This is an alternative method of sharpening and one that only sharpens the 'luminance' of the image - that is to say, its black & white content.

This method has two advantages:
a) It doesn't enhance 'colour noise'.
b) It doesn't have the drawback of 'lost information' which occurs when swapping 'Modes'. This would happen if we swapped to 'Lab Mode' to sharpen in the (Lab) Luminosity Channel.

This method is very quick to apply.

The Method

Duplicate the layer you wish to sharpen (say, 'Backgound').

Working on the 'copy' layer, change the layer blend mode to 'Luminosity' (do this by clicking on the drop-down arrow to the right of 'Normal' in the blend window at the top left of the Layers Palete).

Next, go to Filter > Sharpen > Unsharp Mask and adjust to suit the image.

The effect of sharpening may be reduced by changing the opacity of the copy layer. Changes can also be made by experimenting with different layer bland modes.

As the sharpening has been carried out on a separate layer, a 'Layer Mask' can be added and the sharpening effect removed selectively.