1.8GHz balanced low noise amplifier

Abstract

A low noise amplifier is important to a receiver in the Wireless Communication. As it is the key component to boost the signal and minimize the noise produced by itself, it can much extend the distance between the transmitter and the receiver. A traditional low noise amplifier achieves low noise but without good input matching. Then the balanced amplifier is a practical method to improve the input matching. This report presents a new balanced low noise amplifier with size reduction. The amplifier was designed at the centre frequency of 1.8GHz, and it consists of three main parts, namely input branch-line coupler, two pieces of well biased transistor, and output branch-line coupler. The input branch-line coupler not only divides signal into two paths with equal magnitude and 90° out-of-phase, but also provides optimum source impedance for the biased transistor to minimize noise figure. Similarly, the output branch-line coupler not only combines signals, but also provides optimum load impedance to maximize gain. Both input and output branch-line couplers were designed with the general port impedance but not purely resistive 50Ω port. It results in non-standard values of both characteristic impedances and electrical lengths of the four branch lines, where its size is similar to the conventional coupler. Conventional balanced low noise amplifier was also designed for comparison. The new design achieves about 36% size reduction compared with conventional design with similar performances. The impedances of both input and output ports are matched, where measured S11=-13.4dB and measured S22=-17.3dB. Measured gain and noise figure are 14.5dB and 2.7dB, respectively.