Regenerative and Degenerative Feedback

Author: J.B. Hoag

Fig. 13 N. A simple feedback amplifier

In Fig. 13 N, a tickler coil has been added to a simple amplifier circuit. Some of the amplified energy in the plate circuit is fed back to the grid circuit and is re-amplified. Suppose, for example, that the input signal should for a moment make the grid positive. The increased plate current, flowing through the tickler coil, causes its magnetic field to spread out. As this magnetic field cuts across the grid coil it induces voltage which will have either the same polarity as the incoming signal, or the reverse. If the feedback voltage aids the original signal, the feedback is said to be positive, the circuit is said to be regenerative, and the amplification will be increased. Of course the re-amplified energy is also fed back and the process builds up to a limit set only by the closeness of coupling of the tickler coil to the grid coil and by the losses in the circuits. Positive feedback amplifiers, while having high gain, have a tendency to amplify more at one frequency than at another and hence are used where sharpness of resonance is desirable.

If the tickler coil is reversed from that just described, the voltage fed back to the grid will oppose the original signal voltage and the amplification is decreased. This negative feedback is called degeneration. Negative feedback reduces distortion, widens the frequency response, and stabilizes the amplifier against small fluctuations, but it does lower the voltage gain.