60 dB Wide Dynamic Range, Low Frequency AGC Circuit Using a Single VGA

Low frequency automatic gain control (AGC) circuits are used in audio and power equipment for applications such as sensitive microphone preamplifiers (preamps) and regulators. An AGC circuit, a closed-loop feedback system, is shown in Figure 1. The loop consists of a controllable gain element, a detector, a stable reference and a comparison circuit.

This application note describes a low frequency AGC circuit using a wide dynamic range AD8336 variable gain amplifier (VGA) as the gain control element, an AD736 rms-to-dc converter as the detector, a low cost rail-to-rail AD8551 op amp, and an ADP3339 LDO as the reference. Because of its wide controllable gain range and circuit flexibility, the AD8336 is featured in this application note.