A 1.2 V BiCMOS class AB log-domain filter

Companding improves dynamic range in situations with restricted signal swing, such as in low-voltage circuits, without the need to reduce noise at the cost of increased power dissipation. The log-domain filter principle achieves instantaneous signal companding by taking advantage of exponential device characteristics. Thanks to the high compression ratio, voltage swing and frequency tuning become almost independent of supply voltage, which can be reduced to the minimum required for proper operation. The combination of instantaneous companding with class AB operation permits processing of signals much larger than the bias level and hence increases dynamic range substantially. Class AB log-domain filters are suited to low-voltage and low power applications with moderate signal-to-noise ratio (SNR) and linearity requirements. The authors present a 3/sup rd/-order Chebyshev filter which is based on an integrator and implemented in a 1 /spl mu/m BiCMOS process. The filter can be tuned over more than 3 decades with cut-off frequencies ranging from 10 kHz to 15 MHz at supply voltages between 0.9 V and 2 V. For a 1.2 V supply, cut-off frequencies up to about 1 MHz are achieved.