RF Impedance Matching

RF impedance matching is used so that the maximum power can be transferred from the source of the RF signal to its load. The load can be follow on stages or it can be an antenna.

It can be mathematically proven that the maximum power transfer occurs when the load impedance is the complex conjugate the source impedance.

What is meant by "complex conjugate" here? It means that if the source impedance has a real resistive component and a reactive component,

  • the resistive component of the load is the same as the resistive component of the source
  • the reactive component of the load is of the opposite type as the reactive component of the source load impedance. For example, if the source has inductive output impedance, the load will have a capacitive load impedance for optimum match.

Since the reactance varies with frequency, exact impedance match will occur at only one frequency and good RF power transfer will happen only in a narrow range of frequencies. This might be beneficial where we want to pluck the RF energy from only a narrow signal band. This is a disadvantage where we want to work with RF signals over a wide range. There are impedance matching networks that give us a wider impedance matching range than normally possible.

With the advent of highly integrated circuits, the use of RF impedance matching has come down. However we still need to use impedance matching when connecting an antenna to RF circuit or while working on microwave frequencies where discrete components are still used.

We will examine specific impedance matching networks in forthcoming articles. In the meantime, if you have any questions about impedance matching contact us.

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