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.