EXPONENTIAL STATE-SPACE FILTERS - A GENERIC CURRENT-MODE DESIGN STRATEGY

A new method of analog filter design is proposed where nonlinear activ e components are a natural part of a filter realizing an overall linea r transfer function. This approach to design is articulated in the gen eration of the class of so called ''Exponential State Space'' (ESS) fi lters, An outgrowth of ''log filters,'' ESS filters are created via a mapping on the state space of a linear filter. Specifically, the state variables are equal to simple functions of exponentials of node volta ges. The use of exponential, hyperbolic tangent, and hyperbolic sine m appings is shown to produce realizable nodal equations that result in ''log,'' ''tanh,'' and ''sinh'' filters, respectively, Aspects of inte rpretation and realization of the transformed state equations are disc ussed, It Is shown that sinh filters are class AB filters, which is an intriguing extension of a concept introduced by Seevinck. The differe nt filter types are compared to an analogous standard transconductance -C filter via simulation of a band-pass filter with a Q of 5, All filt ers demonstrate tunability over a 100 to 1 range with excellent freque ncy response stability and accuracy over the tuning range, which exten ds to 5 MHz. These results and IMD and noise performance are given usi ng both ideal transistors and transistors whose cutoff frequency equal s approximately 300 MHz.