An Ultra-Linear LNA for Base stations

Abstract

In the commercial basestation market of today, LNAs are implemented using GaAs pHEMT technology. The popularity of GaAs pHEMT technology is based on its excellent low noise and high linearity properties. In this project, an LNA for basestation applications is designed using SiGe technology, which can provide low noise performance, but requires more work to achieve a competitive linearity when comparing with GaAs pHEMT technology. The main motivation of a LNA implementation in SiGe are the lower cost, higher integration possibilities than GaAs. As such, in this master project an ultra-linear LNA has been designed in QUBiC4Xi technology for base station applications in the 1710-1980 MHz band. The requirement for a base station LNA are low noise, high gain, and high linearity. To achieve these goals simultaneously, a 2-stage topology is used where the first stage is optimized for lownoise and high-gain operation, whereas the second stage aims for high linearity and a high 1dB compression point using negative feedback. The first stage is using a cascode topology, which provides high gain and excellent reverse isolation. Its noise figure has been optimized by proper biasing and noise matching. The output stage is implemented using a differential structure, to enhance linearity and output power handling. The techniques used to improve the linearity of the output stage include overall feedback, Implicit IM3 cancellation, 2nd harmonic termination at the output, and out-of-band matching at its output. The out-of-band matching technique has been also evaluated separately for its linearity potential using a simplified Gummel Poon model to understand the impact of individual transistor parasitic (Cje,t f ,Cjc) on linearity. Since the targeted LNA input and output are single-ended, balun have been used in the interstage connection and at output convert from single-ended to differential. The 1 dB compression point of the output stage, which is typically limited by the maximum voltage swing of the transistor technology is in its configuration enhanced by applying impedance transformation in the output balun.