RF Low Noise Amplifiers (LNAs) Applications & Considerations

Low Noise RF Amplifier

Throughout the signal chain and especially at a signal input stage, RF signals for communications and sensing can be extremely weak. RF Low Noise Amplifiers (LNAs) are the most common method for increasing the signal level of these very weak signals while minimizing the introduction of signal degrading factors. Of these signal degrading factors, added noise, added phase noise, and distortion are generally of the greatest concerns. However, the limited bandwidth of an LNA may also result in lost signal information.

The dynamic range of an RF system often depends on performance on an LNA near the signal input stage. This input stage can be a signal generator, antenna/probe, or possibly even over a length of lossy transmission line/waveguide path. Another use case for an LNA would be as a bi-directional amplifier. If multiple stages of amplification are needed, the added noise figure and distortion characteristics of the LNA become even more critical.

There is a trade-off in LNA performance with added noise figure, gain, bandwidth, linearity/distortion, size, complexity, cost, ruggedness, and longevity. It is possible to make a very high gain LNA with very low added noise, but this amplifier may have a very limited bandwidth. Conversely, a wide bandwidth LNA may have a good added noise figure but be of limited gain. Design of an LNA requires knowledge of the application and the priorities of the signal chain performance factors. For instance, it may be that the bandwidth requirements necessitate multiple gain stages to ensure wide bandwidth performance while achieving the necessary gain. However, it may be preferable to optimize the noise figure with a single High Gain Low Noise Amplifier stage and sacrifice some bandwidth while keeping a moderate gain level.

Other factors to consider may involve the portability of the LNA. Higher gain, power, and performance LNAs are often generally larger than lower performing LNAs. It may be necessary to use an LNA with poorer performance to retain some power margin. Higher frequency LNAs also tend to be less efficient than lower frequency LNAs, as operating at higher frequencies leads to greater RF losses. Hence, it may be necessary to use multiple LNAs that are optimized over different frequency ranges and multiplexers/filter banks to ensure the desired frequency range is adequately accounted for while meeting system level performance figures.

In terms of linearity, the output power for 1 dB compression/1 dB compression point (P1dB), saturated output power (Psat), and output third order intercept point (IP3) are the most common figures-of-merit (FOMs) considered.  Gain flatness and gain variation over temperature or other environmental factors may also be important for some applications that may be exposed to harsh environmental conditions, such as marine, aerospace, extreme climates, or space.