Negative Feedback, Amplifiers, Governors, and More [Historical]

The invention of the negative feedback amplifier by Harold S. Black (1898-1983) in 1928 is considered one of the great achievements in electronics. In fact, it is listed among the IEEE Milestones, where it is credited to Bell Labs. Black was hired by Western Electric in 1921 and assigned to work on the Type C system, a newly introduced three-channel telephone network whose push-pull, vacuum-tube, repeater amplifiers produce too much harmonic distortion when connected in tandem [1]. At that time, telephone networks were spreading, and Bell Labs emerged quickly as the major research company in the telephone industry. The extension of lines over long distances required counteracting signal attenuation, which occurred, though at a reduced level, even in lines provided with Pupin's loading coils, which discretely increased the line longitudinal inductance L so as to match the Heaviside condition L/R=C/G for distortionless transmission (R being the longitudinal resistance and C and G the transverse capacitance and conductance, respectively).