Effects of Negative Current Feedback

Abstract

Methods of applying negative current feedback in R-C coupled circuits are discussed. It is shown that the removal of by-pass condenser at the cathode for the application of negative current feedback does not increase and decrease the upper and lower crossover frequencies respectively by the feedback factor (1+Aβ) as expected though the midband gain is reduced by this factor.For pentodes the upper and lower crossover frequencies remain unchanged and for triodes upper crossover frequency is reduced and lower crossover frequency is reduced only slightly by removal of the by-pass condenser. This unexpected behaviour is reasoned out and it is shown that the addition of a considerably small capacitor across the cathode increases upper crossover frequency by factor (1 + Aβ) and addition of a very high inductance across cathode reduces lower crossover frequency by factor (1 + Aβ) for both triodes and pentodes. Relations for computing the values of capacitance and inductance are derived.Transient response is shown to improve in the modified circuit.Bandwidth reduction for grid-grounded amplifier, in which negative current feedback is inherently present, is proved.For a two-stage R-C coupled amplifier using triodes, it is shown that upper crossover frequency increases for certain values of positive current feedback and for certain ratio of RL/rp.