This concerto makes use of a popular chamber music ensemble of the time (flute, violin, and harpsichord), which Bach used on its own for the middle movement. The harpischord acts both as a concertino and ripieno instrument, alternating between obligatto and figured bass parts. An earlier version, BWV 1050a, exists, and has many small differences from its later cousin, but no major difference in structure or instrumentation.

Reference : https://musopen.org/music/1208/johann-sebastian-bach/brandenburg-concerto-no5-in-d-major-bwv-1050/