Genetically derived filter circuits using preferred value components

In the realisation of discrete-component analogue electronic circuits it is common practice, because of costs, to specify passive component values from a set preferred of values. For the design of integrated circuits it can also be desirable to use a standard set of passive component values. For example, to obtain accurate ratio matching of integrated resistors and capacitors by stacking identical unit valued components. The usual design approaches produce circuits in which the permitted component values are assumed to be unrestricted. The circuit is then converted to a practical circuit by simple rounding of the exact component values to the nearest value in the permitted set. Of course, in general the circuit performance realised will differ from the ideal. It may then be necessary to repeat the design with a more stringent specification or to use a more closely spaced set of permitted values, both of which can have cost implications. However if other combinations of permitted values are considered, a better circuit performance may potentially be achieved than that obtained by simple rounding. The difficulty is that in all but trivially simple circuits the space of all feasible combinations to be searched is huge. Genetic algorithms (GAs) can be used to search this space. There the application is to a simple second order active filter specified by its transfer function parameters. The aim of the present contribution is to show that GAs can be successfully applied to more complex filter structures. Moreover the optimal search is carried out directly on the frequency-response template specification rather than on a specified approximating ideal transfer function, thereby avoiding this additional source of approximation. The next section outlines the basic GA and its implementation for the present application. Results are then given for practical filter examples. An all-pole low pass response is considered with template specified by a 1 dB pass band ripple with a pass band edge at 10/sup 5/ rad/sec, and stop band attenuation of -150 dB at a stop band edge of 10/sup 6/ rad/sec. The GA is used to generate both LC ladder structures and the more complex FDNR active RC structures.<
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