Joan Rivers -- Doctor Who Performed Throat Procedure Fired

The doctor who performed the endoscopy on Joan Rivers has been fired by the clinic's board.

Sources at Yorkville Endoscopy tell us Dr. Lawrence Cohen stepped down after the board asked him to do so -- however the board isn't directly blaming him for Joan's death.

A rep for the clinic confirmed he was ousted from his position as medical director.

According to our sources ... the board's issue with Cohen is that he allowed Joan's ENT doc to step in and perform an unauthorized procedure. Reportedly, that procedure was a biopsy ... and Joan went into respiratory and cardiac arrest while it was being done.