Moody's Investors Service said Tuesday it raised the debt ratings of National Semiconductor Corp. after Texas Instruments Inc. closed its $6.5 billion purchase of the company last month.

Moody's raised the ratings three levels to "A1" from "Baa1." Both are investment-grade ratings.

Texas Instruments and National Semiconductor are two of the world's biggest makers of analog chips and had previously been long-time rivals. Analog chips are widely used in electronics to transform signals such as sound into digital form that computers can understand.

National Semiconductor will become a Texas Instruments subsidiary, and Texas Instruments will assume all three of National Semiconductor's senior unsecured notes as if it had issued the debt. Moody's said in a statement that Texas Instruments' "A1" long-term and Prime-1 short-term ratings reflect its "highly defensible positions as the dominant market leader in a wide range of analog and mixed signal processing markets."

"This strong market position, combined with the expectation of solid long term growth in demand should produce strong profitability and cash flows," the ratings service said.

Shares of Texas Instruments climbed 2.3 percent, or 62 cents, to $27.08 in afternoon trading.