Precision Power Amplifier with Enhanced Current Capacity for Characterization of Magnetic Materials

Direct-coupled multistage linear power amplifiers serve as a low cost power source to provide DC-biased alternating excitations for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B$</tex>-<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$H$</tex> characterization of magnetic materials. The voltage and current capacity of amplifier should be adequate to produce the desired value of B and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$H$</tex> in the test sample. The current capacity of such amplifiers is often limited due to temperature rise of the current-carrying device in the output stage, rather than the rated current of transistor. This work presents the design of the output current gain stage of a precision power amplifier having multiple current-carrying transistors in parallel. The required number of transistors in parallel is evaluated using thermal circuit analysis to safely achieve a peak inductive current of 10 A with a bias rail voltage of 75-0-75 V. The design of the base drive stage for paralleled current-carrying transistors is performed. The designed amplifier is fabricated and tested with inductive load to ensure a safe temperature rise. The capabilities in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B$</tex>-<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$H$</tex> characterization are demonstrated through tests on a 0.35 mm thick Si-Fe steel at multiple frequencies ranging from 10 Hz to 5 kHz.