Fig. 4

Preamplifier. The first stage of our amplifying and filtering stage is based on non-inverted operational amplifier topology. The main goal is to provide low-noise amplification on the extra low-level-induced voltage at the induction coil terminals. Resistors R7 and R3 through feedback connection define the gain value as well as the output RC filter stage (R2, C3) eliminates flicker noise for better performance. We also used bypass capacitors at both positive and negative power supplies. The variable gain amplifier is the last stage of our amplifying and filtering chain. That stage is also based on non-inverted operational amplifier topology having a potentiometer in feedback instead of a fixed resistor value component. That potentiometer offers an efficient way to adjust total gain