LOG-DOMAIN FILTERING - AN APPROACH TO CURRENT-MODE FILTERING

A novel approach to filter design, based on Adams' [1] 'log-domain' fi lters, is proposed that yields a truly current-mode circuit realisatio n. Adams' idea, which was introduced in a limited context, is generali sed to permit a complete distortionless synthesis procedure, which res ults in circuit implementations readily realisable using complementary bipolar processes. It is shown that, by introducing an exponential ma p on the state-space description of the desired linear system, a log-d omain filter can be fully realised with transistors configured in curr ent mirror-type groupings, current sources and capacitors. Owing to th e mapping, the state variables are intrinsically related to current, a nd not voltage, in the resulting circuits, a fact that emphasises the current-mode nature of the design. A general biquadratic filter sectio n is designed, and, following discussion of cascading sections, a seve nth-order Chebychev lowpass filter is designed. All designed circuits are shown to be tunable over a two-decade range in frequency while the ir characteristics are accurately preserved, even for biquad sections whose f0 Q product is greater than f(T)/10. The Chebychev filter is sh own in simulation to possess nearly 60 dB dynamic range relative to 0. 9% THD, with a cutoff frequency of nearly 5 MHz, using transistor mode ls from AT&T's CBIC-R 300 MHz complementary bipolar process.