Is it possible to to take backup of orginal Xbox games without connecting the Xbox console to PC.

No. Making backups requires a modded console and necessary software. Since the XBOX DvD drive does not support burning, a laptop or desktop is needed to transfer the appropriate files to to create the copy of your game.

I have a modded console, but i dont know how to take backup of a orginal game.

Please, don't hesitate to use the search button here. There are a lot of guides for various things that can help you out, like this one: How to Copy/Backup XBOX Games