High Q Capacitors for Impedance Matching

Capacitor Q is almost always a primary design con-sideration in RF matching applications. The capaci-tor’s power dissipation is inversely proportional to its Q factor and directly proportional to the equivalent series resistance (ESR). An input matching network is essential for most RF amplifier designs in order to transform the rela-tively low impedance of the active gain device to the system impedance. The active device’s input impedance is typically in the order of 0.5 to 2 ohms and is generally matched to a 50 ohm system impedance. If we assume that a transistor in a power amplifier has an input impedance of 1 ohm, this will require an impedance transformation of 50:1. Therefore, we must trade off voltage for current as the matching network transforms the impedance from 50 ohms to 1 ohm.