larzo92 on "add a new wpdb class"

hi, I want to add a new class wpdb $wpdb1 = new wpdb(my_db_login, my_db_password, my_DB_NAME,my_DB_HOST); for my wordpress blog but I don't know how.
when I put it the following class in wp-config i got this error message
Fatal error: Class 'wpdb' not found in line 80