The personal tax exemption will increase by $100 to $2,150 next year, the Internal Revenue Service announced Friday.

The exemption is adjusted each year to account for inflation, as are the standard deductions. The standard deduction for couples filing joint returns, which is $5,450 this year, will go up to $5,700. The deduction for singles, now $3,250, will rise to $3,400.Under a 1986 law, income-tax brackets are widened each year to avoid "bracket creep" - the problem of taxes rising faster than real incomes. These inflation adjustments, known as indexing, increase the amount of income a person may earn before additional earnings are taxed at a higher rate.

For example, in 1990 a couple filing jointly could have $32,450 of taxable income before moving into the 28 percent bracket. For 1991, that figure rises to $34,000.

These adjustments result in these tax brackets for 1991:

-For couples, the first $34,000 of taxable income is taxed at 15 percent; the next $48,150 is taxed at 28 percent, and anything over $82,150 is taxed at 31 percent.

-For single people, the first $20,350 will be taxed at 15 percent; the next $28,950 at 28 percent, and anything over $49,300 will be taxed at 31 percent.

As a result of a new tax law, there will be no 33 percent tax rate after 1990, although a limitation on itemized deductions and a phase out of personal exemptions will have the effect of raising rates above 31 percent for upper-income taxpayers.

The IRS also announced that in 1991 the standard deductible mileage rate for business use of a car will be 271/2 cents a mile, up from 26 cents this year. The change reflects higher gasoline prices and a 5.1-cent-a-gallon tax increase that took effect Dec. 1.

None of the changes announced Friday will affect 1990 tax returns, which are due by next April 15.