What is Impedance? Why does it matter?

What is impedance? Why is it important? Remembering Ohm's law, voltage equals resistance times current. When you apply a DC voltage to a resistor, the output is reduced according to the amount of resistance contained in the resistor. When you apply an AC voltage through a resistor, the same result occurs. The output is reduced by the amount of resistance encountered, and the voltage and current are in phase at the output of the resistor. But what happens when you apply an AC voltage to an inductor? What happens is a phase shift occurs. The output voltage leads the current by 90°. This phase angle will result in reduced power delivered to a load resistor. Because power is the product of current and voltage, and when these two are out of phase, the product is reduced over much of a single cycle. The inductor voltage opposes the current by creating a 90° phase shift. This is called reactance. That is opposing the creation of current not by a resistance, but by a 90° phase shift. When we combine a resistor and an inductor in series, the phase shift that occurs will be between 0 and 90°. This combination of a pure resistor and a pure reactance is the definition of impedance. It is the ratio of voltage to current, which has both a magnitude and a phase angle. This phase angle or reactive part is what reduces power delivered to a resistive load. Why is this important? When impedances are unmatched between components, signal degradations can occur. One example is an audio amplifier trying to deliver power to an 8-ohm speaker. When reactance gets large at higher frequencies, signal roll-off will occur where the high frequencies can get suppressed, creating a dull lifeless response dominated by the low-end frequencies. When reactances create a resonance, which is either a very high impedance or a very low impedance at a single frequency, a sharp spike or high frequency audio signal will create a ringing or reverberation of the signal as seen here. This audio distortion will be audible to the human ear. High frequency radio circuits have components typically matched to 50 or 75 ohms to keep a standard impedance through the signal chain. When mismatches occur between components such that impedances stray from 50 ohms, ripple can occur in the frequency response,