A novel approach to filter design, based on Adams' [1] ‘log-domain’ filters, is proposed that yields a truly current-mode circuit realisation. Adams' idea, which was introduced in a limited context, is generalised to permit a complete distortionless synthesis procedure, which results in circuit implementations readily realisable using complementary bipolar processes. It is shown that, by introducing an exponential map on the state-space description of the desired linear system, a log-domain filter can be fully realised with transistors configured in current mirror-type groupings, current sources and capacitors. Owing to the mapping, the state variables are intrinsically related to current, and not voltage, in the resulting circuits, a fact that emphasises the current-mode nature of the design. A general biquadratic filter section is designed, and, following discussion of cascading sections, a seventh-order Chebychev lowpass filter is designed. All designed circuits are shown to be tunable over a two-decade range in frequency while their characteristics are accurately preserved, even for biquad sections whose f0Q product is greater than fT/10. The Chebychev filter is shown in simulation to possess nearly 60 dB dynamic range relative to 0.9% THD, with a cutoff frequency of nearly 5 MHz, using transistor models from AT&T's CBIC-R 300 MHz complementary bipolar process.
[1]
T. S. Rathore,et al.
Current-conveyor realisation of transfer function
,
1975
.
[2]
D. G. Haigh,et al.
Continuous-time and switched capacitor monolithic filters based on current and charge simulation
,
1989
.
[3]
C. A. Desoer,et al.
Nonlinear Systems Analysis
,
1978
.
[4]
Chi-Tsong Chen,et al.
Linear System Theory and Design
,
1995
.
[5]
Gordon W. Roberts,et al.
All current-mode frequency selective circuits
,
1989
.
[6]
E. Seevinck,et al.
Companding current-mode integrator: A new circuit principle for continuous-time monolithic filters
,
1990
.
[7]
Christofer Toumazou,et al.
Universal active filter using current conveyors
,
1986
.
[8]
J. M. Khoury.
Design of a 15-MHz CMOS continuous-time filter with on-chip tuning
,
1991
.
[9]
B. Wilson,et al.
Recent developments in current conveyors and current-mode circuits
,
1990
.
[10]
Yannis Tsividis,et al.
Design considerations for high-frequency continuous-time filters and implementation of an antialiasing filter for digital video
,
1990
.