Δ-Σ Automatic-Gain Controller

Abstract

Automatic-gain control (AGC) is a closed-loop feedback regulating circuit. It is used in many systems to maintain a nearly constant signal amplitude at its output despite variation of the signal amplitude at the input. Its role is to reduce the amplitude dynamic range so that circuits following the AGC can handle a reduced dynamic range. In 1925, Harold A. Wheeler invented and patented automatic volume control (AVC). Karl Küpfmüller published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included AVC [1]. AM radio has used AGC since then. It is used in most radio receivers to equalize the average volume (loudness) of different radio stations due to differences in the received signal strength, as well as variations in a single station's radio signal due to fading. Related applications of AGC are in radar systems, telephone recording, audio/video recording, biological sensory systems, etc. Correct operation of mobile telephone systems is greatly dependent on AGC due to environmental factors such as seasonal change in vegetation, fluctuation of users, overpower of a neighboring base station, etc. A block diagram of a typical AGC is shown in Fig. 10.1. In brief, we will describe its operation as presented in [2]. As can be seen, the output of VGA, signal V1, is fed back to the detector of the amplitude change and to the next device in the signal-processing chain as well. If needed, signal V1 can be amplified. A detector block can be implemented as an envelope (or rectifier), square law, true RMS, or logarithmic. A detected signal V2 is smoothed (low-pass filtered) to produce a nearly DC signal. This signal is compared with a reference signal VR. The result of the comparison is used to generate the control voltage Vc to adjust the gain of the VGA. There are many publications, books, patents, VGA circuit solutions, and Internet sites dealing with theory, implementation, and applications of AGC devices. There are IC chips on the market as well. For instance, Analog Devices Inc. application note describes a low-frequency AGC circuit (AD8336) used in audio and power equipment [3]. A block diagram of a delta-sigma based automatic-gain control circuit is presented in reference [4]. However, rectification of analog input signal is done off delta-sigma IC chip, with the use of analog components.