Op-amp envelope detector - capacitance load issue
The opamp will saturate at the negative rail unless you add some more components. The opamp can take a while to come out of saturation affecting the circuit operation. You may need to lower R1 if you are running at a high frequency.

Note that R2 and R3 will form a voltage divider, so if this is undesirable, make R2 very small. I try to avoid opamps hitting their current limit, which it will if R2 is small.







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