Design and Operation of Automatic Gain Control Loops for Receivers in Modern Communications Systems Tutorial

into the effective operation of variable gain amplifiers (VGA) in automatic gain control (AGC) applications. Figure 1 is a general block diagram for an AGC loop. The input signal passes through the VGA to produce the output level to be stabilized. The detector’s output is compared against a setpoint voltage to produce an error signal, which is then integrated to produce a gain control voltage. This is applied to the control input of the VGA. The attenuator shown between the VGA and the detector is used to align the maximum output level of the VGA with the maximum input level of the detector. In the course of this article several key issues will be addressed, including VGA types, loop dynamics, detector types, the operating level of VGA and the operating level of the detector. Then an example application revolving around an AD8367 VGA will be presented for further discussion of the material. VGA TYPES There are two major classes of VGA in use today. The first is the so-called IVGA (input VGA), which can be regarded as a passive variable attenuator followed by a fixed-gain amplifier. The second type is the output VGA (OVGA), which is essentially equivalent to a fixed-gain amplifier followed by a passive attenuator. An IVGA is the preferred choice for a receive AGC system because the available outDESIGN AND OPERATION OF AUTOMATIC GAIN CONTROL LOOPS FOR RECEIVERS IN MODERN COMMUNICATIONS SYSTEMS TUTORIAL