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A method of applying negative feedback to a frequency changer by using a similar frequency changer in the feedback path is described. When the reduction of conversion gain due to the feedback is adjusted to 6 db, then complete stability is obtained against small and equal changes in the gain (or loss) of each frequency changer. Provided both do change equally, then for ±2-db change in each, the resulting change in over-all conversion gain is only ±0.1 db. A typical circuit is described which uses pentode tubes for the frequency changers. Nonlinear distortion is also reduced by the feedback. The application of the principle to stable-gain linear amplifiers is briefly discussed, and while the constancy of gain compares very favorably with conventional feedback systems, it appears that the conditions for linearity may be more difficult to achieve.