Control of the Effective Internal Impedance of Amplifiers by Means of Feedback

The effective internal impedance of an amplifier may be either increased or reduced by the proper choice of one or a combination of several of the following types of feedback: 1. positive current, 2. negative current, 3. positive voltage, or 4. negative voltage. Impedance-reducing feedback has been applied to audio-frequency power amplifiers to flatten the response and improve the loud-speaker damping. Impedance-increasing feedback has been applied to intermediate-frequency amplifiers to increase selectivity without loss of stability.