AWR eBooks

RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 312 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 9.6, shows the impedances of the input and output matching networks, required at different frequencies to produce a 25W CW output. It should be noted that these values do depend on individual device and the quiescent current, so that in practice the actual values may be slightly different. Equation 9.2 shows that for a 50V supply, V sat = 0, and a 25W output, then R L =50 Ω. That agrees with figure 20 of the MRFE6VS25N [3] datasheet, which shows that R L is close to 50 Ω from 1.8 MHz to 30 MHz. Figure 31 of this datasheet, shown in figure 9.6, uses a different test circuit to obtain a value of R L that is close to 20 Ω from 30 MHz to 512 MHz. Figure 9.6 shows that the input and output impedances do not vary significantly over the 30 to 512 MHz frequency range. At 100 MHz Z source = 10.7 +j1.2 Ω and Z load = 22.3 + j1.3 Ω. The input impedance of the FET is the complex conjugate of these impedances, i.e. Z in = 10.7 -j1.2 Ω and Z out = 22.3 - j1.3 Ω. Those impedances are the impedances that the matching networks must provide to produce 25W from the MRFE6VS25N, using sufficient input power. The input impedance of the FET is the complex conjugate of these impedances, i.e. Z in = 10.7 - j1.2 Ω and Z out = 22.3 - j1.3 Ω. Motorola AN267 uses the device input and output impedances, so that Z in = 10.7 - j1.2 Ω and Z out = 22.3 - j1.3 Ω must be used to calculate the input matching components using equations 9.3 to 9.20. The equations and frequency responses are shown in figures 9.7 to 9.22. At out of band frequencies some networks have very low input impedances and others have very high input impedances. As a result, some matching networks may cause the amplifier to be unstable out of band. If that is the case, then other matching can provide stability. For the Cadence AWR DE project files for figures 9.9 to 9.20, equations 9.3 to 9.20 are included as part of the "Global Variables". Those equations are shown in figures 9.7 and 9. For a Pi network, AN267 does not include the device reactance as part of C1. Equation 9.4 includes any device reactance as part of C1. Including any device reactance as part of C1 results in a lower effective Q and thus gives a wider bandwidth for the matching network. The Q value to be selected for the matching network is a compromise. For a wide bandwidth, a low Q is required and the Q is normally selected to be as low as possible. However, the desired matching may not be possible at low Q values. For this FET for the Lowpass T and Bandpass T networks, the minimum Q is 1.916. For this example, a Q value of Qt = 2.5 is used for the Lowpass T and Bandpass T networks as shown in figure 9.7. The Q value of the Pi and bandpass L matching networks are then chosen to make the S11 bandwidth of the different matching networks similar, by tuning Qp. That results in Qp = 0.65. The same bandwidth for matching will thus require very different Q values of the matching networks for the same device resistance and reactance to be matched. These matching network equations produce components values. That does not mean that these network can be realised and one may need to change Q values or select specific networks to ensure that realisable components are obtained. In this example, all of the component values are practical. For the output matching, the voltage and current ratings and possible resistive losses of these matching network components must be considered, to ensure that the matching components do not overheat and fail. The higher the Q, the higher the circulating currents and the higher the voltages across the components. The choice of network to be selected then depends on the frequency response and the component values for the different networks. In order to have an exact source impedance of 10.7 - j1.2 , it is convenient to make a complex input variable Zin, using Zin = complex (Rd, Xd) = complex (10.7,1.2). Note that if Rd < R L ), the matching network will have a voltage gain, since there is an RF Electronics: Design and Simulation 312 www.cadence.com/go/awr

• Cover