Magnetic amplifier

The magnetic amplifier (colloquially known as a "mag amp") is an electromagnetic device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube amplifiers where robustness and high current capacity were required. World War II Germany perfected this type of amplifier, and it was used in the V-2 rocket. The magnetic amplifier was most prominent in power control and low-frequency signal applications from 1947 to about 1957, when the transistor began to supplant it. The magnetic amplifier has now been largely superseded by the transistor-based amplifier, except in a few safety critical, high-reliability or extremely demanding applications. Combinations of transistor and mag-amp techniques are still used.

Visually a mag amp device may resemble a transformer, but the operating principle is quite different from a transformer – essentially the mag amp is a saturable reactor. It makes use of magnetic saturation 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. Another common design uses a single core shaped like the number "8" with one control winding and two AC windings as shown in the photo above. A small DC current from a low-impedance source is fed into the control winding. 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 the point 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.

Two magnetic cores are used because the AC current will generate high voltage in the control windings. By connecting them in opposite phase, the two cancel each other, so that no current is induced in the control circuit. The alternate design shown above with the "8" shaped core accomplishes this same objective magnetically.

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. The reactor cores of magnetic amplifiers withstand neutron radiation extremely well. For this special reason magnetic amplifiers have been used in nuclear power applications.

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. Solid-state electronic 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.

Magnetic amplifiers introduce substantial harmonic distortion to the output waveform consisting entirely of the odd harmonics. Unlike the silicon controlled rectifiers or TRIACs which replaced them, the magnitude of these harmonics decreases rapidly with frequency so interference with nearby electronic devices such as radio receivers is uncommon.