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# Automatic Gain Control

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#### srobertjames

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What's a good resource for me to get started designing a simple Automatic Gain Control?

I'd like to keep my signal with a DC offset of 3.5 Volts, and 2V peak-to-peak.

I'm not asking "Where can I find someone else's schematic to assemble?" but rather "Where can I find a beginner's introduction, so, knowing a bit about analog and op amp circuits already, I'll be able to design my own simple AGC"?

The internet has an abundance of tutorials on almost everything electronics related. AGC circuits are no exception. AGC had early implimentation in AM radio recievers, so I might start out reading up on those fundamental designs.

Here is a simple circuit, as an example, for a microphone preamp.
https://www.arrl.org/tis/info/pdf/9711036.pdf

Just found it after a moment of browsing.

Which brings up the ARRL, which publishes The Radio Amateur's Handbook. That publication along with many others has fantastic information for the beginner on the concepts behind AGC circuits. And any group of AM radio enthusiasts on internet forums or hobby blogsites will abound with fundamental resources.

I realize you may not be interested in AM radio, but a little light reading on the topic is a good place to get your feet wet.

Analog multiplier ICs, such as built by Analog Devices, are a simple way to get an AGC function at audio frequencies. You connect the audio signal to one input and a DC level to control the gain at the other input.

Analog multiplier ICs, such as built by Analog Devices, are a simple way to get an AGC function at audio frequencies. You connect the audio signal to one input and a DC level to control the gain at the other input.

Thanks - but how will that *Automatically* adjust the gain?

Thanks - but how will that *Automatically* adjust the gain?
You rectify the output level to generate a DC level and then run that through an inverter op amp with some added offset, to get the desired DC for the multiplier (since as the output signal increases you want the multiplier control signal to decrease to give negative gain feedback).

Edit: To get a constant 3.5V dc output offset that you desire, you will need to capacitively couple the multiplier output and add the offset at that point with a resistive divider, since the multiplier dc offset will shift with the gain setting.

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Analog multiplier ICs, such as built by Analog Devices, are a simple way to get an AGC function at audio frequencies. .

Yup, agree 100%
It will have very low distortion, excellent frequency response, be very fast, and have very little dc shift in the output.

Excellent - thanks for the clear explanations.

Since my goal is more to learn analog than to have a marketable product, let me follow up: How can I make my own analog multiplier, using transistors or op amps? How can I discover the special sauce the ICs use and use it in my own recipes (even if it doesn't come out as professional)?

How can I make my own analog multiplier, using transistors or op amps? How can I discover the special sauce the ICs use and use it in my own recipes
Analog IC's are designed by analog engineers with years of experience, Therefore you would need to become an analog engineer with years of experience.

Analog IC's are designed by analog engineers with years of experience, Therefore you would need to become an analog engineer with years of experience.

True. Let me rephrase my question: What is the most basic multiplier circuit, understanding that it won't be the same quality as a real analog engineer's?

True. Let me rephrase my question: What is the most basic multiplier circuit, understanding that it won't be the same quality as a real analog engineer's?
See https://www.electro-tech-online.com/custompdfs/2009/11/7AnalogMultipliers.pdf, although that may be a little more detail then you really want to learn. But multipliers are not beginner's stuff. They depend upon the non-linear characteristics of semiconductors. The simple ones only operate over a limited range of signal polarity and amplitudes. They require complex circuits to make them operate over a wide range of signals.

A more simple AGC circuit can be made using a FET as a variable resistor. The FET is used as part of a voltage divider to ground to vary the signal amplitude. The gate of the FET is fed the rectified output signal being controlled. A larger signal generates a larger rectified DC voltage to the gate, which turns on the transistor, reducing it's resistance, and thus lowering the signal amplitude, performing the AGC function.

Thanks.

I assume I should smooth the rectified signal, perhaps with a LPF passing < 1 Hz or so? (Anything more than that wil get confused by the sound signal variation).

Also: it would seem that a half rectification is enough and I don't need full bridge.

Thanks.

I assume I should smooth the rectified signal, perhaps with a LPF passing < 1 Hz or so? (Anything more than that wil get confused by the sound signal variation).

Also: it would seem that a half rectification is enough and I don't need full bridge.
Yes, generally you want a long filter time-constant so the gain is not constantly going up and down due to short term variations in the signal level. You probably want it to be several seconds or longer.

And half wave rectification is fine.

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