Frequency Discrimination by Inverse Feedback

This paper deals with the application of feedback to amplifiers for the sole purpose of obtaining a frequency discrimination in the gain-frequency characteristic of the amplifier. The theory of the feed-back amplifier is given and the general form of the vector envelope μβ representing the over-all propagation factor of the amplifier and feed-back circuit is described for a feed-back amplifier which is to have a predetermined gain-frequency characteristic. A basis of feed-back amplifier design is indicated in which the desired over-all μβ-vector envelope is formed by choosing and synthesizing the propagation factors of the component circuits in the amplifier and feed-back circuits. The analysis of the amplifier and feed-back circuits into component circuits which have either fixed- or variable-frequency propagation factors is indicated. Various common resistance-capacitance circuit configurations useful in feed-back amplifier design are shown and simple graphical means for determining the propagation-factor-vector envelopes for these circuits are illustrated. The allocation of the frequency scale on these vector envelopes is also done by graphical means. Two specific cases in feed-back amplifier design are then illustrated for obtaining respectively a high-pass and low-pass gain-frequency characteristic to resistance-capacitance-coupled amplifiers utilizing only resistive and capacitive elements in the feed-back circuit. The amplifier circuit arrangements, the μβ-vector envelopes, and the gain-frequency characteristics of the feed-back amplifiers are given in these illustrated cases. The analytical proof for the graphical determination of the behavior of the propagation factor for one of the resistance-capacitance circuits illustrated is submitted in the Appendix.