A Miniature Transformer-Coupled Low-Noise Preamplifier for Low Source Resistance Sensors at Low Frequency

Transformer-coupling technique plays a key role in low-noise design methodology. Recently, there has been an increasing amount of research undertaken on developing dedicated transformer-coupled low-noise preamplifiers for particular sensors or systems. However, no research exists which combines an in-depth analysis of analytical models with a thorough understanding of performance specifications in terms of background noise, input range, bandwidth, and size. In this research, we present a small-signal model and a noise model for these preamplifiers along with fundamental design parameters. We design, construct, and test a prototype miniature transformer-coupled low-noise preamplifier for a specific resistive sensor with a source resistance below 0.1 Omega in a frequency band of interest between several hertz and 1 kHz. The proposed models are validated by the excellent agreement between the simulation and experiment results. Results show the input-referred voltage noise at room temperature settles to about 25 and 35 pV/(Hz)(1/2) at 10 Hz when the source resistance is 6 and 36 m Omega, respectively. This research will serve as a basis for future studies on high-resolution resistive sensors and systems.