# Impedance

### Reference Material

• Horowitz & Hill, The Art of Electronics, 2nd Ed., Ch. 1

## Impedance

The concept of impedance is basically an attempt to extend Ohm’s Law to devices that are not resistors. Ohm’s Law

${\displaystyle V=IR,\,\!}$

relates voltage, ${\displaystyle V}$, and current, ${\displaystyle I}$, by a resistance, ${\displaystyle R}$. Impedance generalizes this by considering ${\displaystyle V}$ and ${\displaystyle I}$ as complex waveforms, and using the symbol ${\displaystyle Z}$ to denote the (possibly complex, and possibly frequency-dependent) number that relates them. Just as a note, since ${\displaystyle I}$ generally means current in electronics, the imaginary unit ${\displaystyle {\sqrt {-1}}}$ is generally denoted as ${\displaystyle j}$ by electrical engineers. We’ll use that notation here as well.

### Capacitive Impedance

In discussing capacitors, we used capacitance ${\displaystyle C}$ to relate current to the time-derivative of voltage:

${\displaystyle {\frac {dV}{dt}}={\frac {I}{C}}.\,\!}$

Knowing from Fourier analysis that we can decompose any waveform into a sum of sin/cos functions, it makes sense to examine how this equation works for a sinusoid waveform like

${\displaystyle V=e^{j\omega t}=\cos {\omega t}+j\sin {\omega t}.\,\!}$

In this case we get:

{\displaystyle {\begin{aligned}j\omega e^{j\omega t}&={\frac {I}{C}}\\j\omega V&={\frac {I}{C}}\\V&=I{\frac {1}{j\omega C}}\end{aligned}}\,\!}

In that last line, where we’ve substituted ${\displaystyle V}$ back in, we have an equation that looks very much like Ohms law, but instead of ${\displaystyle R}$, we have ${\displaystyle V=IZ}$, with ${\displaystyle Z}$, for capacitors, given by:

${\displaystyle Z_{c}={\frac {1}{j\omega C}}\,\!}$

Hence, Ohm’s law can be extended to capacitors if we generalize resistance to a complex impedance, and take the imaginary part of that impedance to be a frequency-dependent quantity. Apart from changing the phase of an incoming wave by ${\displaystyle 90^{\circ }}$, a capacitor can pass a high-frequency wave with very little attenuation, but can completely block a DC voltage with an infinite impedance.

### Inductive Impedance

Inductors, on the other hand, have a voltage that is dependent on the time derivative of current:

${\displaystyle V=L{\frac {dI}{dt}},\,\!}$

This time, let’s take the current to be a sinusoid given by:

${\displaystyle I=e^{j\omega t}=\cos {\omega t}+j\sin {\omega t}.\,\!}$

In this case we get:

{\displaystyle {\begin{aligned}V&=Lj\omega e^{j\omega t}\\V&=Lj\omega I\end{aligned}}\,\!}

Again, the last line looks like Ohm’s Law, if we take the impedance of an inductor to be:

${\displaystyle Z_{L}=j\omega L\,\!}$

So an inductor is like a resistor that changes the phase of the incoming wave by ${\displaystyle 90^{\circ }}$, and resists higher frequencies more strongly than lower frequencies (and behaves just as a wire to a DC voltage).

## Mixed Circuits

There’s actually not a lot to say here. You can pretend that impedances are just complex resistors. They add when wired in series; they add reciprocally when wired in parallel. You’ll see that the rules for adding capacitors in parallel falls out naturally using the expressing for the impedance of a capacitor. Oh, and as we’ll see when discussing RC and LC filters, you can use the frequency-dependence of the impedances of capacitors and inductors to construct filters that purposely attenuate waveforms based on their constituent frequencies.

Finally, the idea of the Thévenin equivalent circuit (involving a voltage source and an equivalent series resistor) generalizes naturally to impedances. Now instead of an equivalent series resistor, we instead have an equivalent (and possibly frequency-dependent) series impedance:

The Thévenin equivalent circuit, using impedances