CodeIgniter ajax responseText加载不起作用

I have not understood how Ajax works with CodeIgniter. I explain you the problem.

In the view "Articles" i initially load all the articles. I have a select box too. When i perform an "onChange" i execute a getData() Ajax call with the classic "xmlhttp". The result would be to view only articles which have the needed tipology. "Tipology" is the connector field behind the View (where i see all articles) and the Ajax file.

The error that Firefox console displays me is the following:

 GET http://localhost/CodeIgniter/index.php?c=ajax_controller?tipologia=Recensioni 404 Not Found 13ms

I don't know if it's the right way to do it, but here i attach you my code, explaining the process:

Main articles view:

 <select onChange="getResults()" name="tipologia" id="tipologia">
      <option>--Seleziona--</option>
      <option>Recensioni</option>
      <option>News</option>
 </select>

In the Main articles view, here is the ajax call

 var xmlhttp;

 function getResults() {

      xmlhttp = new XMLHttpRequest();

      var tipologia = document.getElementById("tipologia").value;
      var url = "./index.php?c=ajax_controller?tipologia=" + tipologia;

      xmlhttp.onreadystatechange = callback;

      xmlhttp.open("GET", url, true);
      xmlhttp.send();
 }

 function callback() {

      if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
           console.log(xmlhttp.responseText);
      }     
 }

The response should replace (innerHTML = responseText) the div that actually contains this code (however, the not filtered View and the filtered View should execute the same foreach):

foreach ($contacts as $contact) {
    echo $contact->id;
    echo " -> ";
    echo $contact->artist;
    echo " : ";
    echo $contact->album;
    echo " -> ";
    echo "<a href='/CodeIgniter/index.php?c=new_controller id=".$contact->id."'>Dettagli</a>";
    echo "</br>";
}

This always happens inside the Main article view.

In the model i have this function

function get_ajax_results() {

    $tipologia = $_REQUEST['tipologia'];


    $query = $this->db->query("SELECT artist, album, cover FROM contacts WHERE tipologia = '".$tipologia."' ");
    return $query->result();    
}

In the Ajax controller...

public function index() {

    $this->load->model("new_model");

    $data['contacts'] = $this->new_model->get_ajax_results();           

    $this->load->view('ajax_view', $data);          

}

Where is the error?