PHP. Fatal error: Using $this when not in object context

I've got a problem

I'm new to PDO, so i'm trying to get all data from a tabla, very easy i know, but $this variable is giving me some problem, i wrote a class called DB to connect to mysql, and i use it in another class called getAll with a function called get;

DB class

<?php 
class DB
{
    private $_dbh;
    private static $_instance;
    private function __construct()
    {
        try 
        {
            $this->_dbh = new PDO("mysql:host=127.0.0.1;dbname=telmex","root","a.d.s.l");                   
        } 
        catch (PDOException $e) 
        {
            die($e->getMessage());  
        }
    }

    public static function Link()
    {
        if(!isset(self::$_instance))
        {
            self::$_instance = new DB();
        }

        return self::$_instance;    
    }
}

?>

getAll Class

<?php 

class getAll
{
    public $_dbh;
    public function __construct()
    {
        $this->_dbh=DB::Link();
    }

    public static function get($tabla)
    {
        $sql="SELECT * FROM ".$tabla."";
        if($sentencia=$this->_dbh->prepare($sql))
        {
            echo "Prepared.";
        }   
    }
} ?>

So when i run

<?php 
$select = new getAll();
    $consultar = $select->get("users");
 ?>

it gives me a fatal error.