i designed an Automatic Gain Control (AGC) circuit using the LT1498 op-amp and a BFW10 N-channel JFET, achieving a perfect output (in LTspice simulation). However, upon replacing the LT1498 with the MCP602, the circuit no longer controls the gain as expected. what might be the reason? [posting the images of the schematic, input and output of the circuit using LT1498 for your reference.] note : my aim was to maintain the output signals between 4vpp and 5vpp for input signals ranging from 0 to 2vpp

 

However, upon replacing the LT1498 with the MCP602, the circuit no longer controls the gain as expected.

Is this in the sim or in real life?
If the sim, show if for the MCP602.

 

Is this in the sim or in real life?
If the sim, show if for the MCP602.

its in the simulation... posting the schematics and output for your reference.(i replaced LT1498 with MCP602 and changed the feedback resistor from 120k to 10k so that my output doesnt hit saturation.)

 

The difference is probably because of the differences in opamp characteristics. Different input impedance and different internal gain.
And, as with all feedback systems, the gain in the feedback portion has a large effect on the output.

 

changed the feedback resistor from 120k to 10k so that my output doesnt hit saturation.)

Why the large change?
They are both RR type op amps so should have the same maximum output.

And that resistor doesn't change the maximum output, it just changes the loop gain.
The input level sensitivity to the feedback loop determines the maximum output.

 

What is the intended application for this Compressor ?

Depending on the funkiness of the selected JFET,
it could be very difficult to achieve the desired end-result.

Are You open to using a different method/Circuit ?
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.
.

 

What is the intended application for this Compressor ?

Depending on the funkiness of the selected JFET,
it could be very difficult to achieve the desired end-result.

Are You open to using a different method/Circuit ?
.
.
.

Thank you for your suggestion. While I appreciate your input, I prefer to continue with the current method/circuit. I would be interested in hearing your thoughts on this approach.

 

Why the large change?
They are both RR type op amps so should have the same maximum output.

And that resistor doesn't change the maximum output, it just changes the loop gain.
The input level sensitivity to the feedback loop determines the maximum output.

Thank you for your insights. When I used a 120k resistor, the output was hitting saturation. To address this, I tried a random lower value.

 

The difference is probably because of the differences in opamp characteristics. Different input impedance and different internal gain.
And, as with all feedback systems, the gain in the feedback portion has a large effect on the output.

The slew rate of the MCP602 is significantly higher than that of the LT1498, and the input offset voltage of the MCP602 is in the millivolt range, whereas for the LT1498, it is in the microvolt range. Do you think these differences might be the reason we are not getting the proper output?

 

To address this, I tried a random lower value.

Random selection of parts in electronic design seldom works well.

To reduce the output voltage try reducing the value of R12 going to the base of Q1.

 

Random selection of parts in electronic design seldom works well.

To reduce the output voltage try reducing the value of R12 going to the base of Q1.

Thank you for your advice and patience. As a beginner in electronics circuit design, I appreciate the guidance. I will try reducing the value of R12 as you suggested.

 

Thank you for your suggestion. While I appreciate your input, I prefer to continue with the current method/circuit. I would be interested in hearing your thoughts on this approach.

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JFETs like to do their own thing, they are not even remotely linear.
This means that You are starting-out with a wild-card and hoping for the best.

There are quite a few other compressor topologies that are much more predictable,
and more importantly, easier to adjust the important parameters.

You didn't mention what the end-purpose will be for this Project.
This could make a big difference in the topology choices that will
tend more towards providing the performance and satisfaction that you're looking for.
.
.
.

 

The question about why a simple AGC can go wrong after replacing an op amp. Why not study a completed project in depth?
But where can you find an example where performance elements and circuit theory are shown.
Is there a decent AGC using inexpensive parts and available pcb boards to make it organized and tidy with good explanation.

Ebay sells cheap LM393 comparator boards, Then the project advances to slightly complex and requires some hands on familiarity.
Seeing the forest from the trees by isolating sections into sub circuits.
I found a white paper that explains what appears to have some performance as an AGC, the application is for voice recognition.
The paper is straight forward and professinal, The subheadings help to identify elements in the design, it has the individual op amp circuit sections layout. The circuits for LM833, the LM393 comparator and TL431 reference test are also given are valuable in evaluating features along with the scope shots. The test setup should reproduce the same results and adjustment should be logical. It is the fine adjustment that improves performance.
and this is shown in this example that it can be done using inexpensive components.

David Finch AGC Whitepaper (davidjfinch.com)