Low-Frequency Analog Integrated Circuit Design using Current-Mode Techniques, Zibing Yang, 2004
Representing and processing signals with currents, the current-mode technique, has traditionally been used in high frequency applications. This dissertation demonstrates the feasibility and advantages of applying the technique in low-frequency, analog VLSI applications. Arithmetic operations, such as addition, subtraction and scaling, are typically difficult to implement and often area- and power-consuming in a voltage-mode system. These operations become trivial and economical in current-mode implementations.
This dissertation focuses on low-frequency, analog integrated circuit design using the current-mode technique. First, the current-mode design methodology is described in detail, including the basic elements, the transformation from voltage-mode circuits to their current-mode counterparts, and the signal flow graph that facilitates the current-mode system design. Existing voltage-mode, low-frequency filters usually require complex design techniques due to the unavailability of large on-chip capacitances. This study presents unique tunable current-mode integrators for low frequency filter designs that separate the control of the unity-gain crossover frequency from the dynamic range. In addition, new compact and yet accurate current-mode gain elements are also presented. Finally, the detailed realization of electronic cochleae, a special type of low frequency system, is presented. Although various research groups have designed many cochlea chips over the last decade, this work is the first true implementation of biologically-based cochlea models -- the classical transmission line and the traveling-wave amplifier models.
Ten VLSI chips have been fabricated to test the theory and hypothesis presented in this dissertation. They include one current-mode gain element chip, two low-pass filters that utilize the proposed current-mode integrators, and seven current-mode electronic cochleae. Among the cochlea chips, three are functional and their performance is presented.