An Ultralow-Noise Fully Differential Amplifier

A general-purpose instrumentation amplifier must be dc-coupled and has a differential input to handle both differential and single-ended input signals. It must also exhibit low input noise in both voltage and current to accommodate a wide range of signal source impedances. Additionally, having a differential output is desirable to allow direct connection to current high-resolution analog-to-digital converters (ADCs), which have differential inputs. There are commercially available devices with $e_n$ voltage noise spectral densities as low as $1~\mathrm{nV} / \sqrt{\mathrm{Hz}}$ but present high current noise spectral densities $i_n$ of a few $\mathrm{pA} / \sqrt{\mathrm{Hz}}$ . On the other hand, there are also devices with $i_n$ as low as a few $\mathrm{fA} / \sqrt{\mathrm{Hz}}$ but presenting $e_n$ around $10~\mathrm{nV} / \sqrt{\mathrm{Hz}}$ . To obtain low values of both $e_n$ and $i_n$ , a fully differential circuit topology combining discrete junction field transistors (JFETs) and operational amplifiers (OAs) is proposed. Design equations, stability analysis, and experimental results are presented. As an example, a fully differential instrumentation amplifier has been designed, built, and tested showing $e_n < 1~\mathrm{nV} / \sqrt{\mathrm{Hz}}$ at 1 kHz and $i_n < 10~\mathrm{fA} / \sqrt{\mathrm{Hz}}$ at 1 kHz. The proposed topology finds applications, such as front ends for measuring and testing instruments, industrial instrumentation, and audio circuits.