AN721: Impedance Matching Networks Applied to RF Power Transistors, Courtesy of Motorola

Some graphic and numerical methods of impedance matching will be reviewed here. The examples given will refer to high frequency power amplifiers. Although matching networks normally take the form of filters and therefore are also useful to provide frequency discrimination, this aspect will only be considered as a corollary of the matching circuit. Matching is necessary for the best possible energy transfer from stage to stage. In RF-power transistors the input impedance is of low value, decreasing as the power increases, or as the chip size becomes larger. This impedance must be matched either to a generator - of generally 50 ohms internal impedance - or to a preceding stage. Impedance transformation ratios of 10 or even 20 are not rare. Interstage matching has to be made between two complex impedances, which makes the design still more difficult, especially if matching must be accomplished over a wide frequency band.