A bias circuit comprises a closed loop gain stage arranged to determine a difference between a first current in a first branch circuit and a second current in a second branch circuit, where the first branch circuit and second branch circuit are coupled to respective terminals of a magnetic resistor (MR). A first set of current mirrors is arranged to provide a source current to the first terminal of the MR and the second set of current mirrors is arranged to provide a sink current to the second terminal of the MR. The first set of current mirrors and a second set of current mirrors are balanced to reduce a difference in setting time between the source current and sink current. The source current and sink current further reduce the difference between the first current and the second current to provide a constant voltage bias to the MR based on a voltage of a voltage source.