Using High Frequency Operational Amplifiers for Low Noise Design

Abstract

In this paper we present a comprehensive noise and stability analysis of both the current feedback and the voltage feedback op-amp when connected as transimpedance amplifiers with capacitive current sources. It is generally assumed that the current feedback op-amp is noisier than the voltage feedback op-amp; we show how external component values play an important role in determining the total equivalent input noise current spectral density and show that the voltage feedback op-amp may, in certain cases, be noisier than the current feedback op-amp. Both amplifier types are compared in terms of stability, noise and bandwidth; we find that the current feedback op-amp is particularly suited for operation as a transimpedance amplifier. Theoretical analysis is confirmed by simulations of equivalent circuits of the current feedback op-amp and the voltage feedback op-amp, and simulations of the macro models of two high performance commercially available op-amps, namely the Elantec EL2260 current feedback op-amp and the Elantec EL2244 voltage feedback op-amp.