Automatic gain control amplifier circuit

Claim 1

An automatic gain control amplifier circuit comprising: variable attenuator means including the source to drain circuit of an n-channel field effect transistor acting as a variable resistor and a resistor forming a voltage divider with said field effect transistor for variably attenuating an input electrical signal; a fixed gain amplifier including a first operational amplifier including a pair of oppositely connected diodes connected between the non-inverting input of said first operational amplifier and ground, said first operational amplifier having its non-inverting input coupled to the output of said variable attenuator means for generating an output signal from said automatic gain control amplifier circuit; a diode for rectifying the output of said fixed gain amplifier; threshold detecting means for generating an output signal when the output of said diode is greater than a predetermined amplitude; and integrating means including a second operational amplifier coupled to the output of said threshold detecting means for integrating the output of said threshold detecting means with respect to time for controlling the atTenuation of said field effect transistor, and an integrating capacitor and a diode connected between the inverting input and output of said second operational amplifier, wherein said integrating means has a time constant such that only slowly varying signals are attenuated while rapidly fluctuating signals do not affect said integrating means.