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RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 305 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Large Signal Parameters For linear devices and small signals, S parameters are useful to determine input and output impedances and thus design complex conjugate matching networks to provide the maximum output power from those devices. S parameters also characterise forward and reverse gain and those are useful in determining the gain and stability of an amplifier. For power transistors, by performing measurements using test fixtures, the large signal input and output impedances and the corresponding forward and reverse gain parameters can be measured. From those, large-signal S parameters can be obtained. They are very different from the small signal impedances and small signal S parameters. For a given transistor, small signal parameters should only be used for small input signals, so that the device behaves in a linear manner, i.e. the superposition and scaling of signals is valid. The large-signal parameters should be used for the same device, if the magnitude of the input signal is such that, the device no longer behaves in a linear manner. The large-signal S parameters for the same device will differ depending on the biasing, modulation method and drive signal level. The large-signal S parameters are thus different for a Class AB amplifier for OFDM transmission for mobile phones, DAB and TV broadcasting compared to those for a class C amplifier for FM broadcasting. The large-signal S parameters or large signal source and load impedances can be used to design suitable impedance matching networks. However, in many cases, the parameters for the required operating conditions are not available and the optimum impedance matching needs to be determined experimentally. Types of Matching Transformer Matching: RF Transformers can be used for wideband impedance- matching. The main limitations are the restricted range of available impedances (due to the turns-ratios being limited) and the frequency limitations on transformers. LC Matching: Inductors and capacitors are used to provide the impedance transformation. LC matching results in a relatively narrow bandwidth match. LC matching is very practical at frequencies up to 1 GHz. Above 1 GHz, transmission-line matching is more economical. LC matching permits easy tuning of the match to allow for device variations. Transmission-Line Matching: A transmission-line of a required length and Characteristic Impedance is used to obtain the required composite match. Such a match tends to be of a broader frequency range than LC matching and can be applied at frequencies above 100 MHz. Below 100 MHz the transmission-lines required tend to be too long to be practical. It is difficult to tune the length and characteristic impedance of a transmission-line once constructed. For each application, each of the above matching techniques must be evaluated against the design criteria and the most appropriate technique selected. Choice of Components and Q Value It is very important that the value of capacitors and inductors used be measured at the operating frequency. Many capacitors are not suitable for RF applications, as their self- resonance frequency is below the required operating frequency, or their losses are too high. For power applications, inductors need to be selected for the required current carrying capability and inductors with ferrite cores may not be suitable as their losses may be too high. RF Electronics: Design and Simulation 305 www.cadence.com/go/awr

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