Although both the reactance (X) and the resistance (R) tend to be the same thing in a circuit, there is a particular distinction between them. The reactance influences the alternating current (AC), while the resistance affects the direct current (DC). In general, they are the components of an impedance Z, a complex quantity that determines the total opposition of a circuit to the current flow:

Z = R ± j × X,

where j = √-1 is an imaginary number (square root of a negative number).

When you have both resistance AND reactance in a circuit, the TOTAL opposition to current flow is called IMPEDANCE.


A value of a DC blocking cap can be calculated from the reactance formula (f= 1/2piRC).

We solve for C. R = the high-pass resistor in series with the cap OR the impedance the cap "sees" at the input of a BJT, for instance.

C= 1/2pi * R * F

So, I want to pass 20Hz signal and above. The resistor divider that biases the BJT is ~500 ohms.

C= 1/6.2 * 500 * 20 = 16uF


On the Windows Calc:

Type 6.2 * 500 * 20 = 62000

press 1/x key to get 1.61e-5.
Multiply by 1,000,000 to get uF

Result = .0000016 * 1Million = 16uf


Windows Calc and negative exponents solution

w(omega) = 2pi * 500ohms * 20Hz = 62000^(-1)(negative exponent) = 1.6e-5 Farads.

.0000016 * Million = 16uF.