Low noise amplifier circuit for a thermal varying resistance

A hard disk drive, comprising:

• a thermal resistor configured to perform sensing and control of fly height of a tip in the hard disk drive, the thermal resistor having a first sense terminal and a second sense terminal;
• and
• a circuit configured to synthesize the thermal resistor, the circuit comprising:
• a first amplifier circuit, the first amplifier circuit comprising a first transistor, a first load resistor, a first capacitor, and a first current generator, the first transistor configured as a first common gate amplifier, a first current path terminal of the first transistor coupled to a first supply voltage terminal through the first load resistor, a second current path terminal of the first transistor coupled to the first sense terminal and a second supply voltage terminal, a control terminal of the first transistor coupled to a first bias voltage terminal and the first capacitor, the first capacitor configured as a low band filter, and
• a second amplifier circuit, the second amplifier circuit comprising a second transistor, a second load resistor, a second capacitor, and a second current generator, the second transistor configured as a second common gate amplifier, a first current path terminal of the second transistor coupled to the first supply voltage terminal through the second load resistor, a second current path terminal of the second transistor coupled to the second sense terminal and the second supply voltage terminal, a control terminal of the second transistor coupled to a second bias voltage terminal and the second capacitor, the second capacitor configured as a low band filter.

A method for operating a circuit used to perform sensing and control of a fly height type in a hard disk drive through a sense resistor, the method comprising:

• having a first amplifier circuit, the first amplifier circuit comprising a first transistor, a first load resistor, a first capacitor, and a first current generator, the first transistor configured as a first common gate amplifier, a first current path terminal of the first transistor coupled to a first supply voltage terminal through the first load resistor, a second current path terminal of the first transistor coupled to a first sense terminal of the sense resistor and a second supply voltage terminal, a control terminal of the first transistor coupled to a first bias voltage terminal and the first capacitor, the first capacitor configured as a low band filter, the sense resistor;
• having a second amplifier circuit, the second amplifier circuit comprising a second transistor, a second load resistor, a second capacitor, and a second current generator, the second transistor configured as a second common gate amplifier, a first current path terminal of the second transistor coupled to the first supply voltage terminal through the second load resistor, a second current path terminal of the second transistor coupled to a second sense terminal of the sense resistor and the second supply voltage terminal, a control terminal of the second transistor coupled to a second bias voltage terminal and the second capacitor, the second capacitor configured as a low band filter;
• applying a constant voltage to the sense resistor corresponding to a selection of a voltage mode operation of the circuit;
• applying a constant current to the sense resistor based on a selection of a current mode operation of the circuit;
• and
• amplifying, using the first amplifier circuit and the second amplifier circuit, input signals from the sense resistor in accordance with a variation in resistance at the sense resistor.