Graphene has been studied for a ballistic rectifier with zero threshold voltage as it has a long mean free path close to a micrometer at room temperature. In this study, a ballistic rectifier based on graphene encapsulated with hexagonal boron nitride was demonstrated with a diode bridge structure. The diode bridge circuit made by graphene channel provided a ballistic current path for the continuous and effective ballistic motions of the electrons and holes. Its unique geometry combined with the tapered and tilted geometric effects enabled the device to rectify small signals (<100 mV) with an excellent performance. The rectification responsivities were estimated at 38,000 V/W for holes and 23,000 V/W for electrons at room temperature, which are among the highest values reported to date. Due to zero threshold voltage of the device, Johnson noise signals generated at a metal resistor were rectified at room temperature. In other words, thermal excitation energy was converted to electrical energy at room temperature. The bandwidth of the device was estimated at ~ 1.1 GHz for holes and 2 GHz for electrons at the ballistic regime. The device developed in this study is believed to be a critical step for a paradigm shift of energy harvesting source from mechanical vibration to thermal energy.