Adjustment Speed of Automatic-Volume-Control Systems

The behavior of an a.v.c. amplifier, following a sudden change of input level, is analyzed on the basis of the following assumptions, which are justified in the case of many practical a.v.c. amplifiers: (1) the open-circuit voltage developed by the rectifier is a linear function of the decibel output level of the amplifier; (2) the decibel gain reduction in the controlled stages is a linear function of the gain-control voltage; (3) only one resistance-capacitance filter section is important in delaying delivery of the rectifier output voltage to the gain-control points. It is shown that the last condition is desirable from the standpoint of stability. The analysis shows that, following a sudden change of input level, the fraction (1-1/e) of the decibel gain change required to reach a new equilibrium occurs in (RC)/M seconds. RC is the time constant of the filter section specified in assumption (3), while M, a dimensionless "flatness factor," is defined as the decibel change of input level required to produce a 1-db change of output level, under equilibrium conditions. Further, M = 1 + K1K2, where K1is the rectifier voltage increase per db increase of amplifier output, and K2is the db gain reduction per volt of control. If the change of input level is sufficient in magnitude to cause amplifier overload (or "underload"-stoppage of rectifier action), the control voltage changes with the time constant RC until the overload disappears. Overload may greatly increase the time required for gain readjustment.