High input impedance circuits for the measurement of bioelectric potentials
Laurel Hulley, D. Robson · Journal of Scientific Instruments · 1966
The factors which limit the performance of high input impedance transistor amplifiers are investigated. Typical results which may be obtained using field effect transistors are shown. The use of positive feedback principles in conjunction with multiple emitter followers is analysed, with particular reference to the use of microelectrodes in electrophysiological research. An equivalent circuit of the microelectrode system is given. For satisfactory recording a large bandwidth is required. It is shown that stray capacitance rather than transistor parameters restrict the performance. Methods of reduction of stray capacitance are discussed. Three methods of measurement of input impedance are compared and details are given of a circuit whose input impedance is greater than 10 9 ohms in parallel with less than 1 pF are given.