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Linear amplifier

 

 
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Linear amplifier
 
 
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A linear amplifier is an electronic
Electronics

Electronics refers to the flow of charge through nonmetal electrical conductor , whereas electrical refers to the flow of charge through metal electrical conductor....
 circuit whose output is proportional to its input, but capable of delivering more power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio
Amateur radio

Amateur radio, often called Etymology of ham radio, is both a hobby and a service in which participants, called "hams," use various types of radio communications equipment to communicate with other radio amateurs for Public services, recreation and self-training....
. Other types of linear amplifier are used in audio
Sound

Sound is vibration transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a threshold of hearing to be heard, or the sensation stimulated in organs of hearing by such vibrations....
 and laboratory equipment
Laboratory equipment

Laboratory equipment refers to the various tools and equipment used by scientists working in a laboratory. These include tools such as Bunsen burners, and microscopes as well as specialty equipment such as operant conditioning chambers, spectrophotometers and calorimeters....
.

F linear amplifier can be based on either solid state
Solid state (electronics)

Solid-state electronic components, devices, and systems are based entirely on the semiconductor, such as transistors, microprocessor chips, and the bubble memory....
 or vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
 technology.






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Encyclopedia


A linear amplifier is an electronic
Electronics

Electronics refers to the flow of charge through nonmetal electrical conductor , whereas electrical refers to the flow of charge through metal electrical conductor....
 circuit whose output is proportional to its input, but capable of delivering more power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio
Amateur radio

Amateur radio, often called Etymology of ham radio, is both a hobby and a service in which participants, called "hams," use various types of radio communications equipment to communicate with other radio amateurs for Public services, recreation and self-training....
. Other types of linear amplifier are used in audio
Sound

Sound is vibration transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a threshold of hearing to be heard, or the sensation stimulated in organs of hearing by such vibrations....
 and laboratory equipment
Laboratory equipment

Laboratory equipment refers to the various tools and equipment used by scientists working in a laboratory. These include tools such as Bunsen burners, and microscopes as well as specialty equipment such as operant conditioning chambers, spectrophotometers and calorimeters....
.

Explanation

An RF linear amplifier can be based on either solid state
Solid state (electronics)

Solid-state electronic components, devices, and systems are based entirely on the semiconductor, such as transistors, microprocessor chips, and the bubble memory....
 or vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
 technology. Most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tube
Vacuum tube

In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
s (valves) and can provide between 10 to 20 times RF power amplification. For example, a transmitter driving the input with 100 watts will be amplified to 2000 watts (2 kW) output to the antenna. Solid state linear amplifiers are more commonly in the 500 watt range and can be driven by as little as 25 watts. However, AM radio broadcast transmitters of up to 50 kW are now solid state. Large vacuum valves or klystron
Klystron

A klystron is a specialized Linear particle accelerator vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern particle accelerators....
s are still used for international long, medium, and shortwave broadcast transmitters between 500 kW up to 2 MW. Usually, several 500 kW transmitters are used in parallel.

The legal power limit for licensed amateur operators vary from country to country but in the United States it is legal to transmit up to 1.5 kW peak envelope power
Peak envelope power

Peak envelope power is the average Power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope, under normal operating conditions....
, while in the UK
United Kingdom

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom , the UK or Britain,is a sovereign state located off the northwestern coast of continental Europe....
 the limit is 400 watts PEP.

As most amateur radio transceivers can output between 100 and 150 watts, an amplifier is needed to reach higher power levels. Large vacuum tube linear amplifiers are based on old radio broadcast techniques and generally rely on a pair of large vacuum tubes supplied by a very high voltage power supply to convert large amounts of electrical energy into radio frequency energy. Linear amplifiers need to operate with class A
Class A

The term Class A may refer to:*Class A surfaces in automotive design*Class A office space*Class A television service, system for regulating low power stations in the United States...
 or class AB biasing, which makes them relatively inefficient. While class C
Class C

The term Class C may refer to:*Electronic amplifier#Class C, a category of electronic amplifier* Misuse of Drugs Act 1971##Class C drugs , as defined by the UK's Misuse of Drugs Act 1971...
 has far higher efficiency, a class C
Class C

The term Class C may refer to:*Electronic amplifier#Class C, a category of electronic amplifier* Misuse of Drugs Act 1971##Class C drugs , as defined by the UK's Misuse of Drugs Act 1971...
 amplifier is not linear, and is only suitable for the amplification of constant envelope signals. Such signals include FM, FSK
FSK

FSK can have alternative meanings:* FSK * Frequency-shift keying, a modulation scheme* Forsvarets Spesialkommando , a Norwegian special forces military unit...
, and CW (morse code
Morse code

Morse code is a type of character encoding that transmits telegraphic information using rhythm. Morse code uses a standardized sequence of short and long elements to represent the alphanumeric, punctuation and special characters of a given message....
).

Classes of power amplifier

  • The class A amplifiers are very inefficient, they can never have an efficiency better than 50%. The semiconductor
    Semiconductor

    A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
     or valve
    Valve

    A valve is a device that regulates the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe Piping and plumbing fittings, but are usually discussed as a separate category....
     conducts throughout the entire RF cycle. The mean anode current for a valve should be set to the middle of the linear section of the curve of the anode current vs grid bias potential.


  • Class B amplifiers are more efficient, they can be 60 to 65% efficient. The semiconductor
    Semiconductor

    A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
     or vacuum tube
    Vacuum tube

    In electronics, a vacuum tube, electron tube , thermionic valve, or just valve is a device used to amplifier, switch, otherwise modify, or create an Electricity signal by controlling the movement of electrons in a low-pressure space....
     conducts through half the RF cycle.


  • Class AB1 is where the grid is more negatively biased than it is in class A.


  • Class AB2 is where the grid is often more negatively biased than in AB1, also the size of the input signal is often larger. When the drive is able to make the grid become positive the grid current will increase.


  • In a class B amplifier the grid current drawn will be large, and a large drive power will be required.


  • Class C amplifiers are still more efficient, they can be about 75% efficient with a conduction range of about 120o but they are very non linear. They can only be used for FM or CW use only. The semiconductor
    Semiconductor

    A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
     or valve
    Valve

    A valve is a device that regulates the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe Piping and plumbing fittings, but are usually discussed as a separate category....
     conducts through less than half the RF cycle. The increase in efficiency can allow a given valve to deliver more RF power than it could do so in class A or AB. For instance two 4CX250B tetrodes operating at 144 MHz can deliver 400 watts in class A, but when biased into class C they can deliver 1000 watts without fear of overheating. Even more grid current will be needed.


A side effect of improving the efficiency is that the current drawn from the high voltage supply will vary more as a function of the power input into the amplifier, this can result in unwanted effects such as the output of the HT pack being modulated by the audio
Sound

Sound is vibration transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a threshold of hearing to be heard, or the sensation stimulated in organs of hearing by such vibrations....
 modulated RF driven into the amplifier. An extreme example of this has been observed during radio contests where a large linear is used to amplify morse
Morse code

Morse code is a type of character encoding that transmits telegraphic information using rhythm. Morse code uses a standardized sequence of short and long elements to represent the alphanumeric, punctuation and special characters of a given message....
 (carrier on/off keying), it has been the case under some conditions that the wildly changing load on the petrol-driven 240 volt 50 Hz generator set has been sufficient to cause the petrol motor to change speed (and supply frequency) as it attempts to maintain its AC output voltage at 240 volts. In short any person able to hear the petrol engine will then be able to hear the morse.

A simple cure for this is to always attach a fixed small load such as several light bulbs to the output of the 240 volt AC generator.

Early large amplifiers

The first large amplifier used in the United States
United States

The United States of America is a Federal government constitutional republic comprising U.S. state and a federal district. The country is situated mostly in central North America, where its Contiguous United States and Washington, D.C., the Capital districts and territories, lie between the Pacific Ocean and Atlantic Oceans, Borders of the U...
 for public domestic radio broadcasting was in operation between 1934 and 1939 at WLW
WLW

WLW is a clear channel talk radio station located in Cincinnati, Ohio, run by Clear Channel Communications. The station broadcasts locally on 700 kHz AM broadcasting....
 in Cincinnati. It was an experimental amplifier and was driven by the radio station's regular 50 kW transmitter. Not a linear amplifier, it operated in Class C with high-level plate modulation. The amplifier required a dedicated 33kV electrical substation
Electrical substation

An electrical substation is a subsidiary station of an electricity Electrical generation, electric power transmission and electricity distribution system where voltage is transformed from high to low or the reverse using transformers....
 and a large pond complete with fountains
Fountain

A traditional fountain is an arrangement where water issues from a source , fills a basin of some kind, and is drained away. Fountains may be wall fountains or free-standing....
 for cooling
Cooling

Cooling is the Heat transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:...
. It operated with a power input of about 750 kW (plus another 400 kW of audio for the modulator) and its output was 500 kW.

See also

Amplifiers