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This paper provides the analytical background for the tunability in low noise amplifiers achieved by employing a pi-network where the series matching capacitor is assumed to be variable. The theoretical outcomes suggest that the choice of the tunable capacitor value will eventually result in a tradeoff between the noise figure (NF) and the input return ratio (S11). By pursuing a minimized noise figure approach, equations relating the value of the tunable capacitor at different frequencies with circuit parameters are derived and confirmed by the simulation results. Finally, it is demonstrated that to accomplish the design targets for NF and S11 over the tunable band, variable capacitors having quality factors greater than 10 should be utilized.
Date of Conference: 23-27 Aug. 2009