FIG. 10 is a block diagram of the transmission system of an ordinary cellular phone. The monitor signal is coupled out from the power amplified by a power amplifier 1 through a capacity-coupling capacitor 2. An isolator 3 and then a low-pass filter 4 are connected in the system, and the signal is transmitted from the antenna 6 after removing second harmonic spurious and third harmonic spurious energy in the transmission system when a mode switch 5 is turned to the transmission side.
In the above configuration, however, the number of poles in the low-pass filter 4 may have to be increased to fully attenuate amplified second harmonic spurious energy and third harmonic spurious energy in the system. In addition, the isolator 3 connected for preventing reflection signals, regardless of the input position of the mode switch 5, results in a higher price.