ATLANTA (AP) _ The former chief of staff at the Carter Presidential Center was sentenced to 28 months in prison Tuesday for impersonating former President Carter and a Saudi prince in a scheme to bilk $650,000 from a billionaire.

George G. Schira, 53, of West Palm Beach, Fla., abruptly ended his trial in June by pleading guilty to nine counts of fraud.

The plea came just before Carter was to testify against Schira in U.S. District Court in Atlanta. Carter fired Schira in April 1987.

During the trial, Assistant U.S. Attorney Robert Schroeder said Schira made a series of phone calls in 1988 to a business associate of Cypriot engineering tycoon George Paraskevaides.

Schira first impersonated Carter and, later, Prince Bandar Bin Sultan, the Saudi ambassador to the United States.

He obtained $150,000 from Paraskevaides and asked for another $500,000, claiming falsely the Carter Center was in financial trouble.

Judge Jack T. Camp said Schira could have ''damaged the reputation of an institution that does much fine work.''