A hybrid amplifier topology for low-noise direct-coupled front ends

Hybrid amplifiers combine discrete transistors with operational amplifiers (OAs) achieving superior characteristics compared with commercially available OAs, while keeping simple structures and a reduced number of parts. A single-ended amplifier topology based on a junction field-effect transistor (JFET) is proposed as a means of achieving very low input noise levels, both in current and voltage. It is direct-coupled and allows to place a high-pass filter at its input to preserve input range, or at its output to optimize noise. The proposed topology results in a low input capacitance and low input-current noise, making it suitable for applications where multiple amplifiers are connected in parallel to achieve ultra-low noise levels. The amplifier's gain is set by a resistor ratio, being suitable for instrumentation front ends. The circuit analysis and design equations are provided to adapt it for different applications and, as an example, an amplifier with a gain of 40 dB and a bandwidth from DC to 1 MHz was designed, built, and tested. The dominant noise contribution of the amplifier was the JFET (a JFE2140 device), featuring an input noise voltage of 0.95nV/Hz@1kHz$$ \mathbf{0.95}\ \mathbf{nV}/\sqrt{\mathbf{Hz}}@\mathbf{1}\ \mathbf{kHz} $$, that is reduced to 0.70nV/Hz@1kHz$$ \mathbf{0.70}\ \mathbf{nV}/\sqrt{\mathbf{Hz}}@\mathbf{1}\ \mathbf{kHz} $$ when two amplifiers are connected in parallel. The proposed circuit results in direct-coupled amplifiers with low voltage and current noise features. This allows placing a high pass filter at the input to preserve dynamic range or at the output to optimize noise. It also allows to connect several amplifiers in parallel to implement extremely low noise amplifiers. image