Optimization of LNA's first stage to reduce overall noise figure in multi-stage LNAs

An analytical approach for designing the first stage in multi-stage low noise amplifiers (LNAs) is presented. This paper discusses the first stage optimization for minimizing the overall Noise Figure (NF) with consideration of following stage’s noise. This approach focuses on source impedance optimization of first stage while considering its NF, gain and the effect of the following stage’s NF simultaneously. The proposed approach calculates the first stage’s design parameters to have minimum overall NF in a multi stage LNA while the conventional approach suggests optimum source impedance to achieve minimum NF in first stage. In this paper, it is proved that constant overall-NF contours in Smith chart are different from traditional NF circles and Γopt for best noise performance changes in multi stage LNAs with different NF in adjacent stages. These contours and the optimum source impedance are depicted for specific LNA and compared with its constant NF circles and Γopt. For checking the feasibility of proposed approach and also validation of theoretical and simulation results a discrete LNA in 4–5 GHz using ATF13136 transistor is made. Results show that considering the subsequent circuit’s noise in LNA’s first stage design reduces overall-NF while improves its gain and input matching.