A Compact Low Noise Amplifier with Defected Microstrip Structure for Ultra-Wideband Communications

In this paper, an ultra-wideband (UWB) low noise amplifier (LNA) with notch filter is presented. The proposed LNA implements a passive notch filter in the form of U-Shaped defected microstrip structure (DMS) to eliminate the interference in WLAN band at 5 - 6 GHz. This design is based on negative feedback and inductive source degeneration which gradually increase the stability while providing wideband characteristic as well as gain flatness throughout the frequency band. Furthermore, this design implemented a multi-section quarter wave transformer as wideband matching which to improve the input and output matching. The LNA has been fabricated on FR4 substrate microstrip. The measured gain is better than 10 dB, a noise figure of 1.9 - 3.8 dB with a return loss of less than 8 dB. The interference rejection managed to achieve more than 25 dB at 5.5 GHz. This proposed structure is useful for RF/microwave front-end subsystems, especially in interference rejection.