Balanced current mirrors for biasing a magnetic resistor in a hard disk drive

A method comprising:

• determining a difference between a first current in a first branch circuit and a second current in a second branch circuit, wherein the first branch circuit and the second branch circuit are coupled to respective terminals of a magnetic resistor (MR);
• outputting a reference current based on the difference;
• and
• performing a balanced mirroring of the reference current by a first set of current mirrors mirroring the reference current to provide a source current to one of the terminals and a second set of current mirrors mirroring the reference current to provide a sink current to another of the terminals, the source current and the sink current reducing 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.

A bias circuit comprising:

• a first branch comprising a first resistor and a first transistor, wherein one terminal of the first resistor is coupled to a collector of the first transistor, another terminal of the first resistor is coupled to a voltage reference, and an emitter of the first transistor is coupled to one terminal of an MR;
• a second branch comprising a second resistor and a second transistor, wherein one terminal of the second resistor is coupled to a collector of the second transistor, another terminal of the second resistor is coupled to the voltage reference, and an emitter of the second transistor is coupled to another terminal of the MR;
• a closed loop gain stage comprising a transimpedance amplifier (TIA) with one input coupled to the one terminal of the first resistor and another input coupled to the one terminal of the second resistor and an input of an error amplifier (EA) coupled to an output of the TIA;
• a current driver coupled to an output of the EA;
• and
• a first set of current mirrors coupled to the current driver and the one terminal of the MR and a second set of current mirrors coupled to the current driver and the other terminal of the MR, wherein each set of current mirrors have a same number of current mirrors.