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We review several techniques that make possible the dynamic variation of analog circuits internally, without affecting their input-output characteristics. Particular attention is paid to companding (dynamic gain scaling), dynamic impedance scaling, dynamic biasing, and dynamic structure variation. A mixture of more than one of these techniques is appropriate in some cases. We use filters as a specific example of dynamical analog circuits and place particular emphasis on avoiding or eliminating transients at the output of such circuits, which would normally occur due to such dynamic variations. By allowing for dynamic internal variations, the power dissipation of such circuits can be lowered and can be made to depend on how demanding the task at hand is. This allows for large savings of energy drain over time, thus making possible long battery life in portable equipment.