Measurement of Bioelectric Potentials With Microelectrodes and Neutralized Input Capacity Amplifiers

Abstract

The biological aspects of bioelectric potentials are reviewed, equivalent electric circuits of the bioelectric generator derived, and grid current magnitude deleterious to a living cell evaluated. The pipette microelectrode is superior to the metallic microelectrode for simultaneous recording of resting and action potentials. However, the low-pass circuit formed by the high electrode resistance (1 to 100 megohms) and the amplifier input capacity causes distortion of the action potential waveform. The distortion can be reduced by amplifier equalization, capacity reduction, and capacity neutralization. Some of the limitations of these methods are: 1) Compensation for waveform distortion due to the low-pass circuit results in an increase of the amplifier noise, approximately as the inverse characteristic of the low-pass circuit. A filter for improving the signal-to-noise ratio is considered. 2) Neutralization is accomplished with a generated negative capacity which approximates a pure capacity only over a limited bandwidth. This limitation may be evaluated with a six-element synthetic circuit closely approximating a realizable negative capacity. 3) The accuracy of neutralization adjustment is inherently limited by the difficulty of inserting a calibrating voltage in place of the bioelectric generator. Moreover, the distributed capacitance of the microelectric cannot be exactly neutralized by a pure negative capacity. Despite these limitations, neutralization is the most applicable of the methods discussed, and highly successful neutralized input capacity amplifiers have been designed and used in several laboratories. Vacuum tube and transistorized versions of neutralized amplifiers are briefly described.