A LOW-NOISE FET PREAMPLIFIER FOR WIDE-BAND HYDROPHONES
Richard I. Allman · 1968
The design of a low-noise, field-effect transistor (FET) preamplifier is described, and criteria are given for low-noise operation of relevant FET devices. When used in conjunction with a hydrophone having a sensitivity of -80 dBV/microbar and a capacitance of 60 pF, the design provides a measuring system having a self-noise threshold approximately 10 dB below Sea State 0 from less than 100 Hz to nearly the thermal limit. At the same time, DC bias stability is maintained between 0C and 50C, and voltage gain is 6 dB with frequency response essentially flat from 1 Hz to 1 MHz. Input impedance is approximately 1,000 M ohms at low frequencies, and input capacitance at ultrasonic frequencies is approximately 5 pF.