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  • Automatic Gain Control or AGC is a circuit design which maintain the same level of amplification for sound or radio frequency. If the signal is too low the AGC circuit will increase (amplify) the level and if is to high will lower it to maintain a constant level as possible. The Automatic Gain Control principle is widely use in AM receivers and sometimes AGC is called an compressor-expander because it acts just like one.

    On the electroschematics.com site you can find some AGC schematics, just use the seach box.

    Mic preamplifier with AGC schematic

    mic preamp agc

    
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    13 Responses to "Automatic Gain Control – AGC Circuit"

    1. Nash Nash Garza says: on October 30, 2009 at 5:50 am

      the value of resister above cap 1uf/16v is unreadable..can you tell me whats its value?

    2. What those 100k resistors in series doing?
      Next time check schemo before publish

    3. What is the purpose of the two 100k resistors in series. Is there a node missing in resistors with pin 3 of OP?

      • Jim Keith Jim Keith says: on July 30, 2012 at 2:08 pm

        Yes, there is a line missing between two nodes: 470Ω resistor in series with 4.7uF capacitor and two 100K resistors connected in series. This biases the non-inverting input of the op amp at half battery voltage.

        Note that the Vgs (off) characteristics of junction FETs vary greatly, so if you do not obtain good compression with loud signals, try another device.

        This is almost identical to a circuit I made 42 years ago.

    4. can u explain the entire circuit including the function of all resistor, capacitor, potentiometer, jfet, circuit at non inverting pt. and filters. reply asap!

      tnx!

    5. The op amp is configured as a single supply non-inverting amplifier –max gain = 2.2M/1K -1.

      Two 100K resistors on left are a bias divider that sets the operating point midway between Vcc and ground. The 4.7uf is bypass cap so microphone load resistor (470Ω) operates at AC ground potential. DC feedback is via the 2.2M resistor, and the 1uf cap causes the DC gain to be 1. That way, the quiescent amp output voltage is also midway between Vcc and ground.

      The P-channel JFET acts as a variable resistor at low voltages (e.g. 100mV) –the more negative the gate, the higher the resistance and the lower the gain of the amplifier since it is in series with the 1K max gain element.

      On the lower right, the 1K in series with the 1uf cap feeds an AC voltage signal to the two diodes that are connected as a cascade voltage doubler that outputs a negative voltage across the 22uf cap. It turns off the JFET thus increasing its resistance –so the larger the output voltage, the lower the gain and the AC output voltage remains approx. the same. The lower 100K is the load for the voltage doubler so that it can discharge in the event the AC output signal reduces.

      When the JFET is at the high end of its resistance (amplifier min gain), it tends to act as a current source (non-linear resistance). This would add distortion, so the 100K in series with the 1uf cap above the gate adds negative feedback to make the resistance more linear. The 100K below this allows this negative feedback voltage to develop because the 22uf cap would otherwise bypass this signal.

      On the right, the 10K in parallel with the 0.0047uf is a pre-emphasis high-pass filter that accentuates the high frequency response –the FM receiver has a corresponding de-emphasis filter to attenuate the high frequencies along with annoying high frequency noise (hiss).

      The two capacitors on the right are coupling capacitors to eliminate the DC component in both the output signal, and the input stage of the modulator that has an unspecified DC voltage that may be greater (or lower) than the DC output of the amplifier –otherwise the polarity of the voltage across the electrolytic capacitor would be unknown.

      P1 simply adjusts the output signal level to suit the signal level requirement of the modulator.

      If you make this circuit, use schottky diodes instead of the ancient 1N64 germanium diodes.

    6. Jim I appriciate your comments and explanation of the circuit.What is your opinion of the circuit ,or do you have a better one to share with us or me..as I would like to built one for a recording music 36ch.console. THanks.. Nash…

    7. I think that the circuit is workable–I would use a modern op amp and JFET–input attenuation may be necessary to reduce potential distortion. I might be able to do better, but it would take some experimentation. I’ve made similar circuits in the past, but for industrial rather than audio applications.

    8. Rodney Jackson says: on January 10, 2014 at 2:24 pm

      Hello Jim and thanks for the circuit and description. My need for this is to take a signal from a radio receiver and drop the output enough (~50mV) to feed into a power amp. I hope this will do the job.
      The radio will be fiddled with by people who have their own needs, but the amplified signal (in another room) will need to remain at a somewhat constant level.

      Regards,
      Rodney Jackson

    9. Rodney Jackson says: on January 10, 2014 at 2:26 pm

      P.S. With regard the website, it is fantastic.
      R.J.

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