Impedance is the total passive opposition offered by an electric circuit to the flow of an alternating current (AC). At a basic level, it is a measure of how much a circuit impedes, or disrupts, the flow of charge. It’s made up of a combination of resistance, conductive reactance and capacitive reactance. Impedance reduces to resistance in circuits carrying a steady direct current (DC). It is measured in units of ohms.
The level of impedance in a circuit can vary according to the frequency of the signal entering it. This is because reactance also varies depending on the frequency due to the effects of capacitance and inductance. Resistance, on the other hand, remains constant regardless of frequency.
Resistance vs. reactance
Resistance and reactance are both necessary components to create impedance. Resistance is generated from collisions between the current-carrying charged particles and the internal structure of the conductor. Reactance is further opposition to the movement of the electrical charge in AC circuits that is generated from the shifting magnetic and electric fields.
Input impedance vs. output impedance
The input impedance is the level ‘seen’ by components connected to the input of a circuit. It’s the combination of all resistance, capacitance and inductance. Output impedance is equivalent to the output of any circuit or device in series with a consistent source of voltage.
Input impedances are very high, typically ten times higher than the output impedance of a circuit. This prevents the input from overloading and reducing the strength of the signal.
How to calculate impedance
A mathematical equation is used to determine the level of impedance in a circuit. The total impedance (Z) of a circuit is equal to the maximum value of the voltage (V) across the entire circuit divided by the maximum current (I):
Z = V/I