Balanced Feedback Amplifiers

Author: J.B. Hoag

In the circuit of Fig. 26 E, both positive and negative feedbacks are used simultaneously. The positive feedback is designed so as to be proportional to the input signal and independent of the frequency over that band which is to be amplified linearly. Hence the first two stages are designed to give equal response to all frequencies over the desired range. Since the positive feedback need not be large, the gain in these stages can be small. It is comparatively easy to make a low-gain amplifier flat over a wide frequency range. The response curve of this amplifier is shown in Fig. 26 F.

Fig. 26 F. The frequency response curve of a balanced feedback amplifier. Curve A, amplifier alone, no inductances in plate leads. Curve B, amplifier alone, but with inductances. Curve C, balanced feedback amplifier. Curve D, amplifier with negative feedback only. Note the extremely wide range of frequencies which are amplified. See Proceedings of the Institute of Radio Engineers, Vol. 26, page 1378, Nov. 1935