This paper presents the theoretical modelling of photonic band gap (PBG) filters by using co-planar stripline (CPS) for terahertz PCAs and photomixers. The model uses an analytical, transmission line based approach in order to characterize the band-stop response of the PBG filter that enables obtaining fast and accurate results without need of FEM simulations. To present the performance of the model, we designed a band-stop filter that is optimized to work around a center frequency of 550 GHz with a stopband rejection of 40 dB within 100 GHz bandwidth. The simulation results of the presented model show a good comparison with FEM simulation results. The designed band-stop filter is also simulated together with a terahertz photomixer, which also shows an improvement of the radiation characteristics.