Wheatstone bridge based offset cancelling method utilizing a JFET as a voltage-controlled resistor

Abstract

We designed and investigated an offset cancelling circuit based on a Wheatstone bridge at the output of a Hall-effect sensor. This circuit not only cancelled the offset voltage caused by unintentional Hall probe misalignment but also provided a voltage gain. The Wheatstone bridge circuit consisted of a JFET, two fixed resistors and a variable resistor. In the absence of magnetic field, the JFET was used as a voltage-controlled resistor and its channel resistance was determined by the Hall offset voltage. To compensate the node voltage induced in the JFET channel with the non-zero Hall offset voltage, the variable resistor in the bridge circuit was adjusted to produce the cancelled output voltage between the two bridge nodes. The difference output of the bridge nodes acted as the offset cancelled Hall voltage. When the magnetic field was applied, the branch with the JFET acted as a common source amplifier with the input from the measured Hall voltage. Therefore, the difference output represented the amplified offset cancelled Hall voltage with the magnetic field. The operating point of the amplifier was chosen for linearity and its gain was measured to be 26.4.