AC & Impedance on EE Notebook

Impedance & Reactance

Mon, 01 Jan 0001 00:00:00 +0000

Impedance & Reactance#

At DC, resistance tells the whole story: V = IR, and that’s it. But as soon as a signal changes over time — a switching edge, an audio waveform, an RF carrier — resistance alone doesn’t capture what’s happening. Capacitors and inductors push back against changing signals in ways that depend on frequency, and the voltage and current through them aren’t in phase anymore. Impedance is the concept that extends Ohm’s law to handle all of this.

Frequency-Dependent Behavior

Mon, 01 Jan 0001 00:00:00 +0000

Frequency-Dependent Behavior#

Every passive component has an ideal model and a real model, and the gap between them grows with frequency. A resistor isn’t purely resistive, a capacitor isn’t purely capacitive, and an inductor isn’t purely inductive — each one carries parasitic elements that eventually dominate. Understanding where the ideal model breaks down is essential for any circuit that operates above DC.

Resistors at Frequency#

At DC, a resistor is just R. At higher frequencies, two parasitics appear:

Resonance & Q Factor

Mon, 01 Jan 0001 00:00:00 +0000

Resonance & Q Factor#

Resonance is what happens when capacitive and inductive reactance cancel at a specific frequency. Energy sloshes back and forth between the electric field of the capacitor and the magnetic field of the inductor, and the circuit’s behavior at that frequency is dramatically different from its behavior at any other frequency. Resonance is the basis of filters, oscillators, and tuned circuits — and it’s also the mechanism behind parasitic ringing and unexpected frequency peaks in circuits that weren’t meant to resonate at all.