This work deals with the development of a high sensibility DNA biosensor based on a 50MHz Quartz Crystal Microbalance (QCM) Oscillator Electronic Circuit. The designed QCM oscillator sensor is able to detect the presence of complementary DNAs in a solution that match the sequence on a given strand in function of the changes in the output frequency of the oscillator. The design is adapted so that the Barkhausen condition is satisfied even when the quartz is immerged in liquid media. Also a comparative study of the developed 50MHz biosensor in front of a QCM oscillator with smaller frequency is carried out, with object of checking if the sensibility of the oscillator increases, allowing to detect smaller concentrations of the complementary DNA.