Design of a Dual-Band Low Noise Amplifier (LNA) Utilizing Positive Feedback Technique

This paper presents a dual band low noise amplifier (LNA) for 1.8 GHz DECT and 2.4 GHz WLAN applications employing positive feedback technique. This LNA exploits a positive feedback capacitor connected to a conventional cascaded inductive source degenerated LNA whereby the connection of a capacitor determines the operating frequency of the LNA. For operation at 1.8 GHz DECT, the capacitor is feedback to both transistors of the cascaded LNA and as for 2.4 GHz WLAN application, the capacitor is feedback to only one of the transistors. Two switches are employed to control the connection of the feedback capacitor to the LNA. By switching the connection of the feedback capacitor, the narrow-band gain and impedance matching are achieved at both 1.8 GHz and 2.4 GHz frequency bands. The LNA has been simulated using 0.18 mum CMOS technology and results attained for both two bands are as followed; gain, S21=21 dB, noise figure, NF=0.8 dB and linearity with third-order intercept point, IIP3=-0.13 dBm. Simulation results prove that the technique works and a dual-band LNA with a chip size comparable to a single-band LNA is realized with the proposed topology.