Bernanke said the purchases have shown that the Fed still has "ammunition" to aid the economy even after having cut short-term interest rates to near zero.
Last month, the Fed also said it planned to keep its key short-term interest rate at a record low even after unemployment falls close to a normal level â¿¿ which it said might take three more years. Unemployment remains high at 7.8 percent because the economy is growing too slowly and not creating jobs fast enough.
In December, employers added 155,000 jobs, roughly matching the monthly average in 2011 and 2012. That's just been enough to slowly reduce the unemployment rate, which fell 0.7 percentage point during 2012.