Automatic level control circuits heretofore on the market utilized operational amplifiers that have feedback loop extending from the output terminal back to a gain control terminal so that as a voice signal exceeds a predetermined level zener diodes provided in a feedback circuit cause the gain of the operational amplifier to be reduced to a desirable level. In U.S. Pat. No. 3,459,179 there is disclosed a directly coupled non-linear feedback network for an operational amplifier which includes two pairs of diodes that are connected in the output and input terminals of the operational amplifier for reducing the gain of the operational amplifier in response to input signals having a magnitude exceeding a threshold overload value.
In U.S. Pat. No. 3,166,720 there is disclosed a linear amplifier with a non-linear feedback network. More particularly, this device pertains to a linear amplifier which has a predetermined finite forward impedance, and a feedback circuit adapted to remain open until the output voltage reaches a predetermined magnitude. Provided in the feedback circuit are zener diodes that are connected in a back-to-back arrangement. In U.S. Pat. No. 4,138,612 there is disclosed still another circuit for selecting a clipping level that includes a back-to-back connected zener diode. Similar circuits are disclosed in U.S. Pat. No. 3,153,152 entitled Variable Limitor for Analog Computer Using a Fixed Zener Diode, U.S. Pat. No. 3,268,830 entitled Operational Amplifier Gain Control Circuit Utilizing Non-linear Devices, and U.S. Pat. No. 3,094,675 entitled Degenerative Feedback Amplifier Utilizing Zener Diode. One problem with the above disclosed circuits is that after the circuits have been out of use for a period of time, if it receives a signal, this signal due to the high gain of the amplifier will initially appear at the output as a high amplitude signal or spike prior to the operational amplifier settling back to its normal operating amplitude.
USPTO Backgrounds
USPTO Backgrounds