REACTANCE AND IMPEDANCE
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Resistors have RESISTANCE, measured in ohms, which opposes the flow of DC current.

Capacitors have CAPACITIVE REACTANCE, measured in ohms, which opposes the flow of AC current.

Inductors have INDUCTIVE REACTANCE, measured in ohms, which opposes the flow of AC current.

Ohms Law can be applied to all of these. VOLTS divided by OHMS gives AMPS.

In a circuit which contains all three of the above, the total opposition is called IMPEDANCE (Z) and is measured in ohms. Again Ohms Law can be applied.

Reactances and Impedances can be calculated from formulae. They depend upon the values of the components and the AC frequency.

Capacitive reactance decreases as the frequency increases and also as the value of the capacitor increases.

Inductive reactance increases as the frequency increases and also as the value of the inductor increases.

For optimum transfer of power from one stage to another the impedances must match.
In the diagram the amplifier has an input and output impedance of 10k.
The microphone has an impedance of 300 ohms and the loudspeaker an impedance of 8 ohms. 
They are connected to the amplifier via IMPEDANCE MATCHING circuits.


Copyright Graham Knott 1999