A Selective Circuit

Author: J.B. Hoag

Fig. 26 K. A band-pass amplifier

Figure 26 K shows a circuit with a degenerative network whose frequency response has a selective peak, as in Fig. 26 L.

Fig. 26 L. Response curve of the circuit of Fig. 26 K

By changing the values of R₅, R₆, and R₇ (with a single knob), the peak of curve B in Fig. 26 L can be shifted to the various frequencies throughout the range of the amplifier, without changing the relative sharpness of the curve.²

The selective amplifier of Fig. 26 K, without the regenerative feature at the bottom, can be used as a frequency analyzer. Suppose, for example, that the input signal consisted of the voltages from a microphone and that it was desired to know the relative strengths of the fundamental and various harmonics of a sound wave impinging on the microphone. The meter M (a cathode-ray oscilloscope will do) is read as R₅₆₇ is changed. It will be large when the amplifier passes the fundamental frequency, and proportionally large as each of the harmonics is passed over. Obviously R₅₆₇ must be calibrated in frequency; and allowance must be made for the frequency response of the microphone by calibrating the apparatus with a succession of frequencies all of the same amplitude.

When the regenerative feedback of Fig. 26 K is added, the circuit can be made to oscillate at a frequency determined chiefly by its selectivity curve. An unusually pure sine wave is generated by this simple circuit.

1	See Proceedings of the Institute of Radio Engineers, Vol. 26, page 216 (1938).
2	Proc. I. R. E. 26, 233 (1938).