This circuit has an accuracy of 1%. The components used must be precision as well. The input frequency range is 40 Hz to 10 kHz with the indicated components.
This configuration that appears several times with variations on this site is from old American...
This configuration (somewhat critical, as the imbalance of characteristics between transformers can cause...
This circuit is from a Popular Electronics of the 90's. Any power MOSFET can be used. The circuit...