Automatic Gain Control (AGC) circuits

In the early years of radio circuits, fading (defined as slow variations in the amplitude of the received signals) required continuing adjustments in the receiver's gain in order to maintain a relative constant output signal. Such situation led to the design of circuits, which primary ideal function was to maintain a constant signal level at the output, regardless of the signal's variations at the input of the system. Originally, those circuits were described as automatic volume control circuits, a few years later they were generalized under the name of Automatic Gain Control (AGC) circuits [1,2]. With the huge development of communication systems during the second half of the XX century, the need for selectivity and good control of the output signal's level became a fundamental issue in the design of any communication system. Nowadays, AGC circuits can be found in any device or system where wide amplitude variations in the output signal could lead to a lost of information or to an unacceptable performance of the system. The main objective of this paper is to provide the hypothetical reader with a deep insight of the theory and design of AGC circuits ranging from audio to RF applications. We will begin studying the control theory involved behind the simple and primary idea of an AGC system. After that, with the theory as our guide, we will study and describe the characteristics and performance of the most popular AGC system components. Finally, a few practical AGC circuits will be presented and analyzed. At each section, emphasis will be made on those parts of the circuit that could be potentially implemented in an integrated circuit form. A list of references will be presented at the end of this paper for the eventual reader who could be interested in learning or reading more about this subject.