Stable AGC transimpedance amplifier with expanded dynamic range

An automatic gain control (AGC) circuit, comprising:

• a gain stage having,
• a gain stage amplifier coupled to an input of the AGC circuit,
• a gain stage current source configured to bias the gain stage amplifier, and
• a gain control mechanism coupled to the gain stage amplifier and configured to vary a gain of the gain stage amplifier;
• and
• a gain control circuit having an input coupled to an output of the gain stage amplifier and configured to produce a gain control voltage based on the output of the gain stage amplifier, said gain control voltage is coupled to the gain control mechanism and used to adjust the gain of the gain control mechanism;
• wherein:
• said gain control circuit comprises,
• a peak detector coupled to an output of said gain stage and configured to detect a peak output of said gain stage amplifier, and
• a comparator configured to compare the peak output to a gain control reference voltage and apply an output of the comparison to the gain control mechanism;
• said AGC circuit further comprising:
• a dummy gain stage having gain characteristics similar to said gain stage and configured to produce a gain stage reference voltage at an output of the dummy stage, including,
• a dummy amplifier,
• a dummy current source configured to bias the dummy amplifier, and
• a dummy gain control mechanism coupled to the dummy amplifier and configured to vary the gain of the dummy amplifier;
• wherein:
• said peak detector is also coupled to the output of the dummy gain stage and said peak detector is configured to detect said peak output by a comparison of the gain stage output and the dummy gain stage output;
• and
• said gain control circuit is also configured to adjust the gain of the dummy gain stage variable gain control mechanism;
• said AGC circuit further comprising a gain compensation circuit configured to reduce bias current in each of said gain stage amplifier and a corresponding dummy gain stage amplifier in said dummy gain stage based on the gain control voltage.

A method of controlling a gain of a gain staga amplifier over a wide dynamic range of inputs applied to the gain stage amplifier, comprising the steps of:

• determining a gain control signal based on an output of the gain stage amplifier;
• and
• varying an amount of gain of the gain stage amplifier by,
• varying an amount of feedback from an output of the gain stage amplifier to an input of the gain stage amplifier based on the gain control signal, and
• reducing a bias current of the gain stage amplifier based on the gain control signal.

An automatic gain control (AGC) circuit, comprising:

• means for producing a gain control signal based on an output of the gain stage amplifier;
• means for varying an amount of gain of the gain stage amplifier, including,
• means for varying an amount of feedback from an output of the gain stage amplifier to an input of the gain stage amplifier based on the gain control signal, and
• means for reducing a bias current of the gain stage amplifier based on the gain control signal.

An AGC circuit, comprising:

• a gain stage amplifier biased by a bias current and having an input coupled to an input of the AGC circuit;
• a feedback device coupled between an output of the gain stage amplifier and the input of the gain stage amplifier and configured to feedback a portion of an output signal of the gain stage amplifier to the input of the gain stage amplifier;
• a gain control circuit configured to produce a feedback control signal based on the output signal of the gain stage amplifier, said feedback control signal coupled to the feedback device,
• a gain stage compensation circuit configured to drain a portion of the gain stage amplifier bias current;
• wherein the feedback device is configured to regulate the portion of said output signal fed back into the input of the gain stage amplifier and regulate the portion of bias current drained from the gain stage amplifier based on the feedback control signal.