Some embodiments regard a circuit comprising: a high voltage transistor providing a resistance; an amplifier configured to receive a current and to convert the current to a first voltage that is used in a loop creating the current; and an automatic level control circuit that, based on an AC amplitude of the first voltage, adjusts a second voltage at a gate of the high voltage transistor and thereby adjusts the resistance and the first voltage; wherein the automatic level control circuit is configured to adjust the first voltage toward the first reference voltage if the first voltage differs from a first reference voltage.