#### Coupling capacitor

A capacitor is open at low frequencies and shorted at high frequencies.

Capacitive reactance (X_{C}) is inversely proportional to frequency (f) and to capacitance (C). For a coupling capacitor to work properly, it has to act like an ac short at the lowest frequency that the ac source can have. To realize that we can use the following rule:

*Make the reactance at least 10 times smaller than the total resistance in series with the capacitor.*

Ex: Calculate the capacitance of C_{1} for a proper ac transmittance. Frequency range: 20-20000 Hz

*Fig. 7-1: Use of coupling capacitor*

Total resistance: 1KΩ + 500Ω = 1.5KΩ

X

_{C} <= 10 * R -- >

_{}

_{} -- > _{}

_{}

C = 53μF

The capacitor to choose should be bigger than 53μF. The next standard value is: