As the Federal Reserve prepares to end its third round of bond buying next week, the central bank plans to hang on to the record $4.48 trillion balance sheet it has accumulated since announcing the first round of purchases in November 2008. Holding bonds on the Fed’s balance sheet limits the supply of securities trading on the public markets, which helps keep prices up and yields lower than they otherwise would be. “The Fed message is we think we’ve done enough to generate momentum and keep the economy on the right track.