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Automatic gain control circuit for controlling gain of video signal in television receiver

  • US 4,827,511 A
  • Filed: 05/12/1987
  • Issued: 05/02/1989
  • Est. Priority Date: 05/12/1986
  • Status: Expired due to Term
First Claim
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1. An automatic gain control (AGC) circuit for controlling a gain of a video signal in a television receiver, comprising:

  • tuner means having input terminal means for receiving a video signal at a radio frequency (RF), RF amplifier means for amplifying the RF video signal which is received at said input terminal means, frequency-converting means for frequency-converting the RF video signal amplified by said RF amplifier means, into an intermediate-frequency (IF) video signal, and output terminal means for outputting the IF video signal coming from said frequency-converting means;

    IF amplifier means for receiving said IF video signal output from the output terminal means of said tuner means, and for amplifying the received IF video signal;

    detecting circuit means for receiving the output signal of said IF amplifier means, and for video-detecting the received output signal;

    IFAGC circuit means for receiving the detected output signal of said detecting circuit means, and for generating a voltage signal for controlling the gain of said IF amplifier means, in accordance with the level of the detected signal received;

    first time-constant circuit means, for receiving the voltage signal generated by said IFAGC circuit means, for applying a first time-constant to the received voltage signal, and for supplying the received voltage signal with said first time-constant, as an IFAGC voltage signal, to said IF amplifier means;

    RFAGC circuit means for receiving said IFAGC voltage signal, and for generating a voltage signal for controlling the gain of said RF amplifier means of said tuner means, in accordance with the level of said received IFAGC voltage signal;

    second time-constant circuit means, for receiving the output voltage signal of said RFAGC circuit means, for applying either a second or third time-constant to the received output voltage signal, and for supplying the received output voltage signal with the second or third time-constant, as an RFAGC voltage signal, to said RF amplifier means of said tuner means, said second and third time-constants being longer than said first time-constant; and

    time-constant control means for receiving the output voltage signal of said RFAGC circuit means, and for controlling the time-constant of said second time-constant circuit means, in accordance with the change in level of the received output voltage signal.

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