Bi<sub>4</sub>Ge<sub>3</sub>O<sub>12</sub> magneto-optic crystal was chosen as sensitive materials, and a current sensor based on Faraday rotation of
materials was designed. A ferromagnetic collector with an air gap and feedback windings were utilized in the sensor to
construct a closed-loop current sensor. The magneto-optic crystal showed stronger magneto-optic effect and smaller
temperature coefficient when it was exposed to the light, which was produced by a semiconductor laser with a working
wavelength of 655nm. The ferromagnetic collector was used to collect the magnetic field generated by the measured
current. The collector could greatly reduce the influence of the position variation of the conductor on the output. The
measurement system was a closed-loop system due to introducing the negative feedback coil to the measuring system, so
the linear range, linearity, temperature stability and dynamic characteristic index of the sensor were improved greatly.
The principle of this sensor was introduced in detail. The structure of this sensitive component, as well as the signal
conditioning circuit, was given. Some experimental data and the interpretation of these data were presented.