A D.C. Amplifier for Biological Application

Abstract

The limitations of direct‐coupled valve amplifiers are discussed and a method is given for assessing their performance. A multichannel high gain direct‐coupled amplifier is described. This was designed for use in biological applications requiring a high input impedance. The input impedance at the grid is greater than 100 MΩ at 10 kc/sec. The frequency response is flat from zero to over 10 kc/sec. Alternative condenser coupling is provided so that the large amplitude very low frequency noise elements may be eliminated. When the amplifier is direct‐coupled the zero shift is of the order of 100 μv peak‐to‐peak over a period of 30 min. Sufficient voltage gain is available for the limit of useful amplification to be set by the noise generated in the input circuit and first amplifying valve. A parallel balanced circuit is used giving a very high discrimination against in‐phase signals. Both the input and output are at earth potential.