Automatic Gain Control Pre-Amplifier Circuit Diagram

Automatic Gain Control Pre Amplifier Circuit

Automatic Gain Control PreAmplifier Circuit Diagram

The preamp circuit uses an easily obtained 741 op amp set for an internal gain about 200.

Automatic Gain Control Pre-Amplifier Circuit Diagram

R1 100kΩ
R2 470Ω
R3 10kΩ
R4 10kΩ
R5 560kΩ
R6 100kΩ
R7 10kΩ
R9 100kΩ
C1 1µF 16V
C2 1µF 16V
C3 10µF 16V
C4 1µF 16V
C5 0.1µF (104)
C6 10µF 16V
C7 0.22µF (224)
D1 1N4148
D2 1N4148
Q1 BF245
IC1 LM741

A portion of the op amp’s output signal is rectified by the 1N4148 diodes, then filtered and fed to the gate of the FET input shunting circuit. As the output rises, more and more input shunting takes place. That is, more of the input signal is bypassed, effectively keeping the output level constant.



This preamp circuit offers a 100:1 limiting action. The input level can change over a 100:1 ratio with little or no effect on the output level.



The output output level itself can be set from less than unity all the way up to nearly the gain of the amplifier, making the circuit usable in other applications as well.

Automatic Gain Control PreAmp Circuit Automatic Gain Control PreAmplifier Circuit Automatic Gain Control Pre Amplifier Circuit Diagram

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20 comments on “Automatic Gain Control Pre-Amplifier Circuit Diagram
  1. rahul singh says:

    very good

  2. rohan says:

    Please send me your Automatic Gain Control PreAmplifier Circuit Diagram.Thanks.

  3. Inese says:


    Is there a way how to calculate all the necessary element values? Or it’s done just by trying? Thanks!

  4. Vox says:

    Can the electrolytic caps be MMLC instead?

    • Anthony Brown says:

      no, they are there to block the DC voltage. As it is a single supply voltage circuit some parts of the circuit will not work if there is a DC voltage present.


    hello , can you suggest me the replacement or subtitute the FET BF 245,,,no available this kind of transistor in our place,Please reply me soon,
    Hoping your kind consideration regarding this matter,


  6. Lorelee says:

    We have tried the circuit, but it does not function as AGC. It could produce sound , maybe its pre-amp circuit works fine. what could be the problem?
    Thanks a lot

  7. مهندس حسین عظیمی says:

    ok very good

  8. muzammil says:

    what does this means what is the exact specification on the these two capacitor 0.22µF (224)
    and 0.1µF (104).

  9. Henry Dheel says:

    there is a book named FORST MIMMS mini note books
    THERE IS A PAGE ON CALCULATEING THE GAIN shack sold the book long ago

  10. Elaavie Emem says:

    can this circuit work on catv network and how do i make it an inline amplifier to be powered with 18vdc.Thanks.

  11. Anthony Brown says:

    In Theory it should work, but I thought the drain of the Fet should go back directly to the 12 Volt supply ?

  12. Jman says:

    may you explain how the circuit works??

  13. sarath says:

    whats actually the function of mosfet here i cant understand whats happening

  14. Chris says:

    The performance of this circuit depends heavily on the JFET used. The BF245A or BF245B will give different results than the BF245C pictured. The BF245C gate-source OFF voltage is -3.2 to -7.5v. If you substitute, find a JFET with a similar OFF voltage. The BF245C is now obsolete but can be found on eBay. Almost any N-channel JFET with similar OFF voltage will work almost the same.

    Also the supply voltage has some effect on the performance because varying this changes the voltage at the junction of R3 and R4. This determines the gate-source voltage of the JFET with no signal at the audio input: This ideally should be greater than the gate-source OFF voltage.

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