A Phase-Shifting Device for the Rapid Determination of Audio-Frequency Amplifier Characteristics

A description is given of a phase-shifting device which can be used with a beat-frequency oscillator and a cathode-ray oscilloscope to determine quickly the essential points on the frequency-response curve of an audio-frequency amplifier. Use is made of the fact that the phase shift through a single-stage amplifier is ±135 degrees when the gain has dropped to 70.7 per cent of the mid-frequency value, if the amplifier coupling circuit has no resonances. By properly introducing a 45-degree phase shift into the cathode-ray oscilloscope circuit from a network equivalent to an ideal cable a straight-line Lissajous figure is obtained at the 70.7 per cent relative-gain frequencies. The method is extended to include the determination of other significant frequencies in the commonest types of single- and multiple-stage audio-frequency amplifiers.