Improved noise analysis of distributed preamplifier with cascode FET cells

Improved noise analysis of a distributed amplifier (DA) with cascode unit cells is presented. The analysis is based on the lossy m-derived configuration, considering the effects of field-effect transistor resistances, as well as parasitic and intentional inductances, which have been ignored in previous research. Analytical expressions for the noise figure and equivalent input noise current density of the cascode DA are derived. The analysis also identifies the individual contributions from each noise source inside the cascode DA, providing insight into the noise of a DA. It is found from the analysis that the output noise current of cascode cell (~i_de²) is the most significant contributor to the total noise of the DA over most of the operating frequency band. The gate termination noise (~i_Zg²) has a large impact at the low-frequency end, while the input noise current of the cascode cell (~i_ge²) degrades the high-frequency noise performance substantially. The resistive and inductive effects of the unit cells have been carefully studied by comparing the simulation results with those based on the lossless constant-k models, which shows that ignoring these effects may result in misleading results at high frequencies. To validate the noise analysis, the simulated noise figure of nine-section cascode DA monolithic microwave integrated circuit is compared with the measured data, yielding an excellent agreement over the entire operating frequency band.