Adjustment Speed of Automatic-Volume-Control Systems

Abstract

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 + K 1 K 2 , where K 1 is the rectifier voltage increase per db increase of amplifier output, and K 2 is 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.