AWR eBooks

RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 306 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For a simple parallel tuned network, like the Pi network, the circulating current in a resonator is approximately Q times the external load current. For a 50 Watt amplifier, driving a 50 load, the load current is 1 Amp. For Q = 10, the current circulating in the Pi network is thus of the order of 10 Amps. This can cause significant heating and failure of the components. For some networks like both the T networks and the Bandpass L network, very high voltages can be experienced at some nodes and this can lead to voltage breakdown of components. The higher the Q, the higher the stress on the components. A high Q value can also cause the matching network to drift out of match as the components change due to temperature. In general a Q value as low as possible is desirable, as this will give the widest bandwidth match and the lowest component stress. If the network is to be used as an output-matching network of an amplifier and harmonics are to be filtered as well, a higher Q value can be considered, since the higher the Q, the higher the attenuation at the amplifier harmonics. For amplifier output networks, where harmonic filtering is required, cascaded matching networks are preferred as a means to achieve the required harmonic attenuation. To obtain a wide bandwidth and use a low Q, successive impedance transformations are normally used, as described in this chapter, in the section on broadband matching. Since the current and voltage limits in the output-matching networks are very important, a trade-off between the Q value and the order of the filter will need to be considered, in order to achieve the required harmonic attenuation and bandwidth, as well as desirable voltage and current levels. LC Matching In LC matching, inductors and capacitors are used to obtain the required conjugate impedance match. The equations for calculating the components required are contained in NXP (Motorola) Application Note AN267 [1]. LC matching is very practical for power amplifiers up to 1 GHz. A good indication of the suitability of LC matching for a transistor can be obtained by noting the type of matching used for test circuits in the manufacturer's data sheet for the transistor. Because it is easy to tune the matching network using variable capacitors, these LC matching networks are normally used by the manufacturers to determine the input and output impedance of the devices. The device is placed in a test jig and the matching network is adjusted to obtain the required output power at a good efficiency, a low input return loss and amplifier stability. The values of the resulting LC matching components are then used to determine the input and output impedance of the device. AN267 describes 4 types of matching networks. In all the equations used in this chapter, the device impedance is R d +jX d and the load impedance is R L . That notation is different from AN267, where R 1 and R L can readily be confused, particularly if a lowercase L is used. In the equations presented here, it is not assumed that the device is capacitive, so that X d is used for the reactive part of the device impedance, instead of –X cout , as in AN267. For the Pi Network, in AN267, the reactive part of the device impedance must manually be included in the matching network capacitance. The reactive part is included in equation 9.3. In AN267, different expressions for A and B are used for the bandpass T and the lowpass T networks. In this book, consistent expression for A and B are used. In AN267, the term A for the bandpass T network is wrong. The LC matching equations 9.3 to 9.19 presented in this book are consistent and correct. RF Electronics: Design and Simulation 306 www.cadence.com/go/awr

• Cover