Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N2, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (Vb) or nitrogen partial pressures (PN) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp3 fraction were formed when the Vb was in the range of -10 to -50V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low PN (PN<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of Natoms in ta-C network are identified and discussed.