Quick-response and sensitive magnetic field sensor using a 200 MHz resonant field effect transistor multivibrator with amorphous wire magnetoinductive elements

Abstract

A circumferential flux change in an amorphous wire (a-wire) caused by a high frequency wire current induces an inductive a.c. voltage e L between both ends of the wire. The amplitude of e L decreases with increasing external field H ex. We call the effect “the magnetoinductive (MI) effect”. This effect sensitively appears in a nearly zero-magnetostriction wire which was cold drawn followed by annealing under tension. A multivibrator resonance oscillator having an oscillation frequency higher than 200 MHz was constructed using two a-wires and two N-channel junction field effect transistors (FETs) with some resistors and capacitors. The internal inductance L of the a-wire and a capacitance C 0 between the source and the drain of the FET determine the oscillation frequency given by 1 2 π(LC 0) 1 2 at resonance. According to the equivalent circuit, the values of L and C 0 are estimated to be 43 nH and 11 pF respectively. The condition of stable oscillation for the MI FET oscillator was analysed. A small magnetic field less than several oersteds can be linearly detected by adding a bias field inversely at each a-wire.