Optimization Of LNA's First Stage To Reduce Overall Noise Figure in Multi-Stage LNAs
Abstract
An analytical approach for designing the first stage in multi-stage low noise amplifiers (LNAs) is presented. This paperdiscusses the first stage optimization for minimizing the overall Noise Figure (NF) with consideration of followingstage's noise. This approach focuses on source impedance optimization of first stage while considering its NF, gainand the effect of the following stage's NF simultaneously. The proposed approach calculates the first stage's designparameters to have minimum overall NF in a multi stage LNA while the conventional approach suggests optimumsource impedance to achieve minimum NF in first stage. In this paper, it is proved that constant overall-NF contours inSmith chart are different from traditional NF circles and $\Gamma_{opt}$ for best noise performance changes in multistage LNAs with different NF in adjacent stages. These contours and the optimum source impedance are depicted forspecific LNA and compared with its constant NF circles and $\Gamma_{opt}$. For checking the feasibility of proposedapproach and also validation of theoretical and simulation results a discrete LNA using ATF13136 transistor is made.Results show that considering the subsequent circuit's noise in LNA's first stage design reduces overall-NF whileimproves its gain and input matching.
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