I'd recommend using sour cream instead. I have better luck with sour cream not curdling than yogurt. Temperature control is the key to success. heat the SC over the cooking pasta. Whip the egg in a separate bowl. Temper the egg by adding a small amount of the warmed SC to the eggs as you whisk or stir it in. Repeat until the two are completely combined. Return the bowl with the SC and egg to the boiling pot and heat while stirring until it's just warm enough to melt the cheese. Stir the cheese in gradually.
If you get the sauce too hot, the SC and the cheese are subject to curdling.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan