Draw the circle with a compass or drill a hole in the center. You can use a ruler or a string to attach to the center with a nail or screw. Then once your line is drawn use a drill and drill ever couple of inches all the way around the circle. Use the jigsaw to connect the dots. Should be much less of a curve now between points. Clean it up with a sanding paper around a handle or something to make a small sanding block. Or you could clean it with a dremel.