You have now been shown various ways that electron tubes (NEETS, Module 6) and transistors
(NEETS, Module 7) can be used to amplify signals. You have also been shown the way in which this is
done. There is another type of amplifier in usethe MAGNETIC AMPLIFIER, sometimes called the
The magnetic amplifier has certain advantages over other types of amplifiers. These include (1) high
efficiency (up to 90 percent); (2) reliability (long life, freedom from maintenance, reduction of spare parts
inventory); (3) ruggedness (shock and vibration resistance, high overload capability, freedom from effects
of moisture); and (4) no warm-up time. The magnetic amplifier has no moving parts and can be
hermetically sealed within a case similar to the conventional dry-type transformer.
However, the magnetic amplifier has a few disadvantages. For example, it cannot handle low-level
signals; it is not useful at high frequencies; it has a time delay associated with the magnetic effects; and
the output waveform is not an exact reproduction of the input waveform (poor fidelity).
The magnetic amplifier is important, however, to many phases of naval engineering because it
provides a rugged, trouble-free device that has many applications aboard ship and in aircraft. These
applications include throttle controls on the main engines of ships; speed, frequency, voltage, current, and
temperature controls on auxiliary equipment; and fire control, servomechanisms, and stabilizers for guns,
radar, and sonar equipment.
As stated earlier, the magnetic amplifier does not amplify magnetism, but uses electromagnetism to
amplify a signal. It is a power amplifier with a very limited frequency response. Technically, it falls into
the classification of an audio amplifier; but, since the frequency response is normally limited to 100 hertz
and below, the magnetic amplifier is more correctly called a low-frequency amplifier.
The basic principle of a magnetic amplifier is very simple. (Remember, all amplifiers are current-
control devices.) A magnetic amplifier uses a changing inductance to control the power delivered to a
BASIC OPERATION OF A MAGNETIC AMPLIFIER
Figure 3-29 shows a simple circuit with a variable inductor in series with a resistor (representing a
load). The voltage source is 100 volts at 60 hertz.
Figure 3-29.Variable inductor in series with a load.