Resistors aren't resistors

July 30, 2015 // By Kenneth Wyatt
Resistors aren't resistors - at high frequencies, that is. Many designers don't realise that parasitic elements in real components affect their values. [Or they likely “know that” - but still get taken by surprise each time a “secondary effect” has an impact on circuit operation.]

When frequencies reach hundreds of megahertz, basic components such as resistors, inductors, and capacitors take on non-ideal characteristics. Such changes can become critical when you design filters or attempt to optimise power delivery networks, bypass networks, or bias circuits.

We will discuss capacitors and inductors another time. For now, let's talk about the lowly resistor. Here's the plot of an ideal impedance of a resistor, which, as you might expect, is a straight line.

Figure 1: The impedance plot of an ideal resistor versus frequency shows the same value at all frequencies.

Now let's consider a carbon-composition resistor with short leads. If you add the parasitic inductance of the leads and parallel capacitance between the end caps, you should get this simplified model at high frequencies.

Figure 2: A simplified model of a typical resistor at high frequencies shows parallel capacitance and series inductance.

Typical values for the carbon-composition resistor (with 6-mm leads) might be 14 nH of series inductance and 1-2 pF parallel capacitance.

Now, if you plot this simplified model versus frequency, you should see the following idealised impedance plot.

Figure 3: An idealised impedance plot of a real resistor shows the different points where resistance dominates, capacitance reduces impedance, and inductance increases impedance.

At lower frequencies... (continues)