The Principle Of CFA And Its Application In The Voltage-Mode Filter

CFA(Current Feedback Amplifier), an operational amplifier structured in a totally new way, is of not only providing high linearly bandwidth, but also high conversion rate. With better performance in frequency property and dynamic range, CFA is more potential in processing high-linear, wide-range signal than VFA(Voltage Feedback Amplifier). CFA's good dynamic properties have broken through the obsessions on the conventional VFA by traditional puzzles and it is of very significance in practical application and theory. In the development of the Circuit and System, the current feedback technology and current mode will become more useful in the more complicated integrated analogue subsystems, and CFA has become the key unit in some single integrated structures. CFA yet have some disadvantages, its special topology structure make lower its static accuracy and amplitude accuracy than that of VFA. CFA and VFA have almost the same theoretical static accuracy, however, the current parameters of CFA are inferior to those of VFA due to practical technology level. It is impossible for the present that CFA wholly replaces VFA in application in all kinds of circuits, therefore study of the theory and the application of CFA make senses.In this paper, the principle of CFA and its application in the voltage-mode filter are studied. Firstly, we briefly introduce the development of CFA and then discuss its structure and model circuit, the operational principle and the typical circuit of CFA and its advantages and disadvantages. Then, the analogue signal processing and operation circuits with CFAs as the active elements are induced overall: linear controlled sources, impedance transformer, amplifier, adder, integrator and differentiator, and the circuit formulas are inferred. Based on the domestic and foreign research results, firstly, a sinusoidal oscillator circuit based on CFA is proposed which consists of two CFA and six inactive components. The oscillator is simple in structure and has an adjustable capability in oscillation frequency and oscillation condition; Secondly, we present a second-order voltage-mode universal filter circuit based on CFA with three inputs and single output, which consists of two CFAs, two capacitances, three resistances, and the natural frequency and the quality factor can be tuned independently. The device can perform the functions of low-pass, band-pass, high-pass, notch and all-pass filter; Thirdly, three different methods for Nth-order voltage-mode low-pass filters only with single active component, CFA, are proposed. The circuit has characteristics of simple structure and less number of components. The principles of three design methods and their realized circuit structures are studied and their advantages and disadvantages are compared. Finally, the proposed methods and the experimental circuits are simulated by the PSPICE . The simulation results show that the proposed methods are correct and the circuits are practical.