Differential Amplifier for Inductive Sensors

The performance of inductive sensors is limited by the noise and bandwidth of the induction coils and associated amplifier. This article presents a differential amplifier based upon a JFE2140 junction field effect transistor (JFET) long-tail pair. Performance at high frequencies is optimized by designing the amplifier with a moderate input capacitance of 13 pF and negligible current noise. Performance at low frequencies is set by a common mode rejection ratio of 83 dB and voltage noise density of 1.25 nV/(Hz)1/2. Feedback is used to provide artificial cold damping of the induction coil with an effective noise temperature of 5 K, thus reducing thermal noise near the resonant frequency. When connected to an inductive source, the amplifier has ten times lower noise between 20 and 500 kHz than similar amplifiers commonly used in the electromagnetic exploration.