Development and Characterization of 5.8 GHz Reflective Amplifier with Variable Gain for Active Reconfigurable Intelligent Surfaces

Active Reconfigurable Intelligent Surfaces (RISs) have been proposed to generalize and to overcome some limitation of passive RISs. This work introduces a simple but effective reflective amplifier with voltage-controlled gain, suitable for active RISs. It implements a commercial SiGe BJT as active device, and a PN varactor diode as control unit. The paper describes the design method and provides experimental validations of prototypes in the 5.8 GHz band. The data report reflective gain levels of about 20 dB for the two prototypes, a gain control in excess of more than 10 dB, and a noise figure of 1.85 dB.