Resonant circuit, filter, and antenna duplexer

1. An antenna duplexer comprising:

an antenna terminal:

a receive filter and a transmit filter that are connected to the antenna terminal, the receive filter being a ladder type filter and the transmit filter being a ladder type filter;

a resonator connected between the receive filter and the antenna terminal;

an inductor connected in parallel with only the resonator; and

a capacitor connected in parallel with only the resonator;

wherein the capacitor has a capacitance value equal to or more than 0.2 times a capacitance value of the resonator.

2. The antenna duplexer as claimed in claim 1 , wherein the resonator is either a surface acoustic wave resonator or a piezoelectric thin-film resonator.

3. The antenna duplexer as claimed in claim 1 , wherein at least one of the capacitor and the inductor is formed on an identical substrate on which the resonator is formed.

4. The antenna duplexer as claimed in claim 3 , wherein the capacitor is a surface acoustic wave interdigital transducer.

5. The antenna duplexer as claimed in claim 3 , wherein the capacitor is a MIM capacitor.

6. The antenna duplexer as claimed in claim 1 , wherein the capacitor is a MIM capacitor having a dielectric material that is shared with a material of a package on which the resonator is mounted.

7. The antenna duplexer as claimed in claim 1 , wherein at least one of the capacitor and the inductor is formed on a substrate different from the substrate on which the resonator is formed.

8. The antenna duplexer as claimed in claim 1 , wherein at least one of the capacitor and the inductor is a chip capacitor or a chip inductor provided outside of a package on which the resonator is mounted.

9. The antenna duplexer as claimed in claim 1 , wherein the inductor is formed as a line pattern in a package on which the resonator is mounted.