![]() This is only true, however, when the application is operating at low enough frequency bands such that the bond wire connections to the off-chip components are minimally reactive.įIGURE 3.14. Therefore, in applications requiring low system noise figure, it becomes necessary to employ an off-chip inductor to perform the matching. When fabricated in an MMIC (monolithic microwave integrated circuits) process, spiral inductors used in the input matching network add a significant amount of resistive loss to the LNA and contribute directly to the noise figure. #Qucs noise voltage source seriesThe resulting effect is known as series inductive feedback, which tends to decrease the gain in favor of improving the stability factor. In order to optimize for noise figure and gain performance, a small inductor is placed between the transistor source contact and the ground brings Γ G, opt and S 1, 1 ∗ closer together as shown in Figure 3.14. This makes it ideal for the first stage of a high power LNA. HEMT devices are essentially FET transistors that provide the highest gain for the lowest noise figure. As a case in point, consider the popular high-electron-mobility transistor (HEMT) device for designing LNAs. However, procedures, such as the one described in, exist that optimize for both gain and noise figure. As it turns out, the optimum gain occurs when Γ G, o p t = S 1, 1 ∗, whereas the minimum noise figure occurs when the admittance is Y G = Y G, opt, which implies that these two conditions are not met at the same time. The analysis concerning noise figure for two-port networks has indicated that for every LNA, itself a two-port network, there exists an optimum noise figure derived from optimum source resistance. Conversely, if a trade-off between the input and output matching is a target in the specification table-i.e., a better input match although not with a perfect match at the output-a blind optimization step has to be considered, unless more sophisticated approaches (such as those detailed in the following sections) are known to the designer. The intersection (assuming that it exists) of those three families of circles is drawn on a Smith chart, representing the region of source complex loads that allows the fulfillment of all the specifications. This can be accomplished by the corresponding constant noise, constant available gain, and constant input matching family of circles in the Γ S plane. The typical LNA design flow is as follows: the source termination is selected to trade off the amplifier's noise, gain, and input matching performance when the output is considered perfectly matched. Typical microwave amplifier block diagram with the corresponding reflection coefficients. ![]()
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