Input and output return losses, S11 and S22, are also available for trading off to achieve improved gain and NF performance. Maximizing return losses rarely yields optimum gain and NF performance. Typically, the input and output matches are designed to afford good gain and NF performance while maintaining adequate return losses for RF power transfer through the LNA. It is a balance of performance parameters.

Another trade-off is current drain and dynamic range. LNA gain must be adequate so that the noise added by elements in the receiver lineup following the LNA are minimized. Gain, however, must be traded off with linearity concerns in the elements following the LNA.

Emitter degeneration (degen) opens up an entire range of opportunities for trading off performance parameters. The addition of emitter degen can help move gamma opt closer to the complex conjugate gain match, which make the decision on trading off gain and NF easier. Added emitter degen can also bring the match required for good return losses closer to the match for better NF. Adding emitter feedback, however, trades off gain with IP3 and P1dB performance, usually evenly, so output IP3 remains constant.

Emitter degen also trades off linearity and stability, especially at higher frequencies. At the lower frequencies of the cellular bands, stability improves with emitter degen, while at frequencies greater than around 5 GHz, stability decreases as degen increases. So the trade off between linearity and stability must be examined across a full range of frequencies.

Small changes in inductance added to the emitter of a LNA can have large impacts on gain, NF and stability. As inductance increases, stability increases at the expense of gain. NF can also improve. However, continued increases in inductance can soon lead to degraded gain and NF. Simply adding tenths of nHs to the emitter can result in significant movement in the gain and noise circles.

In the LNA transistor, device size can be increased to improve linearity at the expense of current draw. Increasing current density in the LNA device can improve gain and NF, but again it is at the expense of current draw.

Typically, LNAs used in cellular applications must meet competing performance requirements, which becomes increasingly difficult as each specification moves closer to the optimum performance of each individual parameter. The art of LNA design is in exploring the variables available to delicately balance competing parameters. Being cognizant of the performance trade-offs and the variables available to the designer are the first steps in the art of successful LNA design.

Thomas Baker is a LNA designer and applications engineer at Freescale Semiconductor, Inc. Tempe, AZ.

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