The Detection of Extremely Small Voltages and Currents Using the Second-Harmonic Type of Magnetic Modulator

The dc amplifier using the second-harmonic-type magnetic modulator has been considered to be inferior to the chopper-stabilized amplifier with a high input resistance. However, both equivalent input noise power and zero drift caused by the Barkhausen effect are extremely small and can be less than the order of 10-18W. In this paper, the equivalent input circuit of a magnetic modulator is shown to obtain a high signal-to-noise ratio of the dc amplifier. On the other hand, an increase in the input resistance is required for detection of small voltages and a decrease in the input resistance is required for detection of small current. Our dc amplifier used a magnetic modulator with a source resistance of 26 Ω and exhibits a minimum input noise voltage of 9 × 10-9 Vpp/√Hz for detection of voltage signal and a minimum input noise current of 7 × 10-l0App/√Hz for detection of current signal. The input resistance of 50 kΩ for detection of voltage signal and the input resistance of 10 mΩ for detection of current signal can be easily obtained using a negative feedback circuit.