A simple NJFET preamplifier was constructed from commercial parts using parallel input devices in a cascode configuration. The equivalent input noise resistance was 8.5 OHgr; (0.38 nV/(Hz)1/2) at 1 kHz, and 12 OHgr; (0.45 nV/(Hz)1/2) at 100 Hz, measured at room temperature, independent of the source resistance. For 50hyphen;OHgr; sources, a gain of 29 dB was achieved from 3 Hz to 13 MHz. The input noise equivalent resistance is verified by measuring the thermal noise of lowhyphen;valued wirehyphen;wound resistors. Circuit utility is demonstrated by noise measurements performed on GaAs Ohmic contacts at room temperature, under various bias conditions. Design considerations for using parallel input devices, the bias criteria for them, and possible design extensions are discussed.
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