Fundamental Concepts of Feedback Control

In the early twenties the idea of positive feedback was incorporated in one-tube receivers in order to increase the receiver's gain and selectivity since the amplification factor of the vacuum tube was so poor. In 1934, H. S. Black [1] wrote a paper explaining the theory of the feedback principle and how stability of amplification as well as reduction of modulation distortion follow when stabilized feedback is applied to an amplifier. For example, Black proved experimentally that with negative feedback it was possible to control the gain of the amplifier to 0.01 dB with a change in plate voltage from 240 to 260 volts and maintain the modulation products at 75 dB below the signal output at full load.