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RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 304 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Chapter 9 Power Amplifier Design Introduction In order to obtain the most power from a bipolar or field effect transistor, the input and output impedances of the device must be matched to that of the circuit in which it is placed. Similarly, if an antenna is to be driven effectively, the antenna needs to be matched. For the maximum power transfer, the source impedance and the load impedance must be a conjugate match, that is the resistive parts must be the same and the imaginary parts must be the same magnitude but opposite polarity. RF Power Device Impedances In order to design a power amplifier and obtain the correct output using typical 50 ٠source and load impedances, the required input and output impedances of the transistor must be known. For maximum output power from a generator or a transistor, the load impedance should be, the complex conjugate of the output impedance of the generator. That results in the same power being dissipated in the load as is dissipated in the generator. For high power applications, the output impedance of the source should be much lower than the load impedance, so that most of the available power is dissipated in the load. The mains power network is one example where the desired generator's output impedance is much smaller than the loads applied. Similarly, the output impedance of an RF transistor should be much lower than the load impedance, to minimise the transistor's dissipation. The impedance that the source or load matching networks should present at the transistor are thus not the impedances of the transistor, but they are the impedances that must be presented in order to obtain the desired output power at a specified frequency and transistor quiescent currents. For an RF transistor operating at a supply voltage of Vcc, the collector or drain is normally connected to the supply voltage through an inductor. The output voltage can thus vary from close to 2*Vcc to Vsat. The maximum peak voltage is thus (Vcc-Vsat), where Vsat is the saturation voltage of the device. The corresponding RMS output voltage is: ��� �� � �� � ��� Eqn. 9.1. The load resistance for an output power of P is thus: � � 2 � � �� �� �� ��� � � �� Eqn. 9.2. Similarly, the required input matching impedance of a transistor used for RF power amplifiers is not the complex conjugate input impedance of the device. It is the impedance corresponding to the base or gate voltage and the base or gate current, which produces the desired output power from the device, whilst ensuring that the circuit is stable. NXP's (Freescale Motorola) AN1526 [1] and AN282A [2] are good background documents that describes how these desired impedances are measured using test fixtures, and how the subsequent analysis of the impedance matching networks then results in the desired matching source and load impedances to obtain the desired output power from the transistor. RF Electronics: Design and Simulation 304 www.cadence.com/go/awr

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