Q: What complicates the matching combination?Ī: For many real-world designs the frequency range over which the impedances must match is relatively wide, and so the source and load impedances are shift. In some resistive situations, a transformer is used with the appropriate turns ratio needed to provide the resistive-impedance match.
For common resistive source/load pairings, such between a 50- Ω drive as the source and an 8-Ω speaker as load, off-the-shelf devices are available to insert between source and load (these are often called L-pads, T-pads, and pi-pads, depending on their internal arrangement). Q: What types of matching situations are relatively easy to deal with?Ī: Cases where both the source and load are purely or nearly purely resistive are easiest to handle since no reactive elements (inductive or capacitive) are needed. Fig 2: (Image source: National Instruments) The network is designed so that the source sees a load impedance equal to its complex conjugate, while the load looking back, also sees its complex conjugate. For example, the power amplifier output stage of an RF transmitter designed for a 50-Ω purely resistive load may actually be connecting to an antenna with an impedance which is not 50 Ω, and which has a large reactive component due to its physical size, configuration, and placement.Ī: The solution is to devise and insert a matching network, comprised of resistive and reactive elements, between the source and load, Figure 2. Q: Are there non-standard match combinations? These are simple, resistive matches with a reactance (imaginary) value of zero. Q: What are some common impedance-match combinations?Ī: Among the most common pairings are the 50 Ω/75 Ω transmission line of coaxial cable a high-impedance vacuum-tube audio-amplifier output of several hundred Ω (yes, they still sues those) to a low-impedance loudspeaker at 8 Ω.
Among these are a generator and a lamp as load (incandescent, CFL, or LED), an antenna feeding a receiver front end, a power amplifier driving an antenna via a transmission line, or even an audio amplifier powering loudspeakers. Q: What are some source/load pairings or transition locations?Ī: Almost any connection between function blocks or circuit elements can be considered a source/load pairing. These reflections not only distort signals, but their returning energy can actually damage the source circuitry. Q: Why do we need to conjugate-match impedances?Ī: There are two related reason: first, it can be proven analytically that doing so maximizes the transfer of power between the source and load, which is always a good thing second, it eliminates – or at least minimizes – reflections of signal energy from load back to source due to impedance mismatch and “bumps” at impedance-transition points, Figure 1. In a complex conjugate, the imaginary part has the opposite sign: R – jX, Figure 1. Q: Remind me: what is “complex conjugate”?Ī: Every source or load has an impedance which can be expressed by a complex number, with a real (R) and imaginary (X) part: R + jX. In circuit designs spanning low-frequency audio through high-frequency RF, there’s considerable discussion about impedance matching between components or subcircuits, with various tools such as the Smith chart with is used to facilitate the matching.Ī: Impedance matching means that a signal source sees a load impedance which is the complex conjugate of its own impedance.