T he quality of a communication system is mainly infl uenced by the sensitivity of the receiver. A critical receiver stage is the fi rst amplifi er [low noise amplifi er (LNA)] that should have a low noise fi gure (NF) as well as suffi cient gain to provide high sensitivity and also a high dynamic range to avoid intermodulation products from strong signals. The third-order intermodulation products have substantial infl uence on the receiver performance. These undesired signals appear in a band close to the operating frequency and cause interference in the receiver system. In most cases intermodulation can be reduced by increasing the current through the active device [transistor, fi eld effect transistor (FET), etc.], however with mobile terminals, the power consumption is an essential factor and therefore a tradeoff is required. The objective of the LNA Student Design Contest at the 2011 IEEE International Microwave Symposium (IMS2011) in Baltimore was to construct an LNA that also takes into account the linearity and a the overall power consumption. The competition was to build an LNA for 2.45 GHz with a minimum gain of 13 dB and an output compression point higher than 3 dBm. The fi gure of merit (FOM) of the LNA (LNAFOM) was calculated as follows:with OIP3 5 Output third-order intercept point of LNA in mW, Pdc 5 dc power drawn by power supply in mW, and NFdB 5 LNA NF in dB.
Design PreparationWith the goal of achieving a high LNAFOM, a stringent analysis of the competition rules, especially the FOM, was done first. This showed that the intermodulation products would have a much stronger influence on the FOM than the NF or the power consumption. In addition, an extensive evaluation of commercial amplifiers and papers regarding the LNAFOM was done to check the state of the art. As a result, only a few amplifiers with an LNAFOM above ten could be found. In most cases, modern bipolar transistors were used. This is due to their high transconductance at low current levels and their relatively good noise performance. In previous work [1], good results were already made with the SiGe:C Heterojunction wideband RF bipolar transistor BFP720 manufactured by Infineon Technologies. Therefore, this transistor was chosen for the design again.
RF CircuitryLNA design is mainly based on appropriately matching conditions between the active device and the input and output ports of the circuit. Noise matching at the input port is essential to achieve a minimum NF. At 2.45 GHz, the datasheet of the BFP720 promises a minimum NF of 0.5 dB with a current of 5 mA and optimum noise impedance Z opt 5 60 1 j40 V. However, 2 0 1 1