Camera Orbit Rotation Problem

Hello guys, I need some help. This is the peace of code which is used to rotate my camera.

Now if I'm rotating it, my camera makes a circle while rotating, but I want it just to rotate only the direction and not to make a big Orbit.

 if (Input.GetMouseButton(0))
             {
                 xDeg += Input.GetAxis("Mouse X") * xSpeed * 0.02f;
                 yDeg -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;
                 
                 ////////OrbitAngle
                 
                 //Clamp the vertical axis for the orbit
                 yDeg = ClampAngle(yDeg, yMinLimit, yMaxLimit);
 
                 // set camera rotation 
                 desiredRotation = Quaternion.Euler(yDeg, xDeg, 0);
                 currentRotation = transform.rotation;
                 
                 rotation = Quaternion.Slerp(currentRotation, desiredRotation, Time.deltaTime * zoomDampening);
                 transform.rotation = rotation;

             }

For a better clarification please see this picture. https://copy.com/kGTGO9htTL8D