Impedance Matching

Definition

Impedance matching is designing source and load impedances to minimize signal reflection or maximize power transfer. In DC circuits, the source and load should be equal. In AC circuits, the source should either equal the load or the complex conjugate of the load, depending on the goal.

Impedance (Z) is a measure of the opposition to electrical flow, which is a complex value with the real part being defined as the resistance (R), and the imaginary part is called the reactance (X). The equation for impedance is then by definition Z=R+jX, where j is the imaginary unit. In DC systems, the reactance is zero, so the impedance is the same as the resistance.

Why is impedance matching needed?

Impedance mismatch can lead to signal reflection and inefficient power transfer. These reflections cause destructive interference, leading to peaks and valleys in the voltage. Impedance matching is therefore important to obtain a desirable VSWR (voltage standing wave ratio).

Conjugate matching vs. reflectionless matching

Depending on whether the goal of impedance matching is maximizing power transfer or minimizing signal refection, either conjugate matching or reflectionless matching is required.

Maximum power transfer is obtained when the output impedance of the source is equal to the complex conjugate of the input impedance of the load (Z_S=R_L-jX_L). This is called conjugate matching.

Minimal signal reflection is obtained when the source impedance is equal to the load impedance (Z_S=R_L+jX_L), which is called reflectionless matching.

Since reactance is zero in DC systems, this is equivalent to the two resistances being the same in either case. Impedance matching will result in both minimal signal reflection and maximal power transfer in DC systems.

What is an impedance matching device?

Matching networks are configurations used to match source and load impedances, and impedance matching devices are the components that make up these networks. Finding these component values can be done using computer simulations, manual computations, or with tools such as the Smith chart.