A high-gain D-C amplifier for bio-electric recording

This paper describes a d-c amplifier for use with the cathode-ray oscillograph whose maximum voltage amplification is 6 × 10⁶. This amplification is attained with a noise level referred to the input of less than two microvolts over a 5,000-cycle-per-second band, and without the drift due to variations in battery voltages and circuit constants characteristic of such amplifiers. The amplifier is push-pull throughout and provides a choice of two input circuits. Voltage sources in which both terminals are at a high impedance to ground are connected grid-to-grid, symmetrically to ground, while sources in which one terminal is at ground potential are connected to either grid and ground, the first stage, in this case, acting as a phase inverter. Stability and phase inversion are obtained by means of a circuit that makes the output relatively independent of battery voltage and tube-constant variation. The amplifier is simple and rugged, uses ordinary radio-type tubes which do not have to be carefully matched, is constructed of ordinary radio-type components, and has reasonable battery requirements.