# Impedance

## Meanings and phrases

n.

- a material's opposition to the flow of electric current; measured in ohms; electric resistance; electrical resistance; resistance; resistivity; ohmic resistance

n.

- opposition to the flow of sound through a surface; acoustic resistance is the real component of acoustic impedance and acoustic reactance is the imaginary component; acoustic resistance; acoustic reactance

E.g.

- Up to the MHz range only "discrete" properties need be considered; e.g., a terminal has an input impedance.
- Bilateral amplifier input impedance depends on the load, and output impedance on the signal source impedance.
- This was often necessary in the early days of bipolar transistors, which inherently have quite a low input impedance.

E.g.

- Such an element of a distributed circuit is entirely characterised by its length and characteristic impedance.
- USB signals are transmitted using differential signaling on a twisted-pair data cable with characteristic impedance.
- The secondary constants of a line meeting the Heaviside condition are consequently, in terms of the primary constants: Attenuation, Phase change constant, Phase velocity, The characteristic impedance of a lossy transmission line is given by In general, it is not possible to impedance match this transmission line at all frequencies with any finite network of discrete elements because such networks are rational functions of jω, but in general the expression for characteristic impedance is irrational due to the square root term.