Graphene, with its distinctive and outstanding optoelectronic properties, has prompted its use in a wide variety of applications such as transparent conductive electrodes and modulators. Recently, due to advances in graphene synthesis methods, large-area graphene field effect transistors (GFET) have displayed unique modulation of terahertz waves. By transferring chemical vapor deposited graphene films (~1cm × 1cm) onto a high resistivity silicon substrate to fabricate a GFET, we calculate the AC conductivity extrapolated by measuring the terahertz transmission. We demonstrate the tunable modulation of the AC conductivity using a DC gate voltage, in parallel with broadband (0.2-1.2 THz) frequency domain measurements.