Entries found for search: impedance

**characteristic impedance** : See *termination*.
**impedance (Z)** : The opposition (measured in ohms) of circuit elements to an *alternating current*. The impedance includes both* resistance* and *reactance*. In an AC circuit, resistance presented to the current is a function of frequency, called impedance and is also measured in ohms. Ohm’s law can be used for AC if the reactances are zero, that is, if there are no capacitors or inductors in the circuit. See Appendix B.

**source output impedance**: the actual impedance of the output circuit. The output impedance of a device determines just how much current it can provide to a load.

**source load impedance**: the impedance that the source circuit is designed to serve as a load. This is usually the figure given in the specifications, e.g., the impedance value that the equipment was designed to be connected to.

**load input impedance**: the impedance that the load is designed to be fed from. As long as this value is higher than the source output impedance, the rest doesn’t matter. For example, if a tape recorder is connected to a mixer, this value is the impedance that the recorder wants to serve at its input.

**load termination impedance**: the actual impedance of the input circuit.

**impedance-matching** : If the absolute maximum power is to be transferred from a source to a load, the impedances must match exactly. The source impedance output, the cable impedance, and the load impedance input have to be equal. This is called a *matched circuit*. However, when a circuit is matched, the voltage swing is reduced 6dB, lowering the best possible* S/N ratio* by that amount. Because of low-noise preamplifier circuits, there is a better audio trade-off of power transfer for a larger voltage swing. Therefore, the idea that circuits should be impedance-matched is fallacious, and instead modern audio equipment should simply be connected via* balanced* input and output lines. Systems that use high impedance mics are limited to cable lengths of around 10’ or so, but there is no reason to worry about the actual impedance value; it is either high or low. See* bridging circuit*.

**radiation impedance** : The acoustic *impedance* that acts as a load on a loudspeaker, opposing the motion of the cone.

**output impedance** : The output impedance of a device is the actual impedance at the source output terminals. See * impedance-matching*.