Magnetic-Amplifier Limitations

Several limitations may affect the practical usefulness of magnetic amplifiers. Some of these limitations are beneficial in certain applications:

• Residual output with zero input.
• When more than one reactor is used in a circuit, reactor cores must often be matched.
• Zero drift. At low input levels (of the order of 10^-13 watt for toroids of rectangular loop core material) magnetic amplifiers do not track because of hysteresis.
• Amplifiers with feedback or high-gain self-saturated amplifiers are subject to instability when biased to cut-off and may change linear amplifiers into bistable amplifiers.
• When the amplifier operates over a wide range of ambient temperature, variations in resistance of the reactors and rectifiers, and hysteresis loop width, cause changes in gain, output, and balance.
• Response time of a magnetic amplifier is a limitation in comparison with an electronic amplifier.
• Variations in supply frequency and voltage cause variations in gain and output, especially with self-saturated amplifiers.
• Whereas the vacuum tube is a relatively high-impedance device, the magnetic amplifier is better adapted to low impedances, where the turns arc fewer.
• Saturation inductance is greater than the leakage inductance of the reactor, measured as in a transformer. The B-H curve slope at B_s, even with rectangular loop core materials, always gives μ greater than unity at the top. This effect reduces output and gain, and causes a sloping wave front at the instant of firing.

Many ingenious circuits have been devised to overcome one or more of these limitations. For descriptions of these circuits, for refinements of operation, and for fields of application, the reader is referred to the bibliography on magnetic amplifiers.