A Software-Defined Radio Oriented Approach in Constructing RF Front-End Circuit Based on Fully Differential Amplifier

Due to the characteristic of dual feed-back paths in fully differential amplifier, a software-defined radio (SDR) oriented approach in constructing RF front-end circuit with fully differential amplifier is presented. By constructing the small signal mode of dual feedback loops, the detailed input and output relationship of this kind of circuit is deduced. Thereby, the transfer function with symmetrical loops and the noise formula with nonsymmetrical feedback paths are both derived. An optimized approach of termination is also introduced to make the RF front-end circuit more suitable for high-speed and wideband system. The proposed method has been applied in an ultra-wideband digital receiver and the experimental results show the circuit exhibits high gain and good termination in ultra-wideband. The gain flatness (−3dB) beyond 1.8GHz and the stand wave ratio (SWR) is less than 1.15 over 10GHz. It shows that the method proposed here is credibility and practical, also could help to guide the design and improve the performance effectively.