By Rex Crum, MarketWatch

SAN FRANCISCO (MarketWatch) -- With earnings reports from Yahoo Inc. and Intel Corp. set for after the close, tech stocks began to show some signs of weakness as trading progressed and Federal Reserve Chairman Janet Yellen spoke about the economy in front of the U.S. Senate.

Yahoo (YHOO) was off by 16 cents a share at $35.55 in advance of its second-quarter report. Prior to the start of trading, FBR Capital Markets analyst William Bird initiated his coverage of Yahoo with a market perform rating and $37-a-share price target. Bird said the "sins of past management continue to haunt" the company.

Intel (INTC) shares edged up by 9 cents to $31.58 as the chip giant also prepared to deliver its second-quarter earnings results.

Among other leading tech companies reporting earnings this week, Google Inc. (GOOG) fell by $3.26 a share to $591 and eBay Inc. (EBAY) was off by 20 cents at $50.96 a share.

Several social-media companies were in the red after Yellen said that the stock valuations of that sector appear to be stretched. Declines came from Yelp Inc. (YELP), which fell by 5% to $67.59, Pandora Media Inc. (P), down by 3.3% at $26.05 a share, Facebook Inc. (FB) , which fell 2% to $66.58 and Angie's List Inc. (ANGI), which also fell 2% to $10.42 a share.

The Nasdaq Composite Index (RIXF) was down by 34 points at 4,405 and the Philadelphia Semiconductor Index (SOX) was also in negative territory.

More tech news from MarketWatch:

Intel may see modest earnings and sales gains

Yahoo's earnings to focus on Alibaba, display business

Former White House PR chief may be in line for Apple job

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