Matching of low-noise amplifiers at high frequencies

Higher transistor transition frequencies, lower supply voltages and smaller physical dimensions are, nowadays, general trends in the semiconductor industry. Operating at lower supply voltages often results in a low-power design, smaller dimensions allow the use of a large number of transistors and a high transition frequency (fT) opens the way to design of amplifiers with ever-higher gains and lower noise figures. However, such trends make the use of some circuit topologies questionable. Take the inductively-degenerated low-noise amplifier, arguably the most-widely used RF amplifier topology, requiring an impractical inductance in the order of pH for a simultaneous input-power match at high fTs. Therefore, a conceptual change in a design approach has resulted in transformer-feedback degenerated low-noise amplifier, presented in this paper. By controlling the coupling coefficient, the power match is possible even for the highest values of fT, with practical values of primary inductance for the transformer. The analysis gives full insight into the performance of the newly introduced transformer-feedback degenerated low-noise amplifier scheme.