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While the low noise amplifier (LNA) is a relatively simple design compared to other RF components in a cellular receiver lineup, the performance trade-offs it presents challenge the LNA application design engineer. LNA design typically involves making choices between directly competing performance parameters. Seldom do design choices afford simultaneous improvements in two or more parameters. Finding the delicate balance in satisfactorily trading off performance variables becomes the challenge more often than simply maximizing a single key parameter.

The most recognizable trade-off is between LNA gain and noise figure (NF). LNA design often begins with the use of gain and NF circles on the Smith chart to visualize the trade-off. Excursions around the Smith chart that cross gain and NF circles leads to a trade-off decision on which impedance to use. Will it be lower gain to achieve improved NF or perhaps an excellent NF with a gain sacrifice?

The optimum NF, NFmin occurs at gamma opt and where this point on the Smith chart meets the gain circles gives one possible solution for the design. But maximum gain rarely occurs at this same impedance state on the Smith chart, so the trade-off begins. To further complicate the trade-off, stability must be considered in the impedance choice. Input and output stability circles are plotted on the same Smith chart to insure that the impedance chosen does not lie in potentially unstable regions.

As an illustration, the Smith chart shown at right, includes the gain and noise circles and the input and output stability circles for 900 MHz. Gamma opt is near the center of the chart, with gain circles crossing the noise circles and the stability circles at the outsides of the chart.

Today's LNA designs also involve trade-offs in linearity and current drain. Third-order intercept point, IP3, has emerged as an important parameter in LNA design. Most of the time the easiest way to improve IP3 performance for a given frequency is to increase the current density or current draw of the LNA. Until the current density reaches relatively high levels, it will continue to improve with increasing current draw. If current draw is less important than IP3 performance, then it can be increased with the usual slight increase in gain and NF. So, in this case, IP3 improves with the trade-off in current draw and NF.

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