1000 / zt1000
14.1 inch : dv4z / dv2800tae (Artist Edition) / dv2500tse (Special Edition) / dv2000 / dv1000
13.3 inch : dv3z / dv3500t
12.1 inch : tx1000z tablet PC / tx2000z Tablet PC / tx2500z Tablet PC
10.1 inch : Mini 1000 Mi / Mini 1000 XP / Mini 1000 Mobile Broadband Wireless
8.9 inch : Mini 1000 Mi / Mini 1000 XP / Mini 1000 Mobile Broadband Wireless
[edit] Model Number Suffixes
The two or three letter suffix on the model number indicates special information like country or language (dv----xx). The following chart describes each suffix.
t : Intel Processor
z : AMD Processor
ae : Artist Edition (Artist Edition Imprint)
bw : Broadband Wireless Series
se : Special Edition ("Intensity" dv4tse, "Renewal" dv5tse, Special Edition Imprint)
The following sufixes corresponds to the region the notebook is sold.
us : United States
ca : Canada
la : Latin America
ea / ee / (e + a letter) : Europe / Middle East
au / ax : Asia / Australia - AMD Processor (AU = AMD + UMA, AX = AMD + Discrete *UMA and Discrete refers to graphics *)
tu / tx : Asia / Australia - Intel Processor (TU = Intel + UMA, TX = Intel + Discrete)
Other suffixes include nr, cl, and wm.
nr : No Region
cl : Club Model (These models are available only through some clubs like Costco and Sam's Club)
wm : Walmart Model
br : Brazil
ap : Asia Pacific
The HP Pavilion HDX is only sold with Intel Processors, but it doesn't end with the suffix "t". It has no suffix.
The HP Pavilion tx tablet PC series are currently sold with AMD Processors only, but they still end with the suffix "z".
Take Action. Make Art
HP and MTV held a contest for young people around the world to help design a special edition HP notebook. The theme: the cause that is most personal to you. It all starts with your personal view. Imagine that your design is a positive thought, a belief, a message you want to tell the world. Your take on how to make our planet or your community a better place.
The contest went from September 5, 2007 to October 17, 2007 and over 8,500 designs from 112 countries were submitted.
"Asian Odyssey" the personal design of 20-year old João Oliveira of Porto, Portugal, was chosen as the winner of the "Take Action. Make Art" global notebook-design competition. The winning design is featured on the HP dv2800tae Series Notebook. There were 4 other top regional designs and they were featured as exclusive laptop skin covers at skinit.com. The regional finalists are Jeremy Kiraly of Queensland, Australia, Marco Feola of Rome, Italy, Eduardo Arevalo Lopez of Bogota, Colombia and Charles Williams of Ottawa, Canada.
HP Engine Room
“Engine Room” is a series of challenges created by HP and partners in which young artists from around the world can showcase their creativity and bring art to life with technology. HP held another contest to design a special edition notebook.
Artists from 94 countries submitted nearly 17,000 designs with the hopes that their art would be displayed on an HP notebook PC. More than 62,000 votes were received from 159 countries.
Hisako Sakihama, a 27-year-old designer from Japan was the winner of the “Engine Room” Notebook Design Contest. The regional finalists include Rodrigo Daniel Diaz from Argentina, Carlos Alonso Zebenzuy from Spain, Abhishek Goswami from India, and Stacy Pezzola from the United States. The regional finalists will have their art transformed into notebook attachable “skins” from SkinIt, which will be made available to consumers around the world.
Criticisms
Some models of Pavilion laptop have been criticized as having faulty power jacks, which separate from the motherboard under normal use.
Some models have ongoing problems with wireless connectivity
Recently the dv9000 series notebooks have a Hinge problem, where the left side of the screen will snap off when opening. Some say this is due to a design flaw, but has yet to be publicly addressed. a website has been created for this problem, it is called http://www.notebookhingecrack.com/
Users with HP Pavilion laptops report having their volume button jumping erratically and clicking and the sounds moving up and down on its own accord, this can be fixed by uninstalling HP's QuickTouch on-screen display [9]
Friday, October 2, 2009
an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.
In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually voltage or current. In audio applications, amplifiers operate loudspeakers used in PA systems to make the human voice louder or play recorded music. Amplifiers may be classified according to the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).
A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.
Figures of merit
The quality of an amplifier can be characterized by a number of specifications, listed below.
Gain
The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(Pout /(Pin)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).
Example: an audio amplifier with a gain given as 20dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical).
Bandwidth
The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The "satisfactory performance" may be different for different applications. However, a common and well-accepted metric are the half power points (i.e. frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths (otherwise called "frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or "plus or minus 1dB" (roughly the sound level difference people usually can detect).
A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points < 10 and > 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.
Efficiency
Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat.
In RF Power Amplifiers, such as cellular base stations and broadcast transmitters, specialist design techniques are used to improve efficiency. Doherty designs, which use a second transistor, can lift efficiency from the typical 15% up to 30-35% in a narrow bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to 60%, by modulating the supply voltage to the amplifier in line with the envelope of the signal.
Linearity
An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.
Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain.
Linearization is an emergent field, and there are many techniques, such as feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the non-linearities.
Noise
This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. The metric for noise performance of a circuit is Noise Factor. Noise Factor is the ratio of input signal to that of the output signal.
Output dynamic range
Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.[1]
Slew rate
Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.
Rise time
The rise time, tr, of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by:
tr * BW = 0.35, where tr is rise time in seconds and BW is bandwidth in Hz.
