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This is the most basic, bare-bones envelope detector circuit. The biggest limitations come from the voltage drop of the diode, meaning the envelope will be a lower voltage than the waveform peak. Quiet signals (lower than the Vf) will be ignored entirely.
The pot in the lower left is just to change the amplitude of the test signal, and the opamp is just acting as a buffer/voltage follower so that the test signal does not get distorted due to the potentiometer.
The pot in the upper right controls the responsiveness, which is a tradeoff between ripple and response time to sudden volume changes.
The signal goes through the diode, which ignores the negative part of the waveform (half-bridge rectifies). This then goes into a capacitor referenced to ground with a resistor hooked up in parallel. This resistor-capacitor pair have the effect of a lowpass filter. The capacitor will charge quickly through the diode during the positive peak, and then will slowly discharge through the resistor. The result is a rough approximation of the amplitude envelope.
This topology is the most basic demonstation of the theory behind envelope detection. This is likely to be not useful in most real-world applications, for that, please check out my quad opamp version.
The most basic improvement to this circuit would be the addition of a full-bridge rectifier, to capture both the positive and negative peaks. This lowers the ripple by about half and makes the envelope closer to the actual waveform peak. The next improvement would be overcoming the voltage drop of the rectifier, which can be done by replacing it with a precision rectifier (using opamps). Another improvement on that would be amplifying the lowpass output to match the waveform peaks. All of these improvements are in my quad opamp version.
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