Automatic Volume Control

Author: J.B. Hoag

Due to fading, the carrier frequency often varies in amplitude. This causes undesirable changes in the volume of sound radiated from the loudspeaker. The fading sometimes occurs too rapidly to be compensated for manually by a gain-control system. Within limits, an a.v.c. circuit automatically keeps the output at constant volume.

Fig. 32 N. Principle of automatic volume control (a.v.c.)

In Fig. 32 N, the diode-detector rectifies the r.f. (or the i.f.) currents and develops a d.c. voltage across resistor R₁, with negative at its ungrounded end, and of an amount proportionate to the audio signal plus the fading modulation on the carrier. The audio component is bypassed through C₂ (the r.f. goes through C₁ and is kept out of C₂ by resistor R₂), leaving the undesired fading voltage to continue to the left as indicated by the arrow marked a.v.c. If the carrier strength increases, the voltage across R₁ increases proportionately, and that part due to the undesired changes (fading) is fed back as a negative value onto the grids of the r.f. and i.f. amplifiers, to undo the increase. This can be understood from Fig. 32 0 which shows the dynamic curve of a remote-cutoff pentode used in an r.f. or i.f. stage.

Fig. 32 0. Effect of a.v.c. voltage on the r.f. (or i.f.) pentode

With a weak carrier e₁ , the plate current has a certain value i₁. A stronger carrier e₂ swings the grid more, but also increases the C-bias, so that the plate current i₂ remains of about the same strength. The changes in this figure are exaggerated. In practice it is found necessary to apply the a.v.c. voltage to several stages simultaneously in order that the accumulated effect will be satisfactory.

The a.v.c. system just described operates on weak as well as on strong carrier waves, and hence makes the circuit less sensitive than desired.

Fig. 32 P. A modern second-detector circuit with delayed a.v.c.

In the delayed a.v.c. circuit of Fig. 32 P, the a.v.c. action does not occur unless the signal strength is above a predetermined minimum value. In this figure; one of the diodes serves as a rectifier, one for a.v.c, while the grid and the regular plate are used as the first audio stage; the a.v.c. resistor is R₁, the r.f. bypass is C₁, the r.f. blocking resistor is R₂ and the a.f. bypass is C₂. The bias voltage developed across R₃ is applied to the a.v.c. diode plate through R₄, R₅, and L, so that no rectified current flows until the input voltage has become equal to or greater than this bias voltage.