Settling time and ringing
Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is called the settle time, and is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output that cycles above and below its final value, leading to a delay in reaching final value quantified by the settling time above.
Overshoot
In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value.
Stability factor
Stability is a major concern in RF and microwave amplifiers. The degree of an amplifier's stability can be quantified by a so-called stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its two-port parameters.
Electronic amplifiers
Main article: Electronic amplifier
There are many types of electronic amplifiers, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Critical components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows.
Power amplifier
The term "power amplifier" is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier: see power amplifier classes.
Operational amplifiers (op-amps)
An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves.
Fully differential amplifiers (FDA)
A fully differential amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins.
[edit] Video amplifiers
These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented.
Oscilloscope vertical amplifiers
These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.
Distributed amplifiers
These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.
Microwave amplifiers:
Travelling wave tube (TWT) amplifiers
Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.
Klystrons
Very similar to TWT amplifiers, but more powerful and with a specific frequency "sweet spot". They generally are also much heavier than TWT amplifiers, and are therefore ill-suited for light-weight mobile applications. Klystrons are tunable, offering selective output within their specified frequency range.
Musical instrument (audio) amplifiers
An audio amplifier is usually used to amplify signals such as music or speech.
Other amplifier types:
Carbon microphone
One of the first devices used to amplify signals was the carbon microphone (effectively a sound-controlled variable resistor). By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications; analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were also used as amplifiers in telephone repeaters for long distance service.
Magnetic amplifier
Main article: magnetic amplifier
A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the hundreds of kilohertz.
Rotating electrical machinery amplifier
A Ward Leonard control is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy. Changes in generator field current result in larger changes in the output current of the generator, providing gain. This class of device was used for smooth control of large motors, primarily for elevators and naval guns.
Johnsen-Rahbek effect amplifier
The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen-Rahbek effect was not understood until the semiconductor era.
Mechanical amplifiers
Mechanical amplifiers were used in the pre-electronic era in specialized applications. Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.
Optical amplifiers
Optical amplifiers amplify light through the process of stimulated emission.
Miscellaneous types
There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.
In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually voltage or current. In audio applications, amplifiers operate loudspeakers used in PA systems to make the human voice louder or play recorded music. Amplifiers may be classified according to the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).
A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.
Figures of merit
The quality of an amplifier can be characterized by a number of specifications, listed below.
Gain
The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(Pout /(Pin)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).
Example: an audio amplifier with a gain given as 20dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical).
Bandwidth
The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The "satisfactory performance" may be different for different applications. However, a common and well-accepted metric are the half power points (i.e. frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths (otherwise called "frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or "plus or minus 1dB" (roughly the sound level difference people usually can detect).
A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points < 10 and > 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.
Efficiency
Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat.
In RF Power Amplifiers, such as cellular base stations and broadcast transmitters, specialist design techniques are used to improve efficiency. Doherty designs, which use a second transistor, can lift efficiency from the typical 15% up to 30-35% in a narrow bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to 60%, by modulating the supply voltage to the amplifier in line with the envelope of the signal.
Linearity
An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.
Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain.
Linearization is an emergent field, and there are many techniques, such as feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the non-linearities.
Noise
This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. The metric for noise performance of a circuit is Noise Factor. Noise Factor is the ratio of input signal to that of the output signal.
Output dynamic range
Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.[1]
Slew rate
Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.
Rise time
The rise time, tr, of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by:
tr * BW = 0.35, where tr is rise time in seconds and BW is bandwidth in Hz.
Settling time and ringing
Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is called the settle time, and is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output that cycles above and below its final value, leading to a delay in reaching final value quantified by the settling time above.
Overshoot
In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value.
Stability factor
Stability is a major concern in RF and microwave amplifiers. The degree of an amplifier's stability can be quantified by a so-called stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its two-port parameters.
Electronic amplifiers
Main article: Electronic amplifier
There are many types of electronic amplifiers, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Critical components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows.
Power amplifier
The term "power amplifier" is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier: see power amplifier classes.
Operational amplifiers (op-amps)
An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves.
Fully differential amplifiers (FDA)
A fully differential amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins.
[edit] Video amplifiers
These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented.
Oscilloscope vertical amplifiers
These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.
Distributed amplifiers
These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.
Microwave amplifiers:
Travelling wave tube (TWT) amplifiers
Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.
Klystrons
Very similar to TWT amplifiers, but more powerful and with a specific frequency "sweet spot". They generally are also much heavier than TWT amplifiers, and are therefore ill-suited for light-weight mobile applications. Klystrons are tunable, offering selective output within their specified frequency range.
Musical instrument (audio) amplifiers
An audio amplifier is usually used to amplify signals such as music or speech.
Other amplifier types:
Carbon microphone
One of the first devices used to amplify signals was the carbon microphone (effectively a sound-controlled variable resistor). By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications; analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were also used as amplifiers in telephone repeaters for long distance service.
Magnetic amplifier
Main article: magnetic amplifier
A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the hundreds of kilohertz.
Rotating electrical machinery amplifier
A Ward Leonard control is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy. Changes in generator field current result in larger changes in the output current of the generator, providing gain. This class of device was used for smooth control of large motors, primarily for elevators and naval guns.
Johnsen-Rahbek effect amplifier
The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen-Rahbek effect was not understood until the semiconductor era.
Mechanical amplifiers
Mechanical amplifiers were used in the pre-electronic era in specialized applications. Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.
Optical amplifiers
Optical amplifiers amplify light through the process of stimulated emission.
Miscellaneous types
There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.
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