Most any mtb or touring group derailleur should work with a 20T difference (e.g., Shimano Deore of any flavor, Suntour Mountech, etc, as well as others mentioned). If you are considering an FD for a TA-style crankset with really close spacing between the large ring and the crank, best to find a derailleur with a flat cage, not the sculpted type of cage that most modern derailleurs have.