High-Frequency Matching of Low-Noise Amplifiers

Moving to higher transition frequencies, lower supply voltages and smaller dimensions are, nowadays, general trends in the semiconductor industry. While operating at lower supply voltages results in a low-power design, smaller dimensions allow the use of a large number of transistors, a high transition frequency (fT) is accepted as an almighty-good, opening a way to a design of the amplifiers with ever-higher gains and ever-lower noisefigures. However, rather hidden, there are also some drawbacks of such a trend, making the use of some circuit topologies quite questionable. Take the inductivelydegenerated low-noise amplifier, the most-widely used RF amplifier topology, where the input-power match appears to be impossible at high fTs, requiring an inductance in order of pH, that can not be safely designed and integrated on chip. Therefore, a conceptual change in a design approach has resulted in transformer-feedback degenerated low-noise amplifier, presented in this paper. Controlling the transformer coupling coefficient, the power match becomes possible even for the highest values of fT, with at the same time larger and design-safe primary inductor of the implemented transformer. The presented analysis gives full insight into the performances of the newly introduced transformer-feedback degenerated low-noise amplifier scheme.