Voltage buffer for input voltages above a supply voltage or below ground voltage

A power monitoring circuit to monitor a line voltage associated with a power source, comprising:

• a buffer amplifier comprising a source follower and a feedback amplifier, the feedback amplifier configured to control a drain current of the source follower to remain substantially constant independent of a load;
• and
• a chopper circuit coupled to an input of the buffer amplifier.

Apparatus comprising:

• means for controlling a drain current of a source follower in a buffer amplifier to remain substantially constant independent of a load, wherein the buffer amplifier is coupled to an input voltage based upon a line voltage and to a supply voltage derived from the input voltage;
• and
• means for controlling an output impedance of the buffer amplifier.