Preheat oven to 180 degrees C. Grease one 25cm ring tin.
Sift the flour, bicarb soda and salt together. Set mixture aside.
Separate the eggs. In a large bowl beat the egg whites until soft peaks form. Gradually add 1/2 cup of the sugar, beating until stiff glossy peaks form. Set aside.
Cream butter, 1 1/2 cups sugar, egg yolks, lemon zest and lemon juice together until fluffy. Add flour mixture alternately with the yoghurt to the egg yolk mixture. Gently fold in the egg whites and pour the batter into the prepared pan.
Bake at 180 degrees C for 50 to 60 minutes. Let cake cool in pan for 10 minutes then turn out onto a rack to finish cooling. Serves 12.