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In recent years, wide-band wireless communication systems such as IMT-2000 for mobile communication and IEEE802.11 standard for W-LAN applications have received considerable attention [1][2][3]. These wide-band systems use a channel bandwidth up to many tens of MHz to achieve a high bit-rate data transfer. In receiver front-end for such wide-band systems, a down-converter (D/C) is required to output a wide-band IF signal whose level should be significantly flat within its channel band for sufficient BER. A D/C and other function blocks are also required not to use many external components and additional pins for a highly integrated receiver. This paper describes a single-balanced Gilbert-cell type D/C with a small number of pins, which can output a wide-band IF signal converted from a 2-GHzband RF signal.In conventional narrow-band systems, a D/C has an output impedance matching circuit using high-Q LCs to match the following channel selection bandpass filter (IF BPF) [4]. This is not suitable for a wide-band system because the need to flatten the output signal level within its channel bandwidth results in a complicated impedance matching circuit. In order to output a wide-band IF signal and drive the following IF BPF, an emitter follower can be employed as an IF buffer circuit. However, its bandwidth would be limited by parasitic capacitance such as Cπ and CCS if the emitter follower were directly connected to the mixer load resistor (Figure 26.4.1a). To avoid this bandwidth limitation, a transimpedance amplifier (TIA) is inserted between mixer load resistor and the emitter follower to reduce the node impedance at A and B (Figure 26.4.1b). Considering the distortion at the TIA, a large bias current is required in order to receive a large LO signal current from the mixer into the TIA. In order not to increase the current consumption, filtering of the LO signal should be introduced. Bandpass filtering could not be adopted for the above-mentioned reason and simple low-order lowpass filtering could not be adopted because of poor LO signal suppression, so band-rejection filtering is adopted. To suppress the LO signal, an LC series circuit resonated at the LO signal frequency is added in parallel with the mixer load resistor (Figure 26.4.2). This circuit does not need any additional pins because the required inductance and capacitance of the LC resonator are sufficiently small to be realized by on-chip capacitor and spiral inductor. In practical design, this resonator is composed of 2pF and 2nH.An on-chip LC resonator has a low Q value. The impedance of this LC resonator is about 4Ω at the resonated (LO signal) frequency. At the IF signal frequency, the impedance is about400jΩ. To achieve both IF signal level flatness and LO signal rejection, it is desirable that an input impedance of the TIA be several tens of ohms at both frequencies, and this value is easily achieved. The low Q value of LC resonator makes the sensitivity of the values of L and C to the impedance at the resonated frequency lower, and so the deviatio...

SUMMARYDue to requirements for large-capacity data communication, the bandwidth of wireless communication systems such as cellular phones and wireless LAN has been increased. In this paper, the recent trends of wireless communication systems are reviewed with an emphasis on the receiver analog front end. In addition, a design technique for wide-band circuits is described, with the mixer in the receiver as an example.

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