Authors list

Hi!
I’m using this code to show the authors list:

function contributors() {
  global $wpdb;
  $authors = $wpdb->get_results("SELECT ID, user_nicename from $wpdb->users ORDER BY display_name");
	foreach($authors as $author) {
	  echo "<li><a href='".get_bloginfo('url')."/?author=".$author->ID;
	  echo "'";
	  echo " title='";
	  the_author_meta('display_name', $author->ID);
	  echo "'>";
	  echo get_avatar($author->ID);
	  echo "</a></li>";
	}
}

But I want to show only the category “author”.
I don’t want to show “Administrator”, “Editor”…

Can you help me?