Magnetic amplifier

Magnetic amplifier

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The magnetic amplifier is an electromagnetic
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube
Vacuum 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...

 amplifiers where robustness and high current capacity were required. World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 perfected this type of amplifier, and it was used for instance in the V-2 rocket
V-2 rocket
The V-2 rocket , technical name Aggregat-4 , was a ballistic missile that was developed at the beginning of the Second World War in Germany, specifically targeted at London and later Antwerp. The liquid-propellant rocket was the world's first long-range combat-ballistic missile and first known...

. The magnetic amplifier was most prominent in power control and low-frequency signal applications from 1947 to about 1957, when the transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 began to supplant it. The magnetic amplifier has now been largely superseded by the transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

-based amplifier, except in a few safety critical, high reliability or extremely demanding applications. Combinations of transistor and mag-amp techniques are still used.


The magnetic amplifier is a static device with no moving parts. It has no wear-out mechanism and has a good tolerance to mechanical shock and vibration. It requires no warm-up time.
Multiple isolated signals may be summed by additional control windings on the magnetic cores. The windings of a magnetic amplifier have a higher tolerance to momentary overloads than comparable solid-state devices. The magnetic amplifier is also used as a transducer in applications such as current measurement and the flux gate compass
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...



The gain available from a single stage is limited and low compared to electronic amplifiers. Frequency response of a high gain amplifier is limited to about one-tenth the excitation frequency, although this is often mitigated by exciting magnetic amplifiers with currents at higher than utility frequency
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...

. Solid-state amplifiers can be more compact and efficient than magnetic amplifiers. The bias and feedback windings are not unilateral, and may couple energy back from the controlled circuit into the control circuit. This complicates the design of multistage amplifiers when compared with electronic devices.

Principle of operation

Visually a mag amp device may resemble a transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 but the operating principle is quite different from a transformer - essentially the mag amp is a saturable reactor
Saturable reactor
A saturable reactor in electrical engineering is a special form of inductor where the magnetic core can be deliberately saturated by means of a direct electric current in a control winding...

. It makes use of magnetic saturation
Saturation (magnetic)
Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...

 of the core, a non-linear property of a certain class of transformer cores. For controlled saturation characteristics the magnetic amplifier employs core materials that have been designed to have a specific B-H curve shape that is highly rectangular, in contrast to the slowly-tapering B-H curve of softly saturating core materials that are often used in normal transformers.

The typical magnetic amplifier consists of two physically separate but similar transformer magnetic cores, each of which has two windings - a control winding and an AC winding. A small DC current from a low impedance source is fed into the series-connected control windings. The AC windings may be connected either in series or in parallel, the configurations resulting in different types of mag amps. The amount of control current fed into the control winding sets the point in the AC winding waveform at which either core will saturate. In saturation, the AC winding on the saturated core will go from a high impedance state ("off") into a very low impedance state ("on") - that is, the control current controls at which voltage the mag amp switches "on".

A relatively small DC current on the control winding is able to control or switch large AC currents on the AC windings. This results in current amplification.


Magnetic amplifiers were important as modulation and control amplifiers in the early development of voice transmission by radio. A magnetic amplifier was used as voice modulator for a 2 kilowatt Alexanderson alternator
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...

, and magnetic amplifiers were used in the keying circuits of large high-frequency alternators used for radio communications. Magnetic amplifiers were also used to regulate the speed of Alexanderson alternators to maintain the accuracy of the transmitted radio frequency.

The ability to control large currents with small control power made magnetic amplifiers useful for control of lighting circuits, for stage lighting
Stage lighting
Modern stage lighting is a flexible tool in the production of theatre, dance, opera and other performance arts. Several different types of stage lighting instruments are used in the pursuit of the various principles or goals of lighting. Stage lighting has grown considerably in recent years...

 and for advertising signs. Saturable reactor amplifiers were used for control of power to industrial furnaces. Small magnetic amplifiers were used for radio tuning indicators, control of small motor and cooling fan speed, control of battery chargers.

Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

) power supplies, as well as in lighting control. Semiconductor based solid-state switches have largely superseded them, though recently there has been some regained interest in using mag amps in compact and reliable switching power supplies. PC ATX
ATX is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standards like the AT form factor. It was the first big change in computer case, motherboard, and power supply design in many years, improving standardization and interchangeability of parts...

 power supplies often use mag amps for secondary side voltage regulation.

Magnetic amplifiers are still used in some arc welders.

Magnetic amplifier transformer cores designed specifically for switch mode power supplies are currently manufactured by several large electromagnetics companies, including Metglas and Mag-Inc.

Magnetic amplifiers can be used for measuring high DC-voltages without direct connection to the high voltage and are therefore still used in the HVDC-technique.

Magnetic amplifiers were used by locomotives to detect wheel slip, until replaced by Hall Effect
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

 current transducers. The cables from two traction motors passed through the core of the device. During normal operation the resultant flux was zero as both currents were the same and in opposite directions. However, the currents would differ during wheel slip, producing a resultant flux that acted as the Control winding, developing a voltage across a resistor in series with the AC winding which was sent to the wheel slip correction circuits.

Early development

A voltage source and a series connected variable resistor may be regarded as a direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 signal source for a low resistance load such as the control coil of a saturable reactor which amplifies the signal. Thus, in principle, a saturable reactor is already an amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

, although before 20th century they were used for simple tasks, such as controlling lighting and electrical machinery as early as 1885.

In the early 20th Century, the General Electric Company, under the direction of engineer E. F. W. Alexanderson, developed a system of transoceanic radio communications, using continuous wave transmission over great distances. Alexanderson drew upon the work of Nikola Tesla and Reginald Fessenden as the inspiration for his system.

The result of this work was the 2 kW Alexanderson alternator
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...

, which produced radio frequencies from 50 to 100 kHz and which critics had previously denounced as impractical. Later, Guglielmo Marconi took a vested interest in the project and, in 1915, witnessed a demonstration of a new, 50 kW, 50 kHz alternator.

The experimental telegraphy and telephony demonstrations made during 1917 attracted the attention of the US Government, especially in light of partial failures in the transoceanic cable
Transatlantic telegraph cable
The transatlantic telegraph cable was the first cable used for telegraph communications laid across the floor of the Atlantic Ocean. It crossed from , Foilhommerum Bay, Valentia Island, in western Ireland to Heart's Content in eastern Newfoundland. The transatlantic cable connected North America...

 that snaked across the bottom of the Atlantic Ocean. The 50 kW alternator was commandeered by the US Navy and put into service in January 1918 and was used until 1920, when a 200 kW generator-alternator set was built and installed.

Usage in radio

Magnetic amplifiers were used early on to control large, high-power alternators by turning them on and off for telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

 or to vary the signal for voice modulation. However, the alternator's frequency limits were rather low to where a frequency multiplier had to be utilized to generate higher radio frequencies than the alternator was capable of producing. Even so, early magnetic amplifiers incorporating powdered-iron cores were incapable of producing radio frequencies above approximately 200 kHz. Other core materials, such as ferrite cores and oil-filled transformers, would have to be developed to allow the amplifier to produce higher frequencies.

Usage in aircraft

Magnetic amplifiers were used in aircraft systems (avionics
Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...

) before the advent of high reliability semiconductors. They were important in implementing early autoland
In aviation, autoland describes a system that fully automates the landing phase of an aircraft's flight, with the human crew merely supervising the process.-Description:...

 systems and Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

 made use of the technology for the control of its engine air intakes before subsequent development of a replacement system using digital electronics.

Usage in computing

Magnetic amplifiers were widely studied during the 1950s as a potential switching element for mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 computers. Mag amps could be used to sum several inputs in a single core, which was very useful in the arithmetic logic unit
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...

 (ALU). Custom tubes could do the same, but transistors could not, so the mag amp was able to combine the advantages of tubes and transistors in an era when the latter were expensive and unreliable.

However, that era was very short, lasting from the mid 1950s to about 1960, at which point new fabrication techniques were producing great improvements in transistors and dramatically lowering their price points. Only one large-scale mag amp machine was put into production, the UNIVAC Solid State
UNIVAC Solid State
The UNIVAC Solid State was a 2-address, bi-quinary coded decimal computer, with memory on a rotating drum with 5000 signed 10 digit words, spinning at 17,667 RPM in a helium atmosphere. It was announced by Sperry Rand in December 1958, as a response to the IBM 650...

, but a number of contemporary late-1950's/early-1960s computers made some use of the technology, like the Ferranti Orion
Ferranti Orion
The Orion was a mid-range mainframe computer introduced by Ferranti in 1959 and installed for the first time in 1961. Ferranti positioned Orion to be their primary offering during the early 1960s, complementing their high-end Atlas and smaller systems like the Sirius and Argus...


Misnomer uses

In the 1970s, Robert Carver designed and produced several high quality high-powered audio amplifiers, calling them magnetic amplifiers. In fact, they were in most respects conventional audio amplifier designs with an unusual power supply circuit. They were not magnetic amplifiers as defined in this article.

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