Design of a CMOS current feedback operational amplifier

Abstract

The Current Feedback Operational Amplifier (CFA) presents a different architecture from the one used in conventional Voltage Feedback Amplifiers (VFAs). The major advantage of the CFA is the elimination of the constant gain-bandwidth limitation present in most VFA topologies [1]. The inherent structure of the CFA also gives rise to an increase in slew rate, making the CFA an ideal choice for high-speed video applications. The aim of this project is to design and implement a CMOS Current-Feedback Operational Amplifier. The scope of using CMOS technology is to highlight the ways in which a low input impedance can be obtained without resorting to bipolar solutions. Unlike the VFA, the CFA structure has a high-impedance non-inverting input and a low-impedance inverting-input. Current is injected to the output node from the inverting-input. The output voltage of the CFA is therefore formed from the product of the inverting-input current (I) and the circuit transimpedance (Z). Among the main steps carried out in this project is the design of a low-voltage CFA built using CMOS technology. At each step of the implementation, the DC operation of the circuit is examined and the frequency response is analysed in relation to parameters such as the feedback resistance. Implementing a circuit for providing a constant input transconductance is an important stage for providing a constant input impedance in order to minimise distortion. The design of a unity-gain buffer is also required in order to enable the addition of low-impedance loads at the output. The results obtained in this project were mostly focused around the frequency response and closed-loop gain obtained when varying several parameters in the circuit. The transient response and slew rate were also simulated. Plots indicating the operation of the circuit under different conditions, such as worst power and worst speed simulations were obtained. Most of the simulated results are consistent with CFA theory, with highlights including the rail-to-rail operation, the consistency in low input impedance, the bandwidths achieved and the stability of the circuit.