Abstract

Abstract
A Study of Voltage-Mode and Current-Mode Filters Using
Modified Current Feedback Operational Amplifier
Xin Cui
There is a prevalent use of current-mode (CM) circuit techniques in analog integrated circuit design, in view of the fact that CM circuits offer certain advantages over voltage-mode (VM) circuits in terms of certain performance parameters such as propagation delay, dynamic range, and bandwidth. The characteristics of a CM circuit make it not so vulnerable to the current demands of IC design trends, such as continuously decreased size and lower DC supply voltages. Therefore, some active devices that could be exploited in both CM and VM circuits have drawn a lot of attention, such as the second generation current conveyor (CCII) and operational transconductance amplifier (OTA). However, a large amount of effort has been made on VM circuits due to their dominant form of signal processing in analog circuit design for the past several decades. The concept of network transposition, introduced by Bhattacharyya and Swamy as early as in 1971, is a powerful technique to convert a VM circuit to a CM one and vice-versa, with little physical circuit alteration and retaining the same performance as its voltage-mode counterpart. It is especially attractive in transforming those circuits that employ active devices which are transposes of themselves, such as OTA or CCII-.
Recently, it has been shown in the literature that a new active element, the modified current feedback operational amplifier (MCFOA), is also its own transpose, and hence can be used to design both VM and CM circuits. It is also known that using the same MCFOA, four equivalent realizations are possible for synthesizing a VM filter function, and further, corresponding four CM filter realizations can be obtained utilizing transposition. However, no detailed study has been conducted with regard to the relative performance of the four equivalent VM structures or the corresponding four CM structures, particularly from the point of view of the non-idealness or the parasitic effects of MCFOA on the performance.
This thesis presents a thorough study on band-pass filter (BPF) and notch filter (NF) implemented with MCFOA both in the voltage-mode and their transposed current-mode counterparts. The transfer functions of the four configurations of voltage-mode circuits, as well as that of the current-mode circuits, should be the same when the MCFOA is ideal. However, in practice, they are influenced by parasitic parameters. Accordingly, the performances of the band-pass and notch filters are influenced remarkably by the parasitic parameters of the active device, namely, MCFOA, especially the parasitic resistances for low frequency applications. These effects are studied by comparing the theoretical and SPICE simulation results of the four configurations of the voltage- and current-mode BPF and NF using non-ideal MCFOA.
In addition, an improved MCFOA that reduces the effect of parasitic resistances is proposed. Performance of BPF and NF are compared among the four configurations of voltage- and current-mode circuits using the improved MCFOA. They are also compared with those using the original version of MCFOA. It is shown that the proposed MCFOA yields several improvements on the performance of both VM and CM BPFs, such as more attenuation at the low frequencies, and drastic reduction in the ω_p and Q_p errors.
Based on the fact that MCFOA is composed of two CCIIs (CCII+ and CCII-), and FTFN can be realized with minor modifications of CCII-, it is natural to compare the performance of BPF using CCII- and FTFN with that using MCFOA. Thus, BPF using CCII- and FTFN and their transposed circuits are also studied. As mentioned earlier, CCII- is its own transpose. However, FTFN does not have a proposed admittance or a hybrid matrix for us to find its transpose. An attempt to find the admittance matrix of FTFN is explored in this thesis. The results show that FTFN can be used as its own transpose only under ideal conditions. Comparisons of performance of BPFs using the original MCFOA, the proposed MCFOA, and CCII-, as well as among their transposes, are presented. It is shown that BPF using the proposed MCFOA exhibits the best performance.