In a frequency multiplier circuit, the input signal is coupled to the base of a transistor through a capacitor. A radio frequency choke is connected between the base of the transistor and ground. The capacitor is:

In this specific circuit configuration, the capacitor placed in series with the input signal path serves to pass the alternating current (AC) radio frequency signal to the transistor's base. At the same time, it prevents any direct current (DC) from feeding back into the preceding stage or altering the base bias. Therefore, it acts as a DC blocking capacitor (frequently called a coupling capacitor). The radio frequency choke (RFC) provides a DC path to ground for biasing without shorting out the RF signal. The series capacitor is not a bypass capacitor, which would be wired in parallel to shunt signals to ground, nor is it the primary tuning component of an LC tank circuit.