Magnetic Amplifier

an amplifier for electrical signals based on the use of the nonlinear relationship inherent in ferromagnetic materials between the magnetic induction B and the magnetic field intensity H. The control elements in magnetic amplifiers are inductance coils with ferromagnetic cores in which two variable magnetic fields are acting; one varies at the frequency of the power supply, and the other at the frequency of the signal being amplified.

The simplest magnetic amplifier consists of two closed magnetic cores with windings W₁ that are connected in series and are supplied from a source of alternating voltage ~ U (see Figure 1). The secondary windings W₂ are connected in series and opposite to each other, so that closing windings W₂ through a small resistance does not result in any change of the current strength i₁ in the primary windings. If direct current is passed through windings W₂., then, because of the nonlinear nature of the magnetization curve for the cores, the dynamic magnetic permeability and, correspondingly, the inductance L₁ of the primary windings are reduced, thereby increasing the current in them. An arrangement such as that shown in Figure 1 (without the load resistance R_L) is called a controlled choke coil, which becomes an amplifier if R_L is connected in series with its wind-

Figure 1. Diagram of the simplest magnetic amplifier: (~U) alternating voltage, (R_L) load resistance, (W₁ primary windings, (W₂) secondary windings, (MC) magnetic cores, (=U) DC voltage, (i₁) current in primary windings, (i₂) current in primary windings, (i₂) current in secondary windings (singnal being amplified)

ings W₁ and if a signal current i₂ that is direct or alternates slowly (compared with the rate of change of the power-supply voltage ~U), rather than direct current, is fed to winding W₂.

A magnetic amplifier differs basically from vacuum-tube and transistor amplifiers in that the control signal varies the inductance L₁, which is in series with the load R_L, rather than the internal resistance of a tube or transistor, thus causing the load current to vary. It is essentially a modulator in which a higher-frequency load current is amplitude-modulated by the low-frequency amplified signal. To produce a signal at the output of a magnetic amplifier that has the same shape as the amplified signal, a rectifier that acts as a detector is added in the load circuit.

The current amplification factor K_i and the power amplification factor K_p for the simplest magnetic amplifiers are equal to

where R_a is the effective resistance of windings W₂, Δi_1av is the increment in the load current corresponding to an increment in the signal current Δi₂, and n₁ and n₂ are the number of turns in the primary and secondary windings, respectively. Magnetic amplifiers have relatively high inertia compared to vacuum-tube and transistor amplifiers, which is due mainly to the delay of the change in current i₂ through the control winding with respect to the change in the voltage supplied to the input of the magnetic amplifier. Consequently, they are used primarily to amplify DC or slowly alternating signals. The inertia can be reduced (the speed of response may be increased) by introducing transient feedback, by increasing the number of amplifier stages, by connecting a differentiating circuit to the input, or by shunting the load with a capacitance. To extend the frequency range of the amplified oscillations toward the higher frequencies, the use of magnetic amplifiers in combination with vacuum-tube, transistor, and dynamoelectric amplifiers is desirable.

There are hundreds of versions of magnetic amplifier circuits and designs, with various types of load characteristics, feedback methods, numbers and shapes of cores, forms of amplified signals, biasing systems, and modes of operation. The choice of a type of magnetic amplifier depends on the gain requirements, the frequency of the amplified oscillations, and the area of use. Magnetic amplifiers have a wide range of uses, high-precision measuring instruments to automatic control devices for high-power production machines (rolling mills, excavators, and so on). The extensive use of such amplifiers results from their advantages: long service life, high reliability, simplicity of use, substantial amplification, low threshold of sensitivity for DC signals (10~^-19-10^-17 watt), wide range of power amplification (from 10^-13-10^-6 watt up to tens and even hundreds of kilowatts), constant operational readiness, the feasibility of summing several control signals at the input, substantial overload capability, fire and explosion safety, and the stability of their characteristics during operation.