Circuit provides 70 dB of AGC
A comparator autoranges to ensure that the automatic-gain-control section sees a signal between −60 and −25 dBm, keeping the FET in its linear region.
Ben Segal, Telecommunications Technology, Sunnyvale, CA; Edited by Margery Conner and Fran Granville -- EDN, February 2, 2012
Originally published in the May 5, 1979, issue of EDN
The circuit diagrammed in the
figure provides a 70-dB automatic-gain-control range for input
voltages of −60 to +10 dBm over a
bandwidth of 55 Hz to 15 kHz. The
worst-case distortion is 3%, but distortion
typically measures less than 1% in
the 100-Hz to 10-kHz range.The signal autoranges to keep the input to the AGC section between −60 and −25 dBm; this procedure allows the FET to operate in its most linear region. A comparator performs the autoranging, comparing a −25-dBm reference with the signal from A1 and A2. A4 furnishes the AGC action, settling a −15-dBm output as a reference voltage; A5 provides a fixed 15-dB gain and an output of 0 dBm. A6, A7, and the FET constitute the AGC’s feedback circuit.
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