A very low noise preamplifier for extremely low frequency magnetic antenna

Besides the electrode-pair antenna, the magnetic antenna is also used for the extremely low frequency (ELF) submarine communication. To receive the weak ELF signals, the structure of a small sized magnetic antenna determines its specific electrical characteristics. The ELF magnetic antenna shows high internal resistance, alternating-current impedance, and a resonance frequency near the operating bandwidth. In accordance with the electrical characteristics of ELF magnetic antenna, a low noise preamplifier and frequency compensation circuit were designed and realized. The preamplifier is a three-stage negative feedback circuit, which is composed of parallel JFET, common-emitter amplifier with a Darlington structure and a common-collector amplifier in push-pull connection. And a frequency compensation circuit is cascaded to compensate the characteristic in low frequency range. In the operating bandwidth f = 30-200 Hz, the circuit has a gain of 39.4 dB. The equivalent input noise is 1.97 nV/√Hz and the frequency response keeps flat in operating bandwidth. The proposed preamplifier of the ELF magnetic antenna performs well in receiving ELF signals. (semiconductor integrated circuits)