Components in a mobile telephone are required to have reduced sizes and small signal loss.
FIG. 12 is a circuitry block diagram of an antenna front end in a GSM (880–915 MHz for transmission and 925–960 MHz for reception)/DCS (1710–1785 MHz for transmission and 1805–1880 MHz for reception) dual-band mobile telephone, which is available worldwide, particularly in Europe. The antenna front end incorporates a switch for switching signals received and to be transmitted at the antenna. The front end generally includes an antenna port 101, transmitting ports 102 and 103, receiving ports 104 and 105, a diplexer 106 for mixing and splitting over GSM and DCS signals, a transmission/reception switch 107 for GSM signal, a transmission/reception switch 108 for DCS signal, low-pass filters (LPFs) 109 and 110 for removing harmonics from the GSM transmission signal and the DCS transmission signal, respectively, band-pass filters (BPFs) 111 and 112 for passing the GSM band and the DCS band, respectively, control ports 113 and 114 for controlling the GSM signal transmission/reception switch and the DCS signal transmission/reception switch, and a surface acoustic wave (SAW) filter. Each of the transmission/reception switches 107 and 108 may commonly be implemented by a switching circuit including diodes 119 and 120 and a transmission line 121 having an electrical length of λ/4, as shown in FIG. 13.
The front end additionally needs low-noise amplifiers (LNAs) 117 and 118 connected to the receiving ports 104 and 105. The connection may become intricate if a system is graded up to triple or quadruple bands. The triple band system requires three LNAs, while the quadruple band system requires four LNAs.
The conventional circuit shown in FIG. 13 includes transmission/reception switches each composed of the diodes 119 and 120 and is thus unfavorable for reduction of the overall size. Also, a loss in the diplexer 106 is about 0.4 dB in the GSM signal band and 0.6 dB in the DCS signal band. Since a power for signal transmission has to be increased to compensate the loss in the diplexer, a battery lasts short. The signal reception declines in sensitivity by the loss in the diplexer. The conventional circuit is not protected from static-electricity breakdown. Moreover, the conventional circuit of a triple or quadruple band mode includes three or four LNAs and hence has large overall dimensions and cost.