Exact Noise Figure Prediction For Transformer-Coupled Fully-Differential Amplifier

by Michael Steffes, Senior Applications Manager, High Speed Signal Path, Intersil Corporation

While the transformer-input-coupled configuration using very low noise fully-differential amplifiers (FDAs) should give a low input noise figure (NF), measurements have typically shown a NF slightly higher than the expected result. A more detailed analysis revealed the missing terms yielding exact match between theory and measurements. That more accurate result is shown here with the necessary equations to predict the noise figure using any input transformer and FDA to provide a high dynamic range gain stage at very low power. Modern FDA’s can deliver low 7 dB range noise figure using this approach. While not suitable to the first stage LNA, this level of performance can be useful in later stages as a flexible gain, very high efficiency, gain block with acceptable noise figure at that point in the receiver chain. Example results for a range of turns ratios and a very low noise FDA are shown with selected design points closely matching bench results.

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