Realization of current mode filters using the concepts of transposed networks and nullors

Current-mode circuits have become very important in the design of high-speed analog integrated circuits that are highly linear and possess a wide dynamic-range. Efficient design of current-mode (CM) filters is still a major challenge to the designer. Considerable researcher efforts have been made for the realization of current-mode filters from the existing voltage-mode (VM) filters. In 1971, Bhattacharyya and Swamy introduced the principle of network transposition, which has led to a very efficient way to convert a voltage-mode circuit to a current-mode circuit with the same transfer function, and with a one-to-one correspondence between the elements of the two circuits. In this thesis, using the concepts of the transposed networks, and the nullator-norator representation for active devices, it is shown that a voltage-mode filter implemented using an operational amplifier (OA) can be very easily converted to a current-mode filter using the same OA, when the OA is configured as a three-terminal element. The effects of the finite gain bandwidth of the OA on the CM and VM filters are studied. If the OA is configured as a four-terminal device in a voltage-mode filter, it is shown that the corresponding current-mode filter can be implemented using an operational floating amplifier (OFA), which is a practical realization of a four-terminal nullor. The theoretical analysis about the proposed method is verified by using simulation results as well as by practical experiment using discrete components, such as resistors, capacitors and LM741 (OA) devices. Finally, an OFA is designed and laid out using 0.18 om CMOS technology to validate the design of a current-mode filter using the OFA as the four-terminal nullor. Simulations as well as the experimental results show very good agreement with the theoretical analysis