High- and Low-Pass Amplifiers

Author: J.B. Hoag

It is possible to amplify voltages of different frequencies by the same amount over a wide band. It is also possible to use filters to suppress all frequencies above (or below) a sharp "cutoff" frequency. It is now proposed to point the way to the design of amplifiers with negative feedback which will accomplish, in a simple and economical manner, the combined functions of these two units.

Fig. 26 G. An R-C coupled feedback amplifier

Figure 26 G shows an R-C coupled amplifier whose negative feedback unit contains several condensers and resistors, so arranged and of such value as to give the overall frequency response curve of Fig. 26 H, curve 4. Curve 1 is for the amplifier alone.¹

Fig. 26 H. Response curves of the amplifier of Fig. 26 G

Figure 26 I shows a degenerative amplifier which sharply suppresses all frequencies above a certain value, yet passes the lower frequencies with fidelity. Its response curve is given in Fig. 26 J.

Fig. 26 I. A low-pass amplifier. (Proc. I. R. E., Vol. 26, page 219, 1938)

Fig. 26 J. Response curve of the amplifier of Fig. 26 I