Abstract
In this paper a direct-current amplifier is described which employs negative feedback for the measurement of small stellar photoelectric currents. A General Electric type FP-54 electrometer tube is used in the input stage to give maximum sensitivity. The output voltage is fed back negatively into the input stage and the amplifier locks in a condition of equilibrium. In this condition the output voltage matches the input voltage with a tenacity proportional to the voltage gain of the amplifier before the feedback is applied. A high gain stage employing one type 1N5-G tube as a load for another type 1N5-G tube supplies a voltage gain of about 1500. The output stage uses a type 1C5-G tube connected as a triode. The output meter used is a Weston portable microammeter with full scale deflection of 50 microamperes. A 1900-ohm resistance is used in series with the meter to give full scale deflection for 0.1 volt. Thus with an input resistance of 2×1010 ohms, a photoelectric current of 5×10−12 ampere gives full scale deflection.
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