A BiCMOS programmable continuous-time filter using voltage-companding

This paper presents a BiCMOS realization of a programmable continuous-time filter demonstrating the feasibility of the voltage companding technique. The circuit implements four low-pass filters (order 2,3,4,5) and one band-pass filter (order 2) by interconnecting a set of identical non-linear cells. The filter is obtained by a proper component substitution transforming a g_m-C structure into its log-domain equivalent. The g_m-C structures are derived from LC ladder prototypes which are synthesized using the image-parameter method. The programmable filters can additionally be frequency tuned from 10 kHz to 100 kHz with a very low THD despite their class AB operation (1% for a modulation index of 3 corresponding to an input current of 30 μA)