Common-emitter Mode Research Articles

A Simple Low-Noise Low-Frequency Transistor Amplifier

THE properties of the amplifier to be described are entirely due to the type of transistor used, and the purpose of this communication is to report on its suitability for the amplification of electro-physiological wave-forms, particularly those originating in the brain. Of the commonly recorded physiological potentials, the electroencephalogram (EEG) presents the most difficulties because of the low voltage (10–1,000 µV.) and low frequency (1–50 c./s.) of its components. Recordings are usually obtained from electrodes on the scalp which give an output impedance of the order of 10,000 ohms. Hitherto, transistors have been unsuitable for amplifying these potentials direct because their intrinsic noise at low frequencies was too high and because the input impedance of the simpler circuit configurations was too low—about 1,000 ohms in the common emitter mode.