Why Nvidia just got a rare stock downgrade

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• Nvidia stock was downgraded to 'Neutral' by NewStreet Research on valuation concerns.
• Analyst Pierre Ferragu cited limited upside in the shares after a 157% year-to-date rally.
• "We would be buyers again, but only on prolonged weakness," NewStreet Research said.

Nvidia stock got a rare downgrade on Wall Street on Friday.

NewStreet Research analyst Pierre Ferragu downgraded shares of the artificial intelligence powerhouse to "Neutral" from "Buy" in a Friday note...