A Differential Amplifier for Resting and Action Potentials of Biological Systems *

Growing interest in the electrical properties of living organisms has required the development of amplifiers sufficiently sensitive and stable to record the minute changes in potential encountered. Such amplifiers must be of high gain, with differential input (so that several amplifiers may be used simultaneously on one specimen), freedom from voltage drift, and should be direct coupled throughout in order to permit of both AC and DC recording. To meet such requirements amplifiers have been built with the most expensive materials, complicated circuits, and many adjustments, and hence become excessively bulky. Three types of recording instrument are commonly used, viz., the cathode ray tube, ink-writing oscillograph, and 'Westinghouse loop oscillograph. These differ greatly in the requirements of the output stage. The instrument here described is a new type of amplifier which meets all of these conditions, uses few expensive parts, is light, and is inexpensive to maintain. Figure 1 shows the circuit of the amplifier. Figure 2 shows a power pack capable of running four channels at once. The amplifier has three stages, all push-pull directly coupled, with inverse feedback from the plates of the second stage to the cathodes of the first stage. The power supply is regulated not only in the usual fashion, but has additional compensation to provide complete independence of line voltage fluctuations. The first amplifier stage provides a differential action of better than 500 to 1 in spite of usual differences between tubes. That is, thie sensitivity to a voltage E1 is more than 500 times as great as to E2. The gain of the amplifier is variable from about one thousand to more than one million. Inverse feedback is provided by R7 and Ri. The gain control R16 affects the amount of feedback voltage applied to the 6J7 cathodes. At maximum gain, this voltage is zero. Since the gain