a comparator;

a transistor connected to an output of the comparator;

a depletion mode device connected to ground and a first reference voltage (Vdd), and comprising a control gate directly connected to an output of the transistor and to an input of the comparator, the output of the transistor controlling a pinch-off voltage at the control gate of the depletion mode device; and

a first resistor connected directly between a predetermined voltage (Vdd1) and directly connected to the control gate of the depletion mode device, wherein when the transistor is OFF, the control gate body of the depletion mode device is pulled up by the resistor to the predetermined voltage (Vdd1) fed into the control gate.