A wideband dc-coupled amplifier

Abstract

A method is described whereby an ac-coupled high-frequency amplifier and a dc-coupled low-frequency amplifier are connected in parallel in order to obtain a dc-coupled wideband amplifier. By using an operational amplifier which compares the output voltage with the input voltage, the low-frequency amplifier contributes to the overall gain only when the gain of the ac-coupled amplifier droops at low frequencies. Thus, no frequency splitting networks are necessary and the excellent low-frequency features of an operational amplifier are added to those of the ac-coupled wideband amplifier. As an example, a low noise amplifier is described which exhibits a hundredfold gain, a bandwidth from dc to 550 MHz, an input bias current of less than 1 nA, and an output voltage range of ±1 V. © 2003 Elsevier Science Ltd. All rights reserved.