High impedance biasing circuit

A pre-amplifier for a hard disk drive, the pre-amplifier comprising a biasing circuit including:

• a high impedance bias stage coupled to first and second sensor pads;
• a first operational transconductance amplifier having a first bandwidth;
• a second operational transconductance amplifier having a second bandwidth lower than the first bandwidth;
• a transistor coupled to a first supply voltage and the resistive sensor;
• a compensation capacitor coupled between a control terminal of the transistor and the first supply voltage;
• and
• switching circuitry configured to:
• couple the first operational transconductance amplifier to control a control voltage of the transistor during an initial biasing phase, and
• couple the second operational transconductance amplifier to control the control voltage during a continuous operation phase.

A method of biasing a resistive sensor in a hard disk drive, the method comprising:

• receiving a differential reference voltage and a common mode reference voltage at a high impedance bias stage coupled to a first sensor pad and a second sensor pad;
• controlling control voltages of at least one transistor coupled to supply voltages using a first operational transconductance amplifier having a first bandwidth during an initial biasing phase;
• providing compensation through capacitors coupled between control terminals of the at least one transistor and the supply voltages;
• transitioning to a second operational transconductance amplifier having a second bandwidth lower than the first bandwidth for a continuous operation phase;
• and
• maintaining bias conditions across the resistive sensor while allowing signal detection.