Principles of Feedback Amplifiers

Author: J.B. Hoag

Suppose a small voltage is fed back from the output of an amplifier to its input as in Fig. 26 A.

Fig. 26 A. The principle of feedback amplifiers

If this voltage is in the same phase (crest for crest, trough for trough) as the input or signal voltage, the feedback is said to be positive or regenerative and the circuit will likely go into oscillation. If the voltage is in reverse phase (crest for trough of the wave-form), the feedback is negative or degenerative. Let the amount of the feedback voltage be a fraction (F) of the output voltage E. Then the actual input will be FE plus the original signal e. The output voltage is equal to the actual input voltage multiplied by the voltage amplification A. Thus E = A(e + FE). Solving this equation for the effective amplification or gain of a feedback amplifier, we find

When F is positive, the circuit is regenerative, and vice versa. When the " feedback factor " FA is very large, the gain becomes (— 1/F), and the effective amplification is independent of the gain A of the amplifier alone. This means that, with degenerative feedback, the amplifier will have great stability, retaining its overall voltage gain at a constant value for long periods of time despite appreciable changes in battery voltages, temperature, and mechanical vibration.

Degenerative feedback also reduces wave-form or harmonic distortion arising in the amplifier. This is because the distortion is fed back and is itself degenerated. Noises which arise in the amplifier, particularly in the later stages, are similarly reduced in magnitude.

A third advantage lies in the fact that wide ranges of frequency can be amplified, with nearly equal response over the entire band of frequencies.

On the other hand, the total voltage gain is less than with the amplifier alone, unless additional stages are added or a " balanced feedback " is used in the manner to be described later.