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Automatic Gain Control Circuit

IP.com Disclosure Number: IPCOM000095480D
Original Publication Date: 1964-Jan-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Dickinson, WE: AUTHOR

Abstract

The optical feedback gain control circuit maintains the amplifier peak output between two levels and is used in speech amplifying circuitry. The amplifier gain is changed only during silent intervals.

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Automatic Gain Control Circuit

The optical feedback gain control circuit maintains the amplifier peak output between two levels and is used in speech amplifying circuitry. The amplifier gain is changed only during silent intervals.

The amplitude of the output voltage of power amplifier 10 controls the intensity of light source 11, providing optical feedback to vary the impedance of photoresistor 12 at the input of amplifier 10. The intensity of source 11 is controlled by the voltage of storage capacitor 13. This, in turn, is charged or discharged by a pair of trigger circuits 14 and 15 and logic circuitry 16. Both trigger circuits are reset off between spoken words. Trigger 14 is set on by peak detector circuit 17, if the peak amplitude of the response of amplifier 10 to the subsequent word exceeds 75% of the amplifier's capability. Peak detector 18 sets trigger 15 on, if the peak amplitude exceeds 90% of the maximum amplifier capability.

The purpose of the gain control circuitry is to adjust the resistance 12 so that the spoken word turns on trigger 14 but not trigger 15. Therefore, fore, at the end of the spoken word, as the triggers are reset, a pulse operates logic circuitry 16 to discharge capacitor 13 if trigger 14 is not on, to charge the capacitor if trigger 15 is on. Circuitry 16 does neither, leaving the capacitor in its previous state, if trigger 14 is on and trigger 15 is off.

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