Impedance Matching


A major part of RF design is matching one part of a circuit to another to provide maximum power transfer between the two parts. Even antenna design can be thought of as matching impedance of free space to a transmitter or receiver. This chapter describes a few techniques that can be used to match between two real impedance levels. While some comments will be made relative to matching to a complex load, the emphasis will be on real impedance matching. The first part of this chapter will discuss the circuit quality factor, Q. The Q factor is useful in certain matching circuit designs.


The circuit Q factor is defined as the ratio of stored to dissipated power in the following form:

(3.1) c03e001

For a typical parallel RLC circuit, the Q becomes

(3.2) c03e002

where G is 1/R. For a series RLC circuit,

(3.3) c03e003

It should be emphasized that Q is defined at circuit resonance. If the circuit reactance is plotted as a function of frequency, the slope of the reactance at resonance is a measure of Q (Fig. 3.1). This is explicitly given as

(3.4) c03e004

where ...

Get Radio Frequency Circuit Design, 2nd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.