The Use of Transistors in Physiological Amplifiers

Preliminary experiments have indicated that it is feasible to construct low-level dc differential amplifiers using inexpensive hearing aid transistors. The noise level of such amplifiers was found to be roughly proportional to source impedance; it is less than that of a vacuum tube circuit for sources having an impedance of less than 10,000 ohms. A physiological amplifier having a voltage gain of 10,000 was designed on the basis of these experiments. Several of these units have been constructed which show a bandwidth of about 60 kc and a noise level of about 2 ¿v rms with a 50-ohm input. For measuring nerve potentials a pair of electrometer tubes connected as cathode followers are used as an input probe. With this circuit arrangement the noise level is the order of 15 ¿v.