Patent ID: 7755453

Claim:
A duplexer comprising: an antenna terminal; a first filter connected to the antenna terminal; a second filter that has a passband at a high-frequency side of the first filter and is connected to the antenna terminal; and a resonance circuit that is connected between the second filter and the antenna terminal and includes a first inductor and a first capacitor connected in parallel; assuming that a passband frequency of the first filter ranges from f 1Low to f 1High (f 1Low ˜f 1High ) and a passband frequency of the second filter ranges from f 2Low to f 2High (f 2Low ˜f 2High ), the resonance circuit having a resonance frequency f r satisfying: f r =( f 2Low ˜f 2High )±( f 1Low ˜f 1High ), or f r =2( f 1Low ˜f 1High )±( f 2Low ˜f 2High ), wherein at least one of the first filter and the second filter is a ladder type filter, and has a parallel resonator, a fourth inductor being provided between the parallel resonator and ground.