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RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 317 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figures 9.18 and 9.19 show a comparison of the impedance matching performance of the LC networks of figures 9.8 to 9.16. The Pi matching network has the lowest bandwidth, but it does not have a minimum Q and its Q can be adjusted to make the bandwidth comparable to the other networks if needed. The lowpass T, the bandpass T and the bandpass L networks, all present a high impedance at the second harmonic of the input frequency. This results in smaller second harmonic currents to flow in the bipolar transistor or FET and this slightly increases its efficiency. These networks will however result in higher peak voltages, due to the device impedance at the second harmonic being close to an open-circuit. This may then exceed the voltage breakdown ratings of the device or the components used in the matching network. The Lowpass networks are also good for filtering out harmonics produced by the amplifier. Bandpass T, bandpass L and the capacitive transformer networks all have a capacitance in series with the input to output path. This avoids the need for an additional coupling capacitor to set the bias levels correctly. If the required bandwidth or the required harmonic attenuation cannot be obtained, several matching networks can be cascaded, allowing the impedance transformation ratios to be reduced, thus increasing the bandwidth. Cascading several Lowpass networks significantly increases the harmonic attenuation. Transformer Matching In transformer matching, an RF transformer, as described in Chapter 3 of this book is used to provide the impedance transformation required. RF transformers can be used for impedance matching at HF and VHF frequencies. Since the number of turns used must be an integer and the impedance transformation is the turns-ratio squared, the impedance transformation ratios are limited to the square of simple fractions. For the matching of the LDMOS FET in this example, a 10.7 +j1.2 impedance is required. The ideal turns ratio required is (50/10.7) = 2.16. This can best be approximated by a 2:1 turns-ratio transformer, giving a 4:1 impedance transformation, so that a 50 source is transformed into a 12.5 impedance, which is sufficiently close to 10.7 - j1.2 for most of the power to be transferred effectively. Figure 9.20. Values for the Transformer matching network. A practical RF transformer can be represented by an ideal transformer, with a magnetising inductance in parallel with it and a leakage inductance in series with it, as shown in figure RF Electronics: Design and Simulation 317 www.cadence.com/go/awr

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