A 0.18 μm CMOS channel select filter using Q-enhancement technique

A channel select filter is designed in particular for the 3.6 GHz range applications. The main function of the filter is to select a channel via a selecting voltage. It also provides a high gain for the selected bandwidth. The filter consists of an on-chip inductor in conjunction with an on-chip varactor. The varactor acts as a variable capacitor; the selecting voltage changes the junction capacitance of the varactor to shift the center frequency. The filter is implemented as an active BPF and provides about 5 dB gain for the selected bandwidth which operates on 1.8 Vdc. The gain and high passband selectivity are the results of the design of a high quality factor filter (few hundreds). A Q-enhancement circuit generates a negative impedance to compensate the low Q of the onchip inductor and provides further selectivity. The Q of this filter is also a function of an external control voltage (i.e., tunable Q). This option offers bandwidth settings for various applications in the selected frequency range. The filter also provides a simple implementation of the receiver as it replaces the off-chip front-end filter and the LNA.