A Low-Noise Low Input Impedance Amplifier for Magnetic Measurements of Nerve Action Currents

We have developed a low-noise solid-state current-to-voltage amplifier to measure the magnetic fields produced by nerve action currents. The amplifier senses the EMF induced in a toroidal, ferrite-core pickup coil surrounding the nerve by the circumferential magnetic field associated with the axial nerve action currents which propagate past the toroid. In order to obtain low input voltage noise, the input stage of the amplifier consists of 20 preamplifiers in parallel, each containing a discrete bipolar transistor pair. The complete amplifier has an input impedance of 1.33 Ω and a minimum noise figure of 15 dB at 100 Hz for a source resistance of 50 Ω. The amplifier has sufficient sensitivity to detect the magnetic field of the action potential propagating along a single giant axon.