In this paper, we propose a widely tunable CMOS biosensor with an active-inductor-based voltage-controlled oscillator (VCO) for detecting circulating tumor cells (CTCs) and exosomes. The active-inductor-based VCO is employed to achieve broadband operation in a relatively small area. To validate the operation of the proposed biosensor, electromagnetic (EM) and SPICE simulations are performed. The results of the EM simulation indicate that an electric field extends to cells on the biosensor, whereas the SPICE simulation verifies that the oscillation frequencies of the biosensor vary with the presence of cells. These simulation results demonstrate the biosensor’s potential in detecting biomarkers such as CTCs and exosomes.