A classic biased envelope detector, with half-wave rectification. This will give worse results (ripple) than my dual-supply version which uses opamps to make a precision full-bridge rectifier. In addition, changing the response time also changes the envelope voltage levels, meaning tuning may be more complex than using a pot. Another limitation of this topology is that even when the input is at 0 amplitude, the envelope will be biased (above 0).
This design is more helpful for demonstrating a simple method of envelope following/detection/extraction, and is probably not very useful for most real-world applications. See my quad op-amp design for something to use in the real world!
