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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).

Last Update: 2010-11-21