cómo obtener la suma de una columna con la consulta de codeigniter

Puedes usar select_sum() función de codeigniter. Prueba el siguiente código-

$query = $this->db->select_sum("tb1.amount")
                  ->from("table1 as tb1")
                  ->join("table2 as tb2","tb1.id = tb2.id") 
                  ->where("tb2.status",1)
                  ->get();
$query = $query->result();

O

$query = $this->db->query('SELECT sum(tb1.amount) FROM table1 as tb1 join table2 as tb2 on tb1.id = tb2.id where tb1.status = 1'); $query = $query->result_array();