Linear amplifier
Encyclopedia
A linear amplifier is an electronic
circuit whose output is proportional to its input, but capable of delivering more power
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
. Other types of linear amplifier are used in audio
and laboratory equipment
.
or vacuum-tube technology. For an amplifier to be considered linear, the amplified output must be a faithful reproduction of the input. The basic linear amplifier has limitations that are functions of its load impedance, supply voltage, input bias current, and limitations of power output capabilities.
s (valves) and can provide 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 tubes are still used for international long, medium, and shortwave broadcast transmitters between 500 kW up to 2 MW.
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
output, while in the UK
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 or class-AB biasing, which makes them relatively inefficient. While class C has far higher efficiency, a class-C amplifier is not linear, and is only suitable for the amplification of constant-envelope signals. Such signals include FM
, FSK
, and CW (morse code
).
for public domestic radio broadcasting was in operation between 1934 and 1939 at WLW
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 electrical substation
and a large pond complete with fountain
s for cooling
. 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.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
circuit whose output is proportional to its input, but capable of delivering more power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
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 is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
. Other types of linear amplifier are used in audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
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 speciality equipment such as operant conditioning chambers, spectrophotometers and calorimeters...
.
Explanation
An RF linear amplifier can be based on either solid-stateSolid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
or vacuum-tube technology. For an amplifier to be considered linear, the amplified output must be a faithful reproduction of the input. The basic linear amplifier has limitations that are functions of its load impedance, supply voltage, input bias current, and limitations of power output capabilities.
Amplifier classes
There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy. Their use in RF applications are listed briefly below:- Class-A amplifiers are very inefficient, they can never have an efficiency better than 50%. The semiconductorSemiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
or vacuum tubeVacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
conducts throughout the entire RF cycle. The mean anode current for a vacuum tube should be set to the middle of the linear section of the curve of the anode current vs grid bias potential.
- Class B can be 60 to 65% efficient. The semiconductor or vacuum tube conducts through half the RF cycle but require large drive power.
- 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.
- Class-C amplifiers are still more efficient, they can be about 75% efficient with a conduction range of about 120o, but they are very nonlinear. They can only be used for non-AM modes, such as FM, CW, or RTTY. The semiconductor or vacuum tube conducts through less than half the RF cycle. The increase in efficiency can allow a given vacuum tube 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.
Amateur radio
Most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tubeVacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s (valves) and can provide 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 tubes are still used for international long, medium, and shortwave broadcast transmitters between 500 kW up to 2 MW.
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. The United States Federal Communications Commission uses PEP to set maximum power...
output, while in the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
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 or class-AB biasing, which makes them relatively inefficient. While class C has far higher efficiency, a class-C amplifier is not linear, and is only suitable for the amplification of constant-envelope signals. Such signals include FM
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
, FSK
FSK
FSK can have alternative meanings:* Federal Counterintelligence Service, , Federal Counterintelligence Service of Russia)...
, and CW (morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
).
Early large amplifiers
The first large amplifier used in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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...
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 electrical substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...
and a large pond complete with fountain
Fountain
A fountain is a piece of architecture which pours water into a basin or jets it into the air either to supply drinking water or for decorative or dramatic effect....
s for cooling
Cooling
Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:-Techniques:* Air conditioning* Air cooling* Computer cooling* Cryogenics* Conduction * Infrared solar cells* Laser cooling...
. 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.