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RF Electronics: Design and Simulation

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RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 318 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. 9.20. The magnetising and leakage inductances are chosen to give a centre frequency of 100 MHz. The 400:1 ratio between Lm and Ls is on the low side of the typical range, so that the bandwidth typically will be larger than what is shown here. In this example, the impedance to be matched is assumed to be constant at 10.7 – j1.2 . In practice the device impedance changes significantly over the typically three decades of bandwidth of the RF transformer, thus limiting the frequency range over which the device is properly matched. FET amplifiers that are well matched up to 100 MHz can easily be designed, as is evident from many power FET manufacturers' application notes. The performance obtained by the transformer-matching network is shown on figure 9.21. Figure 9.21. Frequency response of transformer matching network with and without reactive tuning capacitor. If needed, the reactive part of the input can be resonated out, using a capacitance across pin 1 and 2 of the transformer in figure 9.20. This capacitor can be tuned to provide an optimum match at 100 MHz. Note that adding the capacitance will reduce the upper corner frequency of the transformer-matching network. In this case, such a capacitance is not needed and a good match is obtained over a wide range of frequencies. Due to the turns-ratio of the transformer being limited to integer values, the impedance match is not as good as that of the LC networks or that of the transmission-line networks described later. The model of figure 9.20 does not contain all the parasitic elements encountered in a practical transformer circuit, and the actual FET characteristics may be slightly different from the values given in the data sheet. The value of any tuning capacitance required is thus best determined experimentally, using the value obtained by the computer simulation of figures 9.20 and 9.21, as a starting point. Transmission-Line Matching Zs ZL Z0 L Figure 9.22. Transmission-line sending and load impedances. RF Electronics: Design and Simulation 318

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