Automatic Gain Control PreAmplifier Circuit Diagram
The preamp circuit uses an easily obtained 741 op amp set for an internal gain about 200.
| PARTS LIST | |
| R1 | 100kΩ |
| R2 | 470Ω |
| R3 | 10kΩ |
| R4 | 10kΩ |
| R5 | 560kΩ |
| R6 | 100kΩ |
| R7 | 10kΩ |
| R8 | 10kΩ PRESET |
| 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.
BF245
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.
741 PIN CONFIGURATION
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.
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Please send your ideas, which are very important for our success…
tanks for sending
very good
Please send me your Automatic Gain Control PreAmplifier Circuit Diagram.Thanks.
good
Hei!
Is there a way how to calculate all the necessary element values? Or it’s done just by trying? Thanks!
Can the electrolytic caps be MMLC instead?
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,
Thanks,
hi, 2N3819 FET should work. It is a very common one and readily available.
AWEGFAWER
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
ok very good
what does this means what is the exact specification on the these two capacitor 0.22µF (224)
and 0.1µF (104).
hi, 0.22uf is 220nf (224) is just the common code for this value.
there is a book named FORST MIMMS mini note books
THE OPAMP 741 1458 AND OTHERS
THERE IS A PAGE ON CALCULATEING THE GAIN
P.S.radio shack sold the book long ago
can this circuit work on catv network and how do i make it an inline amplifier to be powered with 18vdc.Thanks.
In Theory it should work, but I thought the drain of the Fet should go back directly to the 12 Volt supply ?
may you explain how the circuit works??
whats actually the function of mosfet here i cant understand whats happening
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.