The variable gain amplifier of the invention includes an input node pair, a first output node pair, a voltage-current converter, a plurality of first resistances, a first current source, a second current source, a second output node pair, a third output node pair and a switch circuit. Referencing a combined X-Y-Z Cartesian and polar coordinates. The first medium processing apparatus may be structured to carry out writing and reading information to and from the information recording medium.