Magnetic amplifiers

D.C. controlled, saturable-core reactors were used successfully by Germany during the 1939¿45 War for the amplification and mixing of powers down to a microwatt. Typical performance figures are quoted. An explanation of transductor action is given, and the turns-ratio law relating output current and input signal excitation is derived for a core of infinite permeability. Transductor behaviour on short-circuit is treated analytically, and the non-dimensional plotting of characteristics is discussed. The effect of permeability on the performance with self-excitation is illustrated with curves for Mumetal, Radiometal and Stalloy, and arrangements of transductors to give duo-directional magnetic amplifiers are dealt with. After some notes on selenium rectifier behaviour, fundamental design principles are developed in which power output, amplification and time lag are related to supply frequency, core area and self-excitation or feedback. Transductor time-constants are directly proportional to power amplification, and inversely proportional to supply frequency and positive feedback. Therefore, to obtain a high amplification/time-constant ratio, the correct design procedure is to use a high supply frequency and to connect in cascade a number of stages having positive feedback. It is concluded that transductors are a valuable addition to circuit technique for the amplification of d.c. powers down to about 10¿9 watt